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University of Gondar

College of Natural and Computational Sciences

Department of Geology

MSc program in Paleontology and Paleoenvironment

Upper Jurassic Bivalves of Dejen - Gohatsion Section, Blue Nile Basin, Central Ethiopia

WORKU BIRHAN

Advisor: - Dawit lebene (PHD.)

September,2020

Gondar, Ethiopia

Upper Jurassic Bivalves of Dejen - Gohatsion Section, Blue Nile Basin , Central Ethiopia

Approved by

Wuletaw mulualem ______

Head, Department of geology signature Date

Dr. Dawit Lebenie ______

Advisor Signature Date

Dr. Balemwal Atnafu ______

External Examiner Signature Date

Wuletaw Mulualem ______

Assistant Prof

Internal Examiner Signature Date

Zerihun Dawit ______

Cherman Signature Date

Declaration

I declare that this thesis is my original master degree work and has not been submitted to any university or institution in the past for the award of any degree or diploma at any other university. All sources and materials used for this thesis work have been well referenced and duly acknowledged.

Worku Birhan ______Student Signature Date

Approved by Dr. Dawit Lebene ______(Advisor) Signature Date

The study area has excellent exposures of upper Jurassic sedimentary rock which is situated in north western Ethiopian plateau. This research is focused on detail field investigation, description and systematics of upper Jurassic bivalves for the reconstruction of depositional environment and ecology. Different scientific methods were followed in order to achieve and accomplish the stated research problem and the objectives of the study with three main methods. These are pre-field work, detail main fieldwork and post field work.

Totally six hundred fifty-two specimens of fossil bivalves have been collected and fifty-four representative samples were prepared for systematic paleontology by identifying down to species level where possible from the Antalo limestone formations in the Blue Nile Basin from three sections with two associations. The sections from which the specimens have been collected include Filikilik section (31%), Demibeza section (48%) and Geligele section (21%). Twenty-one bivalve species have been identified and described systematically and they are belonging to two superfamily, nineteen family, three subfamily, eighteen genera, four subgenera. From the twenty one species, 16 species belong to Demibeza section (Pholadomya socialis, Bucardiomya somaliensis, Pholadomya hemicardia, Pholadomya lirata, Pholadomya aubryi, Pholadomya murchisoni, Pholadomya beamontensis, Palaeonucula lateralis, Paleonucula cuneiformis, Pterotrigonia scabra, Liostrea acuminate, Eligmus asiaticus, Actinostrean gregareum, Nanogyra nana, Nanogyra fourtaui and Ilmatogyra africana), six species were identified from Filikilik (Modiolus bipartitus, Modiolus imbricatus, Paleonucula cuneiformis, Pholadomya murchisoni, paleonucula lateralis and Nanogyra nana ) and six species from Geligele (Nanogyra fourtaui ,Ilmatogyra africana, Nanogyra nana and Liostrea acuminate, Graphaea balli and Exogyra fourtaui) are described systematically. Bivalves in the study area represent mainly Basinal environments and deep shelf. No endemic species were recorded from the studied sections.

Keywords: - Upper Jurassic, Systematic Paleontology, Paleoecology, Depositional Environment, Blue Nile Basin, Bivalves.

ACKNOWLEDGEMENT

My welfare is only in God, who has been bestowed upon me during this research project, and throughout my life.

First of all, and for being a tremendous mentor for me, I would like to express my special appreciation and thanks to my advisor and instructor Associate professor Dr. Dawit lebene. I would like to thank him for guidance, advice, encouragement and introducing me to the scientific the research work. His thorough work on the thesis is most gratefully acknowledged.

Secondly, I am deeply grateful my Mom, Dad and family members; your prayer for me was what sustained me thus far. I am highly indebted to them for their blessing, guidance, advice, encouragement and support. My gratitude goes to all of my friends and relatives who directly or indirectly helped me to complete this thesis.

Thirdly, I would like to give my sincere gratitude to University of Gondar, department of geology and staff members.

TABLE OF CONTENTS ABSTRACT ...... i ACKNOWLEDGEMENT ...... ii TABLE OF CONTENTS ...... iii LIST OF PLATES ...... ix CHAPTER ONE ...... 1 1 INTRODUCTION ...... 1 Background ...... 1 Description of the study area...... 3 1.2.1 Location of the study area ...... 3 1.2.2 Accessibility of the study area ...... 3 1.2.3 Climate and vegetation of the study area ...... 4 1.2.3.1 Climate ...... 4 1.2.3.2 Vegetation ...... 4 1.2.4 Physiography of the study area ...... 4 1.2.5 Drainage pattern of the study area ...... 5 1.2.6 Population settlement ...... 5 Statement of the problem ...... 6 Limitation of the study ...... 6 Previous works ...... 7 Objectives...... 8 1.6.1 General objectives ...... 8 1.6.2 Specific objectives ...... 8 Significance of the research ...... 8 Thesis Outline ...... 9 CHAPTER TWO ...... 10 2 REGIONAL GEOLOGY ...... 10 Introduction ...... 10 Geodynamic setting and sedimentation in Ethiopian sedimentary basin ...... 10 Jurassic Limestone formation in Ethiopian Sedimentary Basin ...... 13 Stratigraphy of Blue Nile Basin ...... 14 2.4.1 Basement rock ...... 14 2.4.2 Paleozoic and Mesozoic Sedimentary Successions ...... 15

2.4.2.1 Pre-Adigrat I ...... 15 2.4.2.2 Pre-Adigrat II ...... 15 2.4.2.3 Pre-Adigrat III ...... 16 2.4.2.4 Adigrat Sandstone ...... 16 2.4.2.5 Gohatsion Formation ...... 17 2.4.2.6 Antalo Limestone ...... 17 2.4.2.7 Mugger Mudstone ...... 19 2.4.2.8 Debre Libanos Sandstone ...... 19 2.4.3 Tertiary volcanic ...... 20 CHAPTER THREE ...... 22 3 METHODOLOGY AND MATERIAL ...... 22 Materials ...... 22 3.1.1 Field instrument ...... 22 3.1.2 Laboratory instruments ...... 22 Methodology ...... 22 3.2.1 Pre-field work ...... 23 3.2.2 Field work ...... 23 3.2.3 Post field work ...... 24 CHAPTER FOUR ...... 26 4 Systematic Paleontology ...... 26 Background ...... 26 4.1.1 Family Pholadomyidae Gray,1847 ...... 26 4.1.1.1 Genus Pholadomya Sowerby,1823 ...... 26 4.1.2 Family pholadomyid King ,1844 ...... 27 4.1.2.1 Genus Bucardiomya Rollier in Cossmann, 1912 ...... 27 4.1.3 Family Pholadomyidae Gray,1847 ...... 28 4.1.3.1 Genus pholadomya Sowerby,1823 ...... 28 4.1.4 Family Pholadomydae Gray,1847 ...... 29 4.1.4.1 Subgenus Bucardiomya Bucardioma Rollier in cossman ,1912 ...... 29 4.1.4.2 Sub genus Rollier in cossman,1912 ...... 30 4.1.5 Family Pholadomyidae Gray,1847 ...... 33 4.1.6 Family Pholadomydae Gray,1847 ...... 34 4.1.6.1 Subgenus Bucardioma Rollier in cossman ,1912 ...... 34 4.1.7 Family Mytilidae Rafinesque, 1815 ...... 34 4.1.7.1 Genus Modiolus Lamarck,1799 ...... 34

4.1.8 Family Mytilidae Rafinesque, 1815 ...... 35 4.1.8.1 Genus Modiolus Lamarck,1799 ...... 35 4.1.9 Family Trigoniidae Lamarck, 1819 ...... 38 4.1.9.1 Genus Pterotrigonia Van Hoepen ,1929 ...... 38 4.1.10 Family Ostreidae WILKES, 1810 ...... 38 4.1.10.1 Genus Actinostreon BAYLE, 1878 ...... 38 4.1.11 Family Palaeolophidae Malchus, 1990 ...... 39 4.1.11.1 Genus Actinostreon Bayle,1878 ...... 39 4.1.12 Family Malleidae Lamarck, 1819 ...... 41 4.1.12.1 Genus Eligmus Eudes-Deslongchamps, 1856 ...... 41 4.1.13 Family Nuculidae Gray,1824 ...... 42 4.1.13.1 Genus Palaeonucula Uenstedit,1824 ...... 42 4.1.14 Family Nuculidae J. Gray,1824 ...... 43 4.1.14.1 Genus Palaeonucula Quenstedt,1830 ...... 43 4.1.15 Family Pholadidae Lamarck,1809 ...... 45 4.1.15.1 Subfamily Jouannetiinae Tryon,1862 ...... 45 4.1.16 Family Gryphaeidae Vyalov,1936 ...... 45 4.1.16.1 Subfamily Exogyrinae viyalov,1948 ...... 45 4.1.17 Family Gryphaeida Vyalov,1936 ...... 46 4.1.17.1 Genus Ilymatogyra Stenzel, 1971 ...... 46 4.1.18 Family Gryphaeidae Vyalov,1936 ...... 49 4.1.18.1 Subfamily Exogyrinae viyalov,1948 ...... 49 4.1.19 Genus Liostrea Douville,1904 ...... 50 4.1.19.1 Genus Exogyra say,1820 ...... 51 4.1.20 Family Gryphaeidae, vyalov ,1936 ...... 52 CHAPTER FIVE ...... 54 5 PALEOECOLOGY AND PALEOENVIRONMENT ...... 54 Background ...... 54 Bivalve Associations (BA) ...... 55 5.2.1 Modiolus bipartitus Associations ...... 55 5.2.2 Pholadomya Murchisoni Association ...... 57 CHAPTER SIX ...... 66 6 DISCUSSION ...... 66 Introduction ...... 66 paleobiogeography ...... 72

CHAPTER SEVEN ...... 76 7 CONCLUSION AND RECOMMENDATION ...... 76 Conclusion ...... 76 Recommendation ...... 77 REFERENCES ...... 78

LIST OF FIGURES

Fig 1 Location map of study area ...... 3 Fig 2 Physiographic map of a study area ...... 5 Fig 3 Sedimentary Basins of Ethiopia (source: Ethiopian Ministry of Mine, 2011)...... 11 Figure 4 Geological map of the Blue Nile basin (source: Dawit Lebene, 2010) ...... 20 Figure 5 Stratigraphy of central Ethiopia, Blue Nile Basin (Dawit Lebene,2010) ...... 21 Fig 6 Mode of life of M. bipartitus association ...... 57 Figure 7 feeding mode of M. bipartitus association ...... 57 Figure 8 Mode of life ph. murchisoni association ...... 59 Figure 9 feeding mode of ph. murchisoni association ...... 59 Figure 10 percent abundance plot of ph. murchisoni association ...... 60 Figure 11 Percent abundance plot of M. bipartitus association ...... 60 Figure 12 Demibeza section out crop2 (post-storm) ...... 62 Figure 13 Demibeza section out crop1 (storm)...... 62 Figure 14 Demibeza section percent abundance ...... 63 Fig 15 Geligele section out crop 1 ...... 64 Fig 16 Geligele section out crop 2 ...... 64 Fig 17 Filikilik section out crop 1 ...... 64 Fig 18 Filikilik section out crop 2 ...... 64 Figure 19 Geligele percent abundance plot ...... 65 Figure 20 Filikilik percent abundance plot ...... 65 Figure 21 Paleogeographic reconstruction of the Middle- Late Jurassic world ...... 75

LIST OF TABLES

Table 1Measurements (in mm) of Pholadomya socialis (Morris & Lycett,1854) ...... 27 Table 2 Measurements (in mm) of Bucardiomya somaliensis (Cox,1935) ...... 27 Table 3 Measurements (in mm) of Pholadomya hemicardia (hoemer,1836) ...... 28 Table 4 Measurements (in mm) of Pholadomya lirata (hoemer,1836) ...... 29 Table 5 Measurements (in mm) of Pholadomya aubryi (Douville,1886) ...... 30 Table 6 Measurements (in mm) of Pholadomya murchisoni (Sowerby, 1827) ...... 33 Table 7 Measurements (in mm) of Pholadomya beamontensis (Sowerby,1823)...... 34 Table 8 Measurements (in mm) of Modiolus bipartitus (J. sowerby,1818) ...... 34 Table 9 Measurements (in mm) of Modiolus imbricatus (Sowerby,1818) ...... 35 Table 10 Measurements (in mm) of Pterotrigonia scabra (Lamarck,1819) ...... 38 table 11 Measurements (in mm) of Actinostreon erucum (Defrance,1821) ...... 39 Table 12 Measurements (in mm) of Actinostreon gregareum (Sowerby,1816) ...... 39 Table 13 Measurements (in mm) of Eligmus asiaticus (Douville,1916) ...... 41 Table 14 Measurements (in mm) of palaeonucula lateralis (Terq.and Jourdy,1980) ...... 42 Table 15 Measurements (in mm) of Palaeonucula cuneiformis (J. Sowerby, 1840) ...... 43 Table 16 Measurements (in mm) of Genus Jouannetia (Desmoulins,1828) ...... 45 Table 17 Measurements (in mm) Nanogyra fourtaui (Stefanini, 1925) ...... 45 Table 18 Measurements (in mm) of Ilymatogyra (Afrogyra) africana (Lamarck, 1801) ...... 46 Table 19 Measurements (in mm) of Nanogyra nana (J. Sowerby,1822) ...... 49 table 20 Measurements (in mm) of Liostrea acuminate (Sowerby,1816) ...... 50 Table 21 Measurements (in mm) of Exogyra fourtaui (Stefanini,1925)...... 51 Table 22 Trophic nuclei of the bivalve associations ...... 61

LIST OF PLATES Plate 1 photograph of mould species ...... 32 Plate 2 photograph of mould species ...... 37 Plate 3 Photograph of mould and shell species ...... 44 Plate 4 Photograph of mould and shell species ...... 48 Plate 5 Photograph of shell species ...... 53

LIST OF ACRONYMS

A: Actinostrean M: Modiolus

B: Bivalve m: mobile

BA: Bivalve Association N: Nanogyra nana

BND: Blue Nile Demibeza PH: Pholadomya

BNF: Blue Nile Filikilik Sf: Suspension feeder

BNG: Blue Nile Geligele SI: Shallow infaunal

Cf.: Compare with S: Sessile

D: Deposit feeder Sp: Species

E: Epifaunal T: Trigonia

Fig: Figure Tmax: Maximum thickness

Gps: Global positioning system UNESCO: United Nations, Educational

H: Height scientific and cultural organization

H/L: Height/Length Worms: World Registration Marine

H2O2: Hydrogen peroxide species

I: Infaunal

Is: Semi- infaunal

ID: Deep Infaunal

CHAPTER ONE 1 INTRODUCTION Background

Ethiopia contains large out crops of marine Jurassic sedimentary rocks which is eponymous to Jurassic Ethiopian province (Neumar,1885; Uhlig,1911; Arkell,1956; Hallam,1997) and the Blue Nile Basin of bivalve were done by showing the literature of the Ethiopian bivalve province by evaluating biogeographic affinities of the benthic fauna.

Blue Nile Basin contains excellent exposures of upper Jurassic sedimentary succession. They contain diverse benthic macrofauna assemblages, including mollusks (Bivalve, gastropoda and cephalopods ), Brachiopods and some distengrate far apart corals. The fossil record acts as a time machine, providing data on the morphology, ecology and biogeography of ancient species.

Shells of marine molluscs are subjected to alterations by numerous living benthic organisms, not only after death, but also while the individuals are still alive.

Ethiopian province consisting of Ethiopian, Somalia, Jordan, Yemen, Kenya, Madagascar, Saudi and Tunisia is evident. India and Tanzania, however can not be assigned to the Ethiopian province, due to paleolatitude play an important role in governing faunal composition, this is because of the countries plot closer to European countries such as Portugal and France than they plot Ethiopian province, i.e. India and Tanzania have the same faunal compositions, but palaeolatitudinal difference these countries is not include in Ethiopian province. This province contains numerous endemic taxa and has been recognized from the Early Jurassic until the Late Cretaceous (Weir, 1925; Muir-Wood, 1935; Arkell et al., 1952; Arkell, 1956).

During the late Triassic Blue Nile Basin of central Ethiopia was positioned near to the western margin of the Tethys about 20 south of equator (Scotes. et al,.1999). Southeastern margin belongs to the Ethiopian Bivalve Province which is only poorly defined by bivalve genera/subgenera: in particular, its northern boundary is blurred, with the Arabian region occupying an intermediate position between the Mediterranean and the Ethiopian bivalve provinces.

The Jurassic period was characterized by progressive rise in sea level, an equitable climate, the westerly extension of the Tethys ocean, rifting in the central Atlantic and the starting of the splitting of the Pangea supercontinent (SellWood,1978), this continental separations with complex interplay factors, but not a single factor such as sea level changes, climate and

Physical barriers provided a greater scope for the development of provincialism in fauna rather than cosmopolitan (Hallam,1975; 1977; Liu et al.1998). Species are the basic taxonomic unit because evaluation of published taxonomic information is critical for the quality of the data and cannot always be carried out at the desired species level.

According to Hallam (1977) remarked, bivalves are the most diverse and abundant macroinvertebrate group in the Jurassic, occurring in a wide range of environments. Next to Ammonites, bivalve have received far more attention as paleobiogeographic indicators in the Jurassic. They were undergoing great evolution, radiation /and or dispersion (Hallam,1997; Heinze). This confirmed by the wide distribution of the studied Middle-Upper Jurassic bivalves’ species on shelves of the Tethyan ocean. Bivalves are known to be sensitive to environmental parameters and record changes in environment directly in their shells. Partially, these modifications can be observed from morphology.

The current sedimentary exposure of the Blue Nile basin covers nearly 120,000 km2 exposed along the Abay River and its tributaries with a total thickness exceeding 2.6 km (Russo et al., 1994; Ahmed,1997), however according to Wolela,1997 the sedimentary successions in the Blue Nile Basin reach a maximum thickness of 3000 m at the central part of the basin (Dejen– Gohatsion area) and 200 m at the extreme west (basin margin). The basin-fill has allowed correlation with the Ogaden Basin and the Mekele Outlier (Beyth,1972, Getaneh,1991, Russo et al.,1994, Bosellini et al.,1997, Hunegnaw et al.,1998). The Blue Nile Basin is one of a series of NE and NW-trending intracontinental rifts and pull- apart basins, known collectively as Karoo rift basins (Schandelmeier et al., 2004, Papini and Benvenuti, 2008), which were subsequently filled with the Permo-Triassic siliciclastic sediments. The Blue Nile basin of Paleozoic sedimentary rocks are not exposed within in the Gorge Nile. Only Late Paleozoic–Triassic rocks are exposed at the lower reaches of the gorge (Mangesha et al. 1996). This is in contrast to the presence of an extensive Paleozoic sedimentary section at the base of the Ogden Basin, to the southeast of the Blue Nile Basin (Williams 2002). The absence of Paleozoic sedimentary rocks within the Blue Nile Basin might be due to uplift during the Paleozoic Era resulting in extensive erosion throughout this area.

Description of the study area 1.2.1 Location of the study area

The study area is located in the Blue Nile River from Dejen to Gohatsion which is located between Amhara region in the northern side and Oromia region in the southern region and it is geographically bounded between by a grid of Easting from 406063E to 417070Em and 1100986N to 1120956Nm (Fig 1). The study area has coverage of approximately 122 km2 and the area is found at a distance of 229 km and 520 km from Addis Ababa and Gondar respectively.

Fig 1 Location map of study area 1.2.2 Accessibility of the study area

The study area is accessed from Gondar through the main road passed via Bahirdar- debremarkos Dejen Gohatsion to Addis Ababa (727km) and there is no other important Asphalt road which are significant to reach the study area, but the Asphalt road locates down to Abay river and the road is zigzag and difficult, specifically from dejen-gohatsion, having a traffic jam with no turning area except around the bridge. There are some foot trails, rugged and gravel

3 road with no longer continuity that makes difficult to do this thesis. The geographical feature of the study area is characterized by varied geomorphologic features. The basalt, limestone, and upper sandstone units form cliff and the gypsum unit form some dolomite cliff, whereas other units such as the marl and glauconitic sandy mudstone have gently sloped.

1.2.3 Climate and vegetation of the study area 1.2.3.1 Climate

The climate of the study area varies from humid to semiarid. Most precipitation (rainfall) occurs in the wet summer season (June through September). Annual rainfall varies between 1000 mm, in the lowland to 2000 mm, in the highland (Daniel, 1977; Conway, 1997; UNESCO, 2004; etc.). Spatial variation in rainfall amount is controlled by topography. The mean annual temperature from 1961 to 1990 was estimated to be 18.3 °C with a seasonal variation of less than 2°C. The annual potential evapotranspiration was found to be about 1100mm (Kim et al., 2008).

More than 80% of annual flow in the Blue Nile results from the summer monsoon and is concentrated between July and October (Soleiman et al., 2009). This runoff flows directly to downstream countries due to the absence of storage capacity in Ethiopia. The basin has an elevation ranging from 500 m in the western lowland to over 4000 m in the east and northeast.

1.2.3.2 Vegetation

In the Blue Nile basin, the vegetation pattern is sparsely vegetated. Vegetation type and its distribution in the study area vary with soil type, elevation and humidity and/or rainfall. Accordingly, there are areas of slightly forest, woodland and grassland coverage. Forest coverage mostly occurs along its tributary river, and fertile marl sediment deposit. The grassland region occurs on the lowland of marl fertile soil along its tributaries.

1.2.4 Physiography of the study area

The study area is characterized by different topographic features including hills, ridges, deep gorge, rugged terrains, plateaus and flat lands. The hill and ridge of the area is deposited by massive, bedded limestone unit and upper sandstone whereas, the deep gorge is covered by lower sandstone and Karroo sandstone and the flat surface consisted of by marl and shale rock unit which is affected by land slide.

Fig 2 Physiographic map of a study area 1.2.5 Drainage pattern of the study area

The main river in the study area is Abay River and there are tributaries that flow to the main river. These tributaries are: Mekentuta, Ada Wedeb, Kurar and other temporal and permanent rivers. The flow direction of the main river is from NE to SW and the tributaries flow from NW to SE along the Dejen side and from SE to NW along the Gohatsion side. The Blue Nile River, which originates from Gish Abay around Lake Tana, crosses the main road at a point approximately 285 km from Lake Tana. This river shows meandering flow arrangement of the rivers.

1.2.6 Population settlement

The population density in the area is sparsely populated. The closest big town to the study area is Dejen and Gohatsion on both sides and there are small villages that are found within the study area. The two largest ethnic groups in the area were the Oromo and Amhara. The people On the Dejen side Amharic is spoken as a first language and the people on the Gohatsion side is spoken both Amharic and Oromic language. The majority of the inhabitant beliefs are

Christians and slightly some Muslim. In the marl unit due to the presence of fertile soil the population is much more populated than other lithologic unit, because the fertile soil gives advantage to those people which are living on the marl lithologic unit.

Statement of the problem

In the Blue Nile basin, the previous studies have focused on sedimentology, stratigraphy, and geochemistry, micropaleontology, whereas macropaleontology is poorly known. This study area focuses on first detailed description of Upper Jurassic bivalves of central Ethiopia, which is a major benthic invertebrate group found in the Antalo Limestone succession of Blue Nile basin. The study area (i.e. the Blue Nile basin) has not been studied in respect of macropaleontology (e.g. the distribution patterns of Upper Jurassic bivalve genera and subgenera), which is only poorly defined and has its boundary blurred. Therefore, such detailed study can provide, for the first time, data to compare and correlate the Blue Nile basin bivalves with the Ethiopian Bivalve Province, which belongs to the southestern margin of the Tethys and to the Mediterranean Bivalve provinces. The upper Jurassic bivalves of Blue Nile Basin have not been described and figured.

Limitation of the study

The main limitation of the study area are lack of extensive database and extensive literature does not survey to identify paleogeographic distribution of bivalves in the Blue Nile Basin.

Poor preservation of fossil and removal of body shell, the bivalve fossil sample in the Blue Nile Basin found in molds and shell forms, this makes too difficult to taxonomic identification and interpret paleoenvironment. Complementary data were not obtained from the recent specimens housed in institutions, this was important to identify down to species level were possible and to put synonymy lists. A general problem when attempting to carry out paleobiogeographic analyses of marine bivalve fauna of the Blue Nile Basin is the scarcity and poor preservation of many taxa. To make the taxonomy of the bivalve species especially Pholadomya is, in general, difficult and in part confused. The difficulties result from the great variation in moulds outline and number of radial ribs. To study taxonomic and paleobiogeography large and extensive data base is important to know the distribution patterns of Jurassic bivalves in time and space, insufficient data base is one of the weakness of many paleobiogeographic studies.

Previous works

According to Kissling et al (2011) morphological descriptions, taxonomic studies and Paleobiogeography pattern of corals, brachiopod and Bivalvia were done from the Antalo limestone of Mekelle outlier of northern Ethiopia. From the result their age is Oxfordian and which were lived in the shallow sub-tidal environment and small patch reefs. They combined the newly collected data with fossil occurrence data from the Paleobiology database by conducting multidimensional scaling analyses to assess biogeographic patterns and the delineation of the Ethiopia province for Callovian to Kimmeridgian stages.

According to Fursich et al (2019) morphologic and taxonomic studies of middle and upper Jurassic bivalves from morondava basin of Madagascar. As they described most of the bivalves come from Callovian and Oxfordian strata. The bivalves represent mainly shallow-marine environments, but some of them probably occupied brackish settings.

According to Chunlian et al (1997) based on an extensive literature survey and examination of field collections the distribution pattern of Jurassic bivalve genera and subgenera within the Proto- Atlantic and along the southern margin of the Tethys is analyzed using multivariate. The origin of faunal provinciality is thought to be complex and a result of the interplay of several factors, the importance of which varied throughout the Jurassic.

Zakhera et al (2017) Callovian-Oxfordian bivalves from Central Saudi Arabia which were done in Tuwaif mountain Limestone (Callovian) and the Hanifa formations (Oxfordian). paleogeographically the studied bivalve assemblages have a dominantly Tethyan character and shows close relationships with Europe, East Africa, India and Iran. In addition, there were considerable links with middle east, North Africa and china. According to Zakhera in the central Arabia no endemic species were recorded. paleolatitude seen to play an important role in governing fauna composition (Liu et al.,1998; Kissling et al,2011).

Late Cenomanian oysters from Egypt and Jordan by Diabat. (2015), Late Cenomanian oysters occur in great numbers, wide distribution, usually as original shells, and can be used as guide fossils in northeastern Egypt and Jordan. According to Abdelhady et al (2014) Macrobenthic palaeo-communities of the middle and upper Jurassic strata of G. maghara, Egypt, Sinai, they were identified relationships with environments and to trace the temporal changes of ecosystem associated with sea-level fluctuations. They were interpreted their original environment using quantitative analysis of

7 data matrix that comprising 198 macrobenthic taxa in 138 samples and identified nine associations and three assemblages. As they concluded middle ramp setting were found to provide the best conditions for macrobenthos. According to Sabbagh et al. (2017) were interpret paleoecology and paleoenvironments by using microfacies and macroinvertebrate fossils from central Saudi Arabia of middle-upper Jurassic sections of strata. A data matrix comprising 48 macrobenthic species in 35 samples collected from four sections which were grouped in to fifteen assemblage and one poorly fossiliferous interval by means of a Q-mode cluster analysis. As they concluded that the depositional environments ranging from restricted lagoon to moderately deeper open marine basin and providing the perfect conditions for macrofossils.

Objectives 1.6.1 General objectives

The general objective of this study is to provide a detailed investigation and description of Upper Jurassic bivalves in the Blue Nile basin of Central Ethiopia, with the aim of correlating with the Ethiopian Bivalve Province, the Mediterranean Bivalve Province and the southeastern margin of the Tethys.

1.6.2 Specific objectives

The specific objectives of this study are as follows: - ➢ To study systematic paleontology of the Upper Jurassic bivalve. ➢ To describe external morphology of bivalve. ➢ To reconstruct the paleoecology by preparing bivalve fossil species association ➢ To reconstruct the paleoenvironment. ➢ To know the relative abundance of the bivalve association. Significance of the research

In Ethiopia there are many sedimentary basins known but only few of them detail local studies has no even in terms of micropaleontology. The study area (Blue Nile Basin) has not been studied in aspect of macropaleontology and this will be a first detail study.

An attempt is made in this paper to describe and discuss: -

✓ systematic paleontology and morphology description needed to interpret history of the fossil, to reconstructs depositional environment and paleoecology analysis.

Thesis Outline This thesis is compiled by seven chapters. Chapter one is introduction about the overview of the study area location, accessibility, climate, vegetation, physiography, drainage pattern, population settlement, statement of the problem, limitation, objectives, previous studies, and significance of the study. Chapter two is compilation of regional geology which are used to create conceptual framework for the study area. The third chapter states different methodology and approaches used to achieve the objectives of the research and it includes the different materials used for the research. The systematic paleontology result is clearly described under chapter four and chapter five contains paleoecological analysis and paleoenvironmental interpretations and chapter six discussion and paleobiogeography. Final conclusions and recommendations are done under chapter seven.

CHAPTER TWO 2 REGIONAL GEOLOGY Introduction

Sedimentary basins are a region of prolonged subsidence for a long period of time in the history of the earth’s surface. Now a day mechanism of basin formation is predominantly related to complex active plate tectonics in which horizontal forces are thought to be the primary cause for the formations of negative topography (variation of uplift and subside) needed for sediment accommodation (Kingston et al.,1983a; b; Busby et al.,1995; Allen and Allen, 2005).

The nature of the sediments that accumulate in a sedimentary basin is related to the environments of the physiographic basin from which the sediments were derived and in which they were deposited. Sedimentary basins typically begin with a transgressive sequence and end with a regressive sequence but they may have a long and complicated history. Transgressive sequences record a general deepening of the sea, with reduction of the land and migration of the facies towards the land. Regressive sequences record a general shallowing of the sea, with extension of the land and migration of the facies seaward (Selley, 2000).

Geodynamic setting and sedimentation in Ethiopian sedimentary basin

The sedimentary succession of Ethiopia is part of the vast sedimentary succession of East Africa, which was deposited during the Mesozoic transgression (Beyth, 1972; Bosellini, 1989, 1992; Bosellini et al., 1995, 1997). Sedimentary basins of Ethiopia cover a significant portion of the country with the exceptions of Quaternary and unconsolidated sediment deposit, which are mainly exposed in five distinct basins that located in different part of the country which are totally covers 33% surface area of Ethiopia (Ethiopian Ministry of Mines, 2011) (Figure 2.1). These are: Blue Nile basin of central Ethiopia, Mekelle basin of Northern Ethiopia, Ogaden basin of Southeastern Ethiopia basin, Gambela basin of Western Ethiopia and Southern part of the Southern rift basins.

Fig 3 Sedimentary Basins of Ethiopia (source: Ethiopian Ministry of Mine, 2011).

The NW-SE trending of Blue Nile (or Abay) Basin is a Late Paleozoic - Mesozoic intracratonic rift basin covering an area of about 120, 000 sq. km in central and NW Ethiopia and it is sandwiched between the Precambrian basement and the Oligocene - Miocene volcanic succession. The sedimentary succession in the basin reaches a maximum thickness of 3,000 m (Wolela,1997) and 200m at the extreme west (basin margin).

The East African region has been affected by two major phases of rifting. The first phase was during Late Carboniferous to Early Jurassic which extends from Ethiopia to south Africa that leads to the initiation of the break- up of Gondwanaland that were called widespread rifting in Karoo times (Norton and Sclater, 1979; Bosellini, 1989) in that further subsidence took place. This subsidence combined with sea-level fluctuations and produced cyclic patterns of shallow- marine carbonates, shales, evaporites and minor clastic deposits. Karoo is abroad term applied mainly continental rift sediments deposited widely in what was eastern Gondwanaland from late Paleozoic to the early Jurassic. The sedimentary history of the Blue Nile Basin commenced with the deposition of up to 450m thick continental undifferentiated Karroo sediments (Jepsen and Athearn, 1964; Getaneh, 1991; Mengesha et al., 1996). The second rifting relates to the formation of the East African rift system (Cenozoic to Recent).

Tectonic evolutions throughout NE-Africa and sea level fluctuations through geologic time in general have the great roles for the formation of Ethiopian sedimentary basins and thick sedimentary deposits within them. The development of most of these basins is related to the extensional tectonic events that have taken place since the late Paleozoic and continued up to Tertiary (Merla et al., 1979; Blanford, 1970). The Ogaden, Blue Nile and Mekelle Outlier are presumed to be an intracontinental rift related basin formed as a result of extensional stresses induced by the break-up of Gondwanaland starting from the Upper Paleozoic up to Tertiary Period (Mohr, 1962 and Blandford, 1970). According to Bosellini,1989; Russo et al.,1994; catuneanuetal.,2005 karoo Rift developed during the Paleozoic-Mesozoic eras as a result of extensional tectonics in relation to the spreading process along the Tethyan margin of Gondwana leading to its breakup and subsequent geodynamic events. The Karroo rift system passes through Ethiopia, Kenya, Tanzania, Mozambique and South Africa which is initial to the break-up Gondwanaland in the Permo-Triassic under NE–SW oriented stress and forms the intracontinental rifts (karoo rift) leads to the formation of Ogaden, Blue Nile and Mekelle basin and the karoo basins which is used to describe the sedimentary fills of all other basins of similar age across Gondwana. During this time, extended rift structures were formed, linking to create a broad platform depression and filled by thick sequences of clastic and carbonate formations (Salman and Abdula, 1995). The Karoo rifting was the first rifting activity occurred in the African continent since Phanerozoic, representing the initial breakup stage of the Gondwana supercontinent.

During Early Jurassic the separation of India from Africa and the emergence of the Indian Ocean . With increasing subsidence of the Blue Nile Basin, a shallow marine embayment was formed that extended northwestwards from the Indian Ocean and thus, submerged the newly formed NW–trending basin. In the Blue Nile basin this is the initial marine transgression which is marked by glauconitic sandy mudstone followed by basin deepening and the deposition of Early-Middle Jurassic marine sediments (Gani et al,.2008). Blue Nile rift , Malut rift, Muglad rift, Ogaden basin become active depositional basins due to the presence of NW-SE striking structural depressions and between late Jurassic and early cretaceous the extensional tectonics become very active, deepening the NW-SE troughs. Three main phases have evolved in Blue Nile Basin (Getaneh (1981); Gani et al., (2008): (1) pre-sedimentation phase, include pre-rift peneplanation of the Neoproterozoic basement rocks, possibly during Paleozoic time. (2) The Triassic to Early Cretaceous sedimentation phase. (3)

The post-sedimentation phase, including Early–Late Oligocene eruption of 500–2000m thick Lower volcanic rocks, related to the Afar Mantle Plume and emplacement of 300m thick Quaternary Upper volcanic rocks (Gani et al., 2008). Generally, the Blue Nile (Abay) Basin, is one of the failed arms of the Karoo Rift, which consists of the Pre-Adigrat (I, II, and III) sediments (Dawit,2010), Triassic siliciclastic Adigrat Sandstone, succeeded by transgressive Jurassic evaporite of the Gohatsion Formation and the Antalo Limestone, which are then overlain by regressive Cretaceous Muger Mudstone and Debre Libanos Sandstone, and Paleogene-Neogene volcanic rocks (Assefa ,1980, Assefa ,1981, Assefa ,1991).

During late Jurassic regression of the sea from the Ethiopian region began as the result of arching and doming of the Arabian- Somalian massif and sediments of varying facies (restricted marine, lagoonal and supratidal to intertidal) were deposited in structurally controlled domains. By late Cretaceous the sea completely withdrew leaving regressive continental clastics deposits (Reynolds et al., 1997). Later on, the formation several basins related to east Africa rift systems took place such as the Gambela Basin which is part of the central African rift system and it’s the southeast extension of the Melut Basin ( White Nile rift ) of South Sudan rifting along with volcanism was responsible for formation of the N-S trending extensional rift basins in southern, central and northern Ethiopia. Jurassic Limestone formation in Ethiopian Sedimentary Basin

100,000 sqkm Carbonate rocks of Ethiopia are exposed widely in the Mekelle, in the Blue Nile basin and in the Ogaden Basin including the Bale and Western Harrarghe areas (Asfaw wossen 2015). According to the geographic locality of Jurassic limestones in Ethiopia have various names example: Antalo Limestone Formation (in Abbay, Mekele and Dire Dawa area), Hammanlei Formation, Urandab and Gabredarre Formation (in Ogaden basin and Harrar) (Kazmin, 1973; Blanford, 1869; Beyth 1972a;1972b; Bosellini et al,1997; Getaneh Assefa 1991; Russo et al, 1994).

In Ethiopia largest volumes of limestone are located in the eastern part of the country also thick limestone sequences is present in the Blue Nile basin in central and Mekele area in northern Ethiopia. They are the results of the wide spread transgressions and extensional deformation, which is related to the breakup of Gondwanaland that has taken place on the horn of Africa

13 starting form early Mesozoic time (Bosellini, 1989). Carbonate deposits in Ethiopia were terminating during latest Jurassic (Tithonian) and Berriasian times due to overall regression Getaneh Assefa (1991) then after clastic deposits overlies carbonate units throughout most part of Ethiopia starting from this time. Jurassic transgression of facies in Ethiopia is represented by shallow-water carbonates referred to as the Antalo Limestones and the Hammanlei Formation. In Ogaden basin these formations were conformably overlie the Adigrat Sandstones and their earliest occurrences (Pliensbachian/Aalenian) are found in these basin (Beyth, 1971; 1972a; 1972b; Kazmin, 1973; Bosellini, 1999; Bosellini et al., 2001; Russo et al., 1994; Mengesha Tefera et al., 1996; Abiyyu Hunegnaw et al., 1998; Abbate et al., 2015). However, in the Blue Nile basin the Adigrat Sandstones are followed by the Gohatsion marls and evaporites of Liassic to Callovian age (Russo et al., 1994). The younger widespread marine flooding is recorded since the Bathonian and in the Callovian/Oxfordian in both dire Dawa-Harrar and Tigray respectively. These carbonate successions are characterized by several depositional sequences the maximum flooding surface is marked by organic-rich marls and shales and is time-transgressive from Oxfordian (Ogaden area) to Tithonian (Tigray) (Beyth, 1971;1972a;1972b, Kazmin, 1973; Bosellini, 1999; Bosellini et al., 2001; Russo et al., 1994; Mengesha Tefera et al., 1996; Abiyyu Hunegnaw et al., 1998; Abbate et al., 2015).

Stratigraphy of Blue Nile Basin

The geology of Ethiopian rock in terms of their age ranging from (oldest to youngest) are Precambrian basement to quaternary volcanic rocks. Therefore, the general stratigraphy of central Ethiopia is characterized by the Precambrian basement, the Late Paleozoic and Mesozoic sedimentary successions, the Tertiary continental flood basalts (Trap series) and Quaternary sediments from the older to younger in age respectively.

2.4.1 Basement rock

These Rocks form the base of Blue Nile basin and out crop with rugged topography at an altitude of 900-1500m along the entire NW-flowing segment of the Blue Nile basin. The Precambrian basement rocks in the Blue Nile Basin consists of quartzites, granites, granodiorite gneisses, hornblende-biotite gneisses, diorite, meta sediments and metavolcanics (Kazmin,1975, Mengesha et al.,1996) and classified under the Alghe Group (Mengesha et al.,1996).

The Neoproterozoic basement rocks in the Blue Nile Basin is unconformably overlain by the Pre-Adigrat I or Pre-Adigrat II sediments (Dawit, 2010). Precambrian basement rocks are overlain unconformably by a Permo-Triassic “Karroo” succession around 450 m thick, and interpreted as alluvial fan and fluviatile deposits (Wolela, 1997, Wolela, 2002a, Wolela, 2006a, Wolela, 2006b). The age of the basement rock is considered to be Neoproterozoic, ranging from 850 to 550Ma as documented from U-Pb and Rb-Sr geochronologic studies by Ayalew ET at., 1990.Neoproterozoic penetrative.

2.4.2 Paleozoic and Mesozoic Sedimentary Successions

According to Mohr (1963), Assefa (1991) and Russo et al. (1994) the complete succession of Paleozoic and Mesozoic sedimentary succession in the Blue Nile Basin of central Ethiopia can be informally grouped into eight stratigraphic units: pre-Adigrat I, pre-Adigrat II, pre-Adigrat III, Adigrat Sandstone, Gohatsion Formation, Antalo Limestone, Muger Mudstone and Debre Libanos Sandstone. These units are described below.

2.4.2.1 Pre-Adigrat I

Pre-Adigrat I is the oldest sedimentary rock which overlies the basement and composed of poorly sorted, massive to cross-bedded, medium to coarse-grained white sandstones and conglomerates (Dawit, 2010). The oldest sedimentary succession of Blue Nile basin that overlies the crystalline basement is pre-adigrat1 is up to 50 m thick, which occur a small isolated outcrop. The presence of ductile soft sediment deformation structures, large scale trough cross-bedded sandstone, crude horizontal bedding and channel type cut and fill structure impart a similarity with the lower glaciogenic part of Enticho sandstone in northern Ethiopia.

2.4.2.2 Pre-Adigrat II

It is widespread in the Blue Nile basin with a maximum thickness of 400 m in the Fincha valley of central Ethiopia. According to Jespen&Athearn (1961) and Mohr (1963) these sediments are not confined to north-south trending channels. The succession unconformably overlies either the Precambrian basement or pre-Adigrat I.

According to Dawit 2010 in the northwest (e.g. in Bekotabo area, around 40 km south of Bure), it reaches up to 200 m thick. In its lower part, it consists of lateral accretion deposits, floodplain fines and playa-lake mudstones, suggesting deposition in a meandering river-floodplain

15 environment. In its upper part, it is characterized by crevasse-splay, playa-lake and eolian dune sediments. The main sediment transport direction is to the west. The extension of these sediments towards northern and eastern Ethiopia is not yet clear, as well as is their age. Kazmin (1975) assumes a broad interval of Carboniferous-Mesozoic age for this unit.

2.4.2.3 Pre-Adigrat III

Dawit (2010) believed that the Pre-Adigrat III in the Blue Nile Basin is equivalent to the Karoo sediment in eastern and southern Africa (Mohr,1963) and overlies the Pre-Adigrat II. It is composed of three successive cycles of stacked, multi-story sheet sandstone bodies that are covered by crevasse splay deposits and overbank fine clastic sediments.

Its succession ranges in thickness from 350m in Fincha area to less than 50 m in Dejen area. Leaf imprints, coaly streaks and palynomorphs are abundant, indicating favorable conditions of preservation on the flood-plain, such as permanent water saturation and poor drainage. The basal part of the Pre-Adigrat III succession contains fossils indicating a latest Carboniferous to Early Permian age (Kemp et al. 1977, Price 1983, Stephenson et al. 2003). The top of the succession is dominated by Staurosaccites quadrifidus and Ovalipollisovalis, which indicates a Middle Triassic age (cf. Dolby &Balme, 1976, Geletu&Wille 1998).

2.4.2.4 Adigrat Sandstone

The thickness of the Adigrat Sandstone Formation in the Blue Nile Basin is 450 m thick at Dejen-Gohatsion, 800 m thick at Amuru-Jarty, 750 m thick at Fincha River, 200 m thick in the Arjo area, and 150 m thick in Ejera area (Assefa and Wolela, 1986, Serawit and Tamrat, 1996, Tamrat and Tibebe, 1997, Wolela, 1997). It pinches out between the Precambrian basement rocks and the Tertiary volcanic in south-western Ethiopia.

According to Gani et al., 2009 Adigrat Sandstone is overlain by a Liassic Transitional (glauconitic shale-mudstone; ca. 50 m thick) unit composed of glauconitic sandy mudstones and greyish green shales, black shales and Limestones and Sandstone on the top. In the Dangrur Mountain (western Gojam) and in the Didessa area some variegated sandstones, siltstones and shales were reported between the basement and Tertiary volcanic, and are correlatable to the continental Adigrat Sandstone Formation (Jepsen and Athearn,1964).

In the Blue Nile Basin, the formation is mainly represented by texturally and compositionally sub-mature to mature sandstones. According to Russo et al. (1994) the thickness of the ‘Adigrat

Sandstone’ in the Blue Nile Basin measured about 300 m south of Dejen. In contrast, more recently Wolela (2008) gave an account of an 800 m thick continental (alluvial) succession that overlies basement, which he called ‘the Triassic–Jurassic Adigrat Sandstone Formation.

According to Russo et al. (1994) and Wolela (2008) Adigrat sandstone is formed by purely fluviatile origin. In contrast Dawit and Bussert (2009) and Dawit (2010,2016) Adigrat sandstone formation are formed by a storm dominated shore face to a barrier/inlet spit as depositional setting.

2.4.2.5 Gohatsion Formation

The Bathonian-Oxfordian Gohatsion Formation (ca. 420 m thick) composed of different types of gypsum (nodular, chicken-wire, satin spar, alabaster, laminated and algal mat), dolostone, marls, mudstones and shales (Assefa,1980, Assefa, 1981).

The Gohatsion Formation in the Blue Nile Basin represents a complex transitional depositional system in a semi-arid peritidal depositional setting (Assefa, 1981; Russo et al., 1994; Dawit, 2010). According to Getaneh (1981) the stratigraphic position of this formation has given a Liassic to Bathonian age and Assefa assigned the age of this formation is Liassic to Late Bathonian on the basis of foraminifera and stromatoporod. Russo et al. (1994) proposed that the Gohatsion Formation corresponds to the initial flooding of the craton , which is largely related to rifting and subsidence of the future African continental margin. It is overlain by a Callovian-Lower Kimmeridgian calcareous unit (Antalo limestone) and underlain by the Permo-Triassic Adigrat sandstone (Assefa,1981; Russo et al.,1994; Dawit and Bussert,2009; Dawit,2014).

2.4.2.6 Antalo Limestone

Antalo lime stone which is the main focus of study. During the early Callovian -early Oxfordian, a major transgression covered the whole of East Africa and led to the deposition of the 720 m thick (bottom unexposed) Antalo Limestone Formation in the Blue Nile Basin (cf. Gibre Yohanes, 1989, Bosellini, 1989, Russo et al., 1994, Wolela, 1997). This formation is composed of limestones (skeletal packstone-wackestones, oolitic-skeletal packstones)

17 alternating with black mudstones and black shales. Bioclasts include brachiopods, corals, algae, gastropods and echinoids.

According to Russo et al. (1994) and Atnafu (2003) 420 m thick carbonate succession conformably overlies the Gohatsion Formation and it can be subdivided into three parts. The lower part with its thickness 180m which is composed of burrowed mudstones that grade upwards into oolitic and coquinoid limestones with or without intercalated marl beds, and then into massive limestones with scattered patches of corals, nerineids and stromatoporoids, for which a shallow water environment was inferred. The middle part with its thickness 200m which is composed of highly fossiliferous interbedding of marly limestones and marls. Based on Russo et al., 1994 and Atnafu,2003 the middle part of Antalo limestone has ammonite fauna (e.g. Lithacoceras sp. and Subplanites spathi), in association with brachiopods (e.g. Terebratula pelagica and Nanogyra) and other infaunal siphone feeders (Anisocardia, Venilicardia and Somalirhynchia somalica and Zeillleria latifrons) suggests a shelf to open marine environment. The upper part (50m thick) comprises planar laminated oolitic and refal limestones, which was interpreted to indicate the return of shallow water conditions. The presence of Pfenderina sp. and Nautiloculina oolithica at the base of the limestone unit points to a Callovian age (Russo et al. 1994). Kurnubia palestiniensis, Parurgonina caelinensis, Conikurnubia sp. and Salpingoporella annulata at the top of the unit indicates a Kimmeridgian age (Turi et al. 1990, Atnafu 1991, 2003, Russo et al. 1994). According to Canuti and Radrizzani 1975; Russo et al. 1994, Antalo limestone thick approximately 400m unit which comprises thinly bedded (average 10 cm) to massive limestone. On the basis of Callovian to Kimmeridgian benthic foraminifers and microfaunas, its age is Middle -Late Jurassic. The middle part of the Upper Limestone is fossiliferous with alternating yellowish limestone and grey calcareous mudstones. The fossils found within this unit are dominantly brachiopod shells, bivalves and gastropods. The Oxfordian-Kimmeridgian Antalo Limestone (ca. 720 m thick) composed of different types of limestones (oolitic, micritic and sparitic), black shales and marls.

2.4.2.7 Mugger Mudstone

The 320 m thick meandering river sediments of the Muhger Mudstone unit passes up vertically into 280 m thick braided river-dominated Debre Libanose Sandstone unit (Getaneh,1991, Wolela,1997) which is unconformably overlain by Trap volcanic. The succession is 15 m in the Gohatsion area but thickens eastwards to reach up to 320 m in the Jema river valley. In its type locality, it is 260 m thick and conformably overlies the Antalo Limestone. Regarding the stratigraphic position of the Muger Mudstone, Assefa (1991) assumed a broad interval of post- Kimmeridgian to pre-Middle Eocene age. According to Goodwine et al. (1999) proposed the age of the base of these unit is Tithonian age based on the presence of microflora (e.g. Classopollis, Cicatricosisporites, Callialasporites trilobatus, Krauselisporites, Neoraistrickia, Crybelo-sporites cf. C. striatus and Gleicheniidites) and dinoflagellates (e.g. Chytroei- sphaeridia and Leptodinium acneum) as well as the acritarch Micrhystridium.

Based on biostratigraphic evidences that contradict with the attempt made by Assefa (1991) to correlate the unit with the Agula Shale of northern Ethiopia, which was dated as “not older than Kimmeridgian” (Bossellini et al. 1997).

The 260m thick Mugher Mudstone unit (Getaneh, 1991) is the result of withdrawal of the sea from east African Craton during late Jurassic. The unit does not crop out in the Abay gorge however it is confined to the canyon of (e.g. Mugher, Zega wedam) the Blue Nile Basin. This unit has two distinctive features. The lower part consists of 15m of alternating gypsum, dolomite and shales. The upper part constitutes 240m thick mudstone. This part of the Mugher mudstone unit consists of mudstone, fine to medium-grained siltstone.

2.4.2.8 Debre Libanos Sandstone

Previously according to these authors (Mohr, 1962, Kazmin, 1972, Kazmin, 1975, Merla et al., 1973) Debre Libanose Sandstone was described as part of the Upper Sandstone and it was renamed by Assefa (1991) after its type section near the village of Debre Libanos (09°44’ N and 38°52’ E) of central Ethiopia. However Recently, (Getaneh, 1991) classified the Upper Sandstone into two units: the Muger Mudstone after its type section at Muger, and DLS after its type section at Debre Libanose.

Single and multistory sandstone sheets in the upper part of the Debre Libanos Sandstone indicate meandering river sedimentation while the brownish-black shales and mudstone possibly indicate a lacustrine environment (wolela,2009). Furthermore, the Debre Libanos

Sandstone consists of potential reservoir rocks in the basin which have undergone complex diagenetic evolution (Wolela ,2012).

According to Assefa 1991 the environment of debre libanos sandstone is braided depositional environment. Based on lithologic similarly these unit is correlated with Amba Aradam Formation of northern Ethiopia.

According to Wolela ,1997 its age ranges from 120-94Ma years, which is equivalent to Barremian to Cenomanian. This unit also does not outcrop in the Abay canyon; however, it is exposed in the Zega Wodem river and its tributaries.

The major sedimentary structures within this sandstone bodies are large scale planar-tabular and asymmetrical trough cross-beds, small scale trough and planar tabular beds, convolute beds, flat beds, scoured and channel surface, and massive beds (Getaneh,1991).

2.4.3 Tertiary volcanic

The basalts that overlie the sedimentary sequences in Dejen and Gohatsion area revealed 26.9– 29.4 Ma years, approximately equivalent to the Oligocene (Hofmann et al.,1997, kieffer et al.,2004.,), and are coeval to the radiometric age of basalts in the South-western Plateau of Ethiopia (cf. Davidson and Rex,1981).

According to Getaneh 1991 volcanic rocks are composed of mainly basalt, trachyte, rhyolite with beds of tuff, paleosols and lacustrine sediment. Prolific outpourings of mainly basalts have built a subaerial pile, which originally covered an area in excess of 500,000 square km, with total thickness locally exceeding 2,000 m (Hofmann et al. 1997). Emplacement of the Ethiopian traps has been linked to the extinction event that defines the boundary between the Eocene and Oligocene epochs in the Cenozoic era (Courtillot et al. 1997).

Figure 4 Geological map of the Blue Nile basin (source: Dawit 20 Lebene, 2010)

Figure 5 Stratigraphy of central Ethiopia, Blue Nile Basin (Dawit Lebene,2010)

CHAPTER THREE 3 METHODOLOGY AND MATERIAL Materials 3.1.1 Field instrument

In order to accomplish this thesis different materials and equipment were used for field data collection and laboratory analysis to accomplish the stated research problem, the general and specific objectives of this research study. Data on fossil specimens have been acquired from both the literature and fossil collection.

During field investigation of Blue Nile Basin (Dejen to Gohatsion) on carbonate unit, 652 bivalve macrofossil samples were collected on marl and marly limestones. Most of the specimens are preserved as moulds and shell forms showing ornament and having muscle scar (shell) and which exposed in road-cuts, quarries and from weathered surfaces and ploughed fields. In study area the different field instruments were used to perform field data collection. These are Brunton compass, geological hammer, sprinkle, mattock, and GPS, Camera, plastic sample bag, marker, pen, note book, base maps with scale of 1:50,000.

3.1.2 Laboratory instruments

The most basic type of paleontological full material was not fully found, so test was done in petrographic laboratory which are involved in extracting and identification of macrofossil in university of Gondar. The material is 250 ml standard beaker, hand lenses, water, dilute 30% H2o2 and reduced acid 10%H2o2, Camera, Verner caliper, needles, and different software including Arc GIS and Adobe photoshop Cs6.

Methodology

The general works of the research are mainly divided into three methods; pre-field work, field work and post-field work to reconstruct depositional environment, describe detail morphology of bivalve, make taphonomy analysis, investigate paleoecology and to interpret paleobiogeography and to make systematic paleontology (taxonomy). Each method was formally followed by its own detailed activities.

3.2.1 Pre-field work

Before the pre-fieldwork office preparation has been undertaken. These includes review of previous work and available base maps, previous geological map in order to have an overview about the geology of the study area and their surrounding region, collecting of secondary data including meteorological data and published geological map, preparation of working plan, identification of the problem, setting General and specific objectives and scheduling the various research activities, finalization of the methodology to be adopted for the proposed study and preparation for the fieldwork (equipment, base map, transport and personal preparation).

3.2.2 Field work

The Quantitative samples were collected by classifying the study area in to three section and taken in the field, which is marked with their corresponding section code alphanumerically for each individual sample, consisting of three letters and sampling numbers.

In Total 652 individual bivalve samples were collected in marl and marly limestone formations with a mode of preservation ranges from moulds to shell preservation from Dejen to Gohatsion sections and counting of the specimens were also done in the field. For the quantitative analysis of macrofossils, the benthic fauna was counted in order to allow a statistical treatment. Macrofossils of bivalves were also collected especially in three sections, these were Geligele section 234 samples (36%) much of oysters, Demibeza section 263 samples (40%) Pholadomya with beautiful rib patterns and Filikilik area 155 (24%) were both oysters and some moulds samples were collected, the former of the two were in Dejen side and the later were in the Gohatsion side. Field trips were carried out during the study to investigate detail morphology and to make systematic paleontology (taxonomy) to know taphonomic properties of bivalve species and to investigate paleoecology of the specimens that were exposed in the carbonate rock formations especially marl unit in the Blue Nile Basin particularly in the Dejen to Gohatsion sections and the trip were conducted from beginning of December to the end of December. Representative samples of the unit in the study area were collected from surface following the roads cut, concreate road, stream, Quarry site and cliff of carbonate formations. some of the specimens collected in the field have already been removed from the rock those can be easily collected and some fossils remain embedded in the rock that can be separated by geological hammer and Chisels.

3.2.3 Post field work

Prepared and cleaned the sample in laboratory and identified down to species level where ever possible. Fossils have been cleaned with water and hydrogen peroxide. After chemical and/or mechanical preparation of bivalve shells, the length, height, length: height ratio and partially thickness of all individuals were measured by Verner caliper in millimeters. For incomplete specimens, dimensions were recorded at the latest entirely traceable growth line.

Chisels, needle and hammers were used to mechanically prepare the fossils, this part of the work was very tedious and time-consuming. In the cases of, where fossils occur in argillaceous/marly sediments, hydrogen peroxide was used to separate fossils from their matrix. In particular, this method has been applied to fossiliferous levels. 54 representative samples were selected from total 652 specimens in the Blue Nile Basin and the selected samples can be correlated based on the literature and treatise invertebrate paleontology books. In terms of biometric study Measurements after chemical and/or mechanical preparation of bivalve shells, moulds and shells were measured using a Verner caliper recorded in millimeter scale on shells and mould bivalve species and taken on each specimen whenever possible: thickness (T) maximum inflation: and Height (H): maximum shell height perpendicular to length; Length (L): maximum length perpendicular to height. In many published works different universities and museums bivalve material and literature was examined, but Due to scarcity of museum no visitation, this part is important to correlate the present sample with other bivalve province. In terms of taxonomy study, the preservation of shell fossils and molds fossils were described if there is little deference between the available and the literature write (put) the difference as remarks. In the determination and description of the pelecypod specimens and in the considerations on a specific variability the following characters were taken into account: outline (shape), shell dimensions (height, length, thickness), indices of the pair of corresponding parameters, convexity, concavity, shell and mould form, shell and mould structure, Umbo/Beak position , ornamentation, attachment area, external morphology, position, shape and size of muscle scar, character of external and internal valve margin and form of hinge. The World Registration of Marine Species (WoRMS) website is used to access original naming, accepted taxa, synonymized taxa and recent updates in the taxonomic classification. For bivalve systematics paleontology comparison with different literatures are used.

Principal taphonomy were analyzed to know whether the specimens autochtneous and parautochtneous by using articulated, disarticulated, fragmentation and encrustations, bioerosion, abrasion and edge chipping. The calcite shells are relatively resistant to chemical treatment, bathing the fossil sample in water for 3 days and again bathing the sample by hydrogen peroxide by reducing the concentration to 10% with some water for 4 days and then brush the sample. This method provides a great potential for discover details of calcite bivalve shell, since the matrix is a marl and marly limestone. Finally, after cleaning the specimens, capturing sample photo, calibrating the sample photo by measuring ruler on computer and preparation of plate width 15, height 10 and 300 resolution using adobe photoshop Cs6. The paleogeographic distribution of the identified bivalve’s species are mainly based on literature Fursich et al (2004,2006) and Kissling et.al (2011) and other from Ethiopian province. Data on fossil specimens have been acquired from both the literature and fossil collection. Paleobiogeographic distribution of the identified taxa are mainly from the literatures.

CHAPTER FOUR 4 Systematic Paleontology Background

Bivalve taxonomy is challenging because of the high degree of morphological variation between species and within single species. Shell ornamentation is a widely used character for species-level identification. The terminology and systematic classification of Moore (1969) and Screepat Jain (2017) in treatise on invertebrate’s paleontology applied here. For most species, identification can be established from previous detailed descriptions by other authors; Classification of suprageneric taxa is based for Bivalvia, on Moore's Treatise of Invertebrate Paleontology (part N6, 1-3, 1969-1971). Linear measurement is in millimeter, were measured by Verner caliper which is used to make the taxonomy and the abbreviation are as follows.

H/L= height/length

Tmax = maximum thickness

The synonymy lists in the present work contain only entries considered to be of major importance for the study and which have been carefully checked by the authors. More comprehensive synonymies can be found in the references cited.

Order Pholadomyoida Newell,1965 4.1.1 Family Pholadomyidae Gray,1847 4.1.1.1 Genus Pholadomya Sowerby,1823 Type Species Pholadomya socialis (Morris & Lycett,1854) Pholadomya socialis (Morris & Lycett,1854) (Pl.1, Fig. 1)

2014 Pholadomya (Pholadomya) cf. socialis Morris and Lycett, Fürsich and Pan, p. 45, pl. 14, Fig. 2017 Pholadomya (Pholadomya) cf. socialis Morris and Lycett, Sabbagh et al, Fig. 7n Material: One Articulated specimen (BND402)

Table 1Measurements (in mm) of Pholadomya socialis (Morris & Lycett,1854)

Specimen No. H L H/L Tmax BND402 21 37 0.57 27

Descriptions Medium in size, Subtriangular and obliquely oval, the near the beaks the anterior part strongly convex, hinge line sharp at the anteroventral margin, inequilateral, at the umbonal part the ribs are ornamented closely, at the anteroventral part sparsely ribbed, i.e. the ribs space is wide, elongate to ovate forms, triangular in out line and the posterior part is not seen, because it is covered by carbonate rocks. surface is ornamented by 3-4 radial ribs crossed by concentric ridges with wide interspaces between adjacent radial ribs. Remarks In the present specimens the surface has 3 radial ribs but the specimen done by Sabbagh etal.2017 of middle to upper Jurassic sedimentary succession in central arebia has 4 radial ribs which are intersected by concentric lines.

Class Bivalvia Linnaeu,1758 Order Pholadomyoida Newell,1965 4.1.2 Family pholadomyid King ,1844 4.1.2.1 Genus Bucardiomya Rollier in Cossmann, 1912 Type species Pholadomya somaliensis Cox,1935

Pholadomya somaliensis Cox,1935 (Pl.1 Fig. 2)

1935 Pholadomya (Bucardiomya) somaliensis Cox: 192, pl. 2, figs. 1, 2.

1980 Pholadomya (Bucardiomya) somaliensis Cox – Hirsch: pl. 8, figs. 1, 2

2011 Pholadomya (Bucardiomya) somaliensis Cox – Kiessling et al.: 210, fig. 13Q

2014 Pholadomya (Bucardiomya) somaliensis Cox – Asmar et al.: fig. 8D Material: One articulated specimens (BND432) Table 2 Measurements (in mm) of Bucardiomya somaliensis (Cox,1935)

Specimen No. H L H/L Tmax

BND432 36 61 0.59 52

Descriptions Medium in size, slightly convex at posterior and anterior side, umbo placed well anteriorly and prominent, heart shaped, strongly inflated at the dorsoventral margin (mid flank part), equivalve and equilateral, since posteriorly truncated the ribs are not seen at the end of posterior side, anteriorly ribs closely spaced, however mid flanks the specimens sparsely ribbed i.e. the rib space wide and the anterior part and the posterodorsal region are devoid of ribs. Ornamentations consisting of 5 thick radial ribs which are intersected by concentric line, on the ventral part there is concentric lines and on dorsal no clear concentric lines are seen because of its part is truncated. Ribs ornamented in beautiful pattern and the internal characters are unknown. Remarks Pholadomya (Bucardiomya) somaliensis differs from Pholadomya (Bucardiomya) lirata in having more elongated, larger and more pronounced marginal crenulations. The material has been included in Pholadomya (B.) protei (Brongniart the diagnosis indicates that the ornamentation could consists of 4 to 6 radial ribs, in some cases even 7 in the Spanish material, as well as the English material described by Arkell (1935: 335) the commonest number of ribs is 4, but in the available material the ornamentation consists of 3-5 ribs. Class Bivalvia Linnaeu,1758 Order Pholadomyoida Newell,1965 4.1.3 Family Pholadomyidae Gray,1847 4.1.3.1 Genus pholadomya Sowerby,1823 Type species Pholadomya hemicardia hoemer,1836 Pholadomya hemicardia (hoemer,1836) (Pl. 1, Fig.3) l926a Pholadomya hemicardia Roemer. -Jaworski: 193, pl. l, figs. 2-3. l926b Pholadomya hemicardia F. A. Roemer. - Jawonski: 399. Material: One articulated specimen (BND 415) Table 3 Measurements (in mm) of Pholadomya hemicardia (hoemer,1836)

BND415 37 46 0.80 35

Descriptions Medium size, prominent beaks, elongate-ovate and moderately inflated ,lunule, dense concentric lines and few radial ribs and they appear very high on the beak extend all the way to ventral margin ,beaks sharp, equivalve, inequilateral, on the ventral part the hinge line is sharp, posteriorly convex than anteriorly, dorsal and ventral part gibbose, radial ribs extending from anterior to posterior, the ribs are close space anteriorly and at the mid flank the ribs space is wider spaced than anterior side and gaping anteriorly. Remarks Pholadomya hemicardia is differing from by pholadomya lirata by ovate to elongate in shape outline and large in size. In the English specimens of hemicardia the radial ribs are covering whole of the surface of the shell, while in present specimens the whole part is not covered by ribs and concentric lines. 4.1.4 Family Pholadomydae Gray,1847 4.1.4.1 Subgenus Bucardiomya Rollier in cossman ,1912 Type species pholadomya lirata Sowerby, 1827 Pholadomya lirata (Sowerby,1827) (Pl.1 Figs. 7-9) Material: - Three articulated specimens (BND55-BND57) Table 3. Measurements (in mm) of Pholadomya lirata (Sowerby,1827) 1980 Pholadomya (Bucardiomya) lirata Sowerby – Hirsch: pl. 7, fig.10 2011 Pholadomya (Bucardiomya) lirata Sowerby – kissling: fig.13R 2017 Pholadomya (Bucardiomya) lirata Sowerby – Zakhera et al. fig.4P Material: Three articulate specimens (BND455-BND457) Table 4 Measurements (in mm) of Pholadomya lirata (hoemer,1836)

BND455 20 32 0.63 22 BND456 31 38 0.82 18 BND457 24 37 0.65 34

Descriptions Medium in size, Umbones prominent, slightly to moderately inflated, lunule, equivalve to inequivalve, inequilateral posteroventral margin vertically inflated and dorsally gibbose with no radial ribs and concentric lines, but ventrally clear concentric lines. The most anteriorly and

29 the most posteriorly placed rib are weak to barely noticeable, anteriorly the ribs are closely spaced and posteriorly wide spaced., inflated, subtrigonial to ovate shape, strongly curved, dorsally flat, posterior convex, strength of radial ribbing variable. The ornamentation of ribs consisting of 2-4 radial ribs which are intersected by concentric lines and the ribs are continues from anterior to the end of posterior without truncated ribs. Remarks The number of radials is also quite variable in the different populations of Pholadomya lirata recorded from different parts of world- in Bathonian of France from 6-8 (Fischer, 1969), in the Middle Jurassic of Somalia from 6-9 (Cox, 1935) and in the Bathonian Callovian of Kachchh from 6-9 (Pandey et al., 1996), This number (irrespective of strength) ranges from 4-10 in the specimens of North Pachchham “Island”. But in the present specimens the number of radials is 2-4. 4.1.4.2 Sub genus Bucardioma Rollier in cossman,1912 Type species Pholadomya (Bucardiomya) aubryi Douville,1886) Pholadomya (Bucardiomya) aubryi (Douville,1886) (Pl.1. Figs.4-6) 1980 Pholadomya (Bucardiomya) aubryi Douville – Hirsch: pl.7, fig. 13 Material: - Four articulated specimens (BND405-BND408) Table 5 Measurements (in mm) of Pholadomya aubryi (Douville,1886)

BND408 20 35 0.57 32 BND407 31 39 0.80 36 BND406 19 33 0.58 31 BND405 17 40 0.43 32

Descriptions medium in size, equivalve, inequilateral, umbo prominent, recurved, prosogyrate, located anteriorly. ovate to triangular in shape, moderately inflated with the maximum inflation occurring about halfway between the anterior and posterior, dorsally gibbose, ventral part uniformly flat, the strictures in the posterior is not truncated, the ribs are variable in strength. some specimens slightly show devoid of ribs. The ornamentation consisting of 3-4 radial ribs which are intersected by fine concentric lines and these lines continue on both dorsal and ventral part, at dorsal part the ribs are closely spaced and posteriorly wider spaced. The most anteriorly and the most posteriorly placed rib are weak to barely noticeable.

Remarks The specimens defined by Francis Hirch 1980 has 5 radial ribs and ph. aubryi defined by Zakhera et al.2017 collected from Callovian-Oxfordian central Saudi arebia has 4-5 radial ribs, but the present specimens have 3-4 radial ribs which are intersected by concentric lines.

Plate 1 photograph of mould species

1.Pholadomya socialis (Morris & Lycett,1854); 2. Pholadomya somaliensis Cox,1935; 3. pholadomya hemicardia (hoemer,1836) 4-6. Pholadomya (Bucardiomya) aubryi (Douville,1886) 7-9. pholadomya lirata Sowerby, 1827.

4.1.5 Family Pholadomyidae Gray,1847 Type Species Pholadomya murchisoni (Sowerby, 1827) Pholadomya murchisoni (Sowerby, 1827) (Pl. 2, Figs. 1-5) 1973 Pholadomya murchisoni Sowerby; Romanov, p, 137, pl. 14, fig.6 1990 Pholadomya murchisoni Sowerby; Dikani & Makarenko, pl. 29, fig. 5-9. 1999 Pholadomya murchisoni Sowerby; Lazăr pl., fig. 1a,1b,2a,2b. Material: Nine articulated specimens (BND508-BND514) and (BNF101) Table 6 Measurements (in mm) of Pholadomya murchisoni (Sowerby, 1827)

BNF101 55 77 0.71 57 BND508 56 58 0.97 42 BND509 32 38 0.84 31 BND510 22 30 0.73 22 BND511 21 23 0.91 18 BND512 25 36 0.69 28 BND513 19 37 0.51` 29 BND514 25 31 0.80 23

Descriptions small to large in size, equivalve, inequilateral, lunule, umbo prominent, rounded and recurved, dorsally strong gibbose and highly inflated, ventrally flat to gibbose and truncated on some specimens, anterior and posteriorly part slightly convex, ribs continue from anterior to posterior end with truncated ventrally and have sharp ligament, almost all its pars are covered by concentric line. The ornamentation of ribs consists of 2-5 radial ribs which are intersected by concentric lines.

Remarks In the present specimens consists of ribs from 2-5 radial ribs, but specimens defined Lazăr ,1999 has ornamentation 7-9 radial ribs which is strongly developed under the umbo.

4.1.6 Family Pholadomydae Gray,1847 4.1.6.1 Subgenus Bucardioma Rollier in cossman ,1912 Type species Pholadomya {Pholadomya) beamontensis (Sowerby,1823) Pholadomya {Pholadomya) beamontensis (Sowerby,1823) (Pl. 2 Fig.9) 2010 Pholadomya {Pholadomya) beamontensis Sowerby-Cambell, fig 2I-K Material: - One articulated specimens (BND466) Table 7 Measurements (in mm) of Pholadomya beamontensis (Sowerby,1823).

BND466 40 53 0.75 42

Descriptions large in size, lunule, umbo and beak prominent, equivalve, inequilateral, highly inflated, beaks opisthogyrous, incurved, dorsally strong gibbose, ventrally flat, ribs continue from anterior to posterior end without truncated have sharp ligament. The ornamentation of ribs consists of 6 radial ribs which are intersected by concentric growth lines. posteriorly the ribs wide space and thick, in contrast anteriorly the ribs are closely spaced and thin.

Remarks This species is very similar in shape to the English P. {Pholadomya) fidicula (Late Toarcian- Bajocian), but the ornament differs. P. fidicula has more ribs (27) which are more irregular in strength and interspace width and some are sinuous. In the present specimens consists of ribs from 6 radial ribs. Order Mytilidae Ferussac,1822 Superfamily Mytilacea Rafinesque, 1815 4.1.7 Family Mytilidae Rafinesque, 1815 4.1.7.1 Genus Modiolus Lamarck,1799 Type species Modiolus bipartitus (J. sowerby,1818) Modiolus bipartitus (J. sowerby,1818) (Pl2. Figs.6-8) 2017 Mytilus bipartitus J. Sowerby-Mohamed, p. 3, pl. 1, Fig. B 1940 Modiolus cf. bipartitus J. Sowerby, Cox, p. 67, pl. 5, Figs. 11,12. 2017 Modiolus bipartitus J. Sowerby, Zakhera et al.fig.3. B. Material: - six specimens (BNF101-BNF106) Table 8 Measurements (in mm) of Modiolus bipartitus (J. sowerby,1818)

BNF101 8 22 0.36 11 BNF102 11 25 0.44 12 BNF103 16 42 0.38 23 BNF104 11 33 0.33 12 BNF105 9 22 0.40 11 BNF106 16 59 0.27 26

Descriptions Small to large in size, greatly variable in size, equivalve, inequilateral to sub equilateral, ovate to elongate in shape, beaks prosogyre near anterior end, ventrally ridge ligament and sharp ligament on both dorsal and ventral. Anterior margin convex, narrow and posterior margin wide, rounded, ventral margin anteromedially concave, rounded oblique ridge runs from umbonal beak to the ventral margin. Ornamentation consists of thin growth lines and striae as well as thin radial striae and strongly developed in its dorso-medial part.

Remarks The present specimens display some similarity to Modiolus cf. Bipartitus Sowerby described by Tamura (1960, pl. 32, Figs. 19, 20) particularly with regard to general form and ornamentation. The specimens exhibit the characteristic features of Modiolus (Modiolus) bipartitus Sowerby mentioned by Fürsich and Pan (2014, p. 8). 4.1.8 Family Mytilidae Rafinesque, 1815 4.1.8.1 Genus Modiolus Lamarck,1799 Type species Modiolus imbricatus (Sowerby,1818) Modiolus imbricatus (Sowerby,1818) (Pl. 2. Figs.10-11 and pl.3, fig.8) 1980 Modiolus imbricatus Sowerby-Hirsh, pl.1.fig.26 Material: - Two articulated specimens (BNF111-BNF112) Table 9 Measurements (in mm) of Modiolus imbricatus (Sowerby,1818)

BNF111 22 41 0.54 16

BNF112 15 31 0.48 9

Descriptions Medium in size, equivalve, sharply inequilateral, umbo and beak are not clearly prominent elongate and thin in outline, dorsal margin convex and ventral margin concave, dorsally and ventrally smooth, sharp ligament that extend from anterior to posterior. The ribs are not clearly seen, but some ribs seen anterodorsally margin. Some specimens are shows abrasions and rectangular boring. Remarks The specimens exhibit the characteristic features of modiolus imbricatus Sowerby,1818 by Hirsch (1980) pl.1, fig.26. M.bipartitus Sowerby,1818 could be differentiated from the presence specimens by having of fine concentric threads.

Plate 2 photograph of mould species 1-5. Pholadomya murchisoni (Sowerby,1827); 6-8. Modiolus bipartitus (J. sowerby,1818); 9. Pholadomya {Pholadomya) beamontensis (Sowerby,1823); 10-11. Modiolus imbricatus (Sowerby,1818).

Order Trigonioida Dall,1889 4.1.9 Family Trigoniidae Lamarck, 1819 4.1.9.1 Genus Pterotrigonia Van Hoepen ,1929 Type Species Pterotrigonia scabra (Lamarck,1819)

Pterotrigonia scabra (Lamarck,1819) (Pl.3, Fig.10)

1989, Pterotrigonia (Scabrotrigonia) scabra (LAMARCK). - FISCHER, p. 128, fig. 6 1999, Pterotrigonia (Scabrotrigonia) scabra (LAMARCK). - ZAKHERA, p. 184, pl. 12, figs 14, 15 2002, Pterotrigonia (Scabrotrigonia) scabra (LAMARCK). - KORA et al., pl. 2, fig. 12 Material: - One specimens (BND603) Table 10 Measurements (in mm) of Pterotrigonia scabra (Lamarck,1819)

BND603 19 27 0.70 18

Descriptions medium in size, slightly club shaped, equivalve, sharply inequilateral, umbones anterior, narrowly pointed, dorsally gibbose, dorsally skeletal ridge, anteriorly greatly convex, obliquely elongated crenulations and postero-ventral margins broadly rounded, posteriorly truncate, ligament external, posteriorly wide, the ribs are radiating from posterior to anterior with inclined form, the ventral side is having clear ribs (slightly smooth). Remarks According to Orbigny,1844 there is a great similarity between Trigonia crenulata LAMARCK, 1819 and the present species. He stated that T. crenulate differs from T. scabra mainly in the nature of the ornament of the costae. In T. scabra, the costae carry rounded tubercles, while in Trigonia crenulata they show fine and obliquely elongated crenulations. 4.1.10 Family Ostreidae WILKES, 1810 4.1.10.1 Genus Actinostreon BAYLE, 1878 Type species Actinostreon erucum (Defrance,1821)

Actinostreon erucum (Defrance,1821) (Pl.3, Figs. 1-2)

2014 Actinostreon erucum (Defrance,1821) -furisch, pl.5, figs.6-7.

1995 Actinostreon erucum (Defrance 1821) – Jaitley et al.: 186, pl. 14, figs. 9-11, pl. 15, figs. 1-2.

Table 11 Measurements (in mm) of Actinostreon erucum (Defrance,1821) Material: - two articulated complete specimens (BND546-BND560)

BND460 9 38 0.24 8 BND559 11 28 0.4 11

Descriptions Anteriorly convex, equivalve, strongly inequilateral, very narrow, curved, ribs numerous, very sharp, narrow smooth area in the middle of the shell extending from the umbo to the ventral margin, in others part covered with divaricating, partly branching ribs.

Remarks This species is distinguished from falcate congeners and species of Rastellum (Faujas-Saint- Fond, 1799) by its smooth central field in both valves and general absence of shell chambers (with respect to Rastellum). The narrow smooth area on the flank of the shell of some specimens may result from attachment to a stick-like object. Order: Pterioda Newell ,1965

4.1.11 Family Palaeolophidae Malchus, 1990 4.1.11.1 Genus Actinostreon Bayle,1878 Type Species Actinostreon gregareum (Sowerby,1816)

Actinostreon gregareum (Sowerby,1816) (Pl.3, Figs. 7)

2014 Actinostreon gregareum Sowerby – Asmar: fig. 8B 2002 Actinostreon gregareum (Sowerby), Sha et al., p. 433, Figs. 6, 7

2015 Actinostreon gregareum (Sowerby), Koppka, p. 50, Fig. 20

2017 Actinostreon gregareum (Sowerby), Zakhera et al., Fig. 3Q-V

Material: - One articulated complete specimens (BND546) Table 12 Measurements (in mm) of Actinostreon gregareum (Sowerby,1816)

BND546 15 23 0.65 11

Descriptions Medium in size, equivalve, inequilateral, triangularly ovate, ventrally wide and anteriorly convex, slightly curved, ligamental area triangular, attachment area small, umbones small, the ligament is having zigzag appearance and sharp. The ornamentation consists of angular radial plications which is radiating from posteriorly to anteriorly, the thickness of plicae is wide posteriorly and thin anteriorly, number of plicae is 25 that is clearly seen. Remarks The specimens described by Zakhera et al 2017in central Arebia has 10-12 radial plicae, but in the present material the numbers of plicae are 25. Actinostreon namtuensis (Reed,1936) differs from Actinostreon gregareum (Sowerby) in having non-curved shell and finer more plications. Arkell (1933) regarded Actinostreon gregareum (Sowerby) and Actinostreon solitarium (Sowerby) as two different species. Jaintly et al. (2000) mentioned that two taxa are likely representing ecophenotypic variations of one species. Actinostreon is a very common ostreid in the Jurassic, and there are two morphotypes: A. solitarium (J. DE C. Sowerby) is circular in shape and A. gregareum (J. Sowerby) is more elongated. According to Arkell (1933: 186), the two morphotypes differ in shape and number of ribs; gregareum has 30 to 40 and solitarium between 20 and 25 ribs. According to A. solitarium is flatter, broder and less curved shell than A. gregareum: it has also fewer plicae and the beaks short. Arkell (1933a) argued that A. solitarium differs from A. gregareum in the following four ways, cri 1) The shape is less protruded, the umbonal region less acute, the postero-ventral region more rounded; 2) the plicae generally number 20 to 25 instead of 30 to 40 as in A. gregareum and therefore, are coarser, more like those of A. marshii; 3) the plicae are shorter and less dichotomous, the umbonal region on both valves being devoid of plicae, whereas in A. gregareum the umbones are plicate to their extremities; 4) the hinge is longer, usually with a distinct posterior auricle. As discussed by Jaitley et al. (1995), A. costatum is an asynonym of A. gregareum. The holotype of A. costatum (J.de c. sowerby,1825) is characterized by small size, a small attachment area and very steep-sided flanks. Otherwise, it is identical to A. gregareum.

Order Pterioda Newell,1965 4.1.12 Family Malleidae Lamarck, 1819 4.1.12.1 Genus Eligmus Eudes-Deslongchamps, 1856 Type Species Eligmus asiaticus (Douville 1916)

Eligmus asiaticus (Douville 1916) (Pl.3, Fig. 11) 1980 Eligmus asiaticus Douville, Hirsch, pl. 2, Figs. 5-9. 2002 Eligmus asiaticus Douville, Abdelhamid, pl. 5, fig. 10, 11. Material: - One specimens (BND548) Table 13 Measurements (in mm) of Eligmus asiaticus (Douville,1916)

BND548 20 25 0.80 12

Descriptions Medium in size, subcircular, equivalve and inequilateral, umbones small, enlongate in outline, prosogyrous, moderately inflated, umbonal area slightly convex, anterodorsally margin nearly straight, posteroventral curved. The surface of the specimens is ornamented with radial plicae that have folded convex upper surface and are crossed by very fine, faint concentric threads. The radial plicae are thicker at central and posterior part of valve, but are thinner and faint towards the anterior side. Remarks The present species differ from Eligmus weiri in having more radial plicae and subcircular in shape, but Eligmus weiri have flat shell with little numbers of radial plicae and triangular shape.

Order Nuculoida Dall,1889 4.1.13 Family Nuculidae Gray,1824 4.1.13.1 Genus Palaeonucula Uenstedit,1824 Type species palaeonucula lateralis (Terq.and Jourdy,1980) palaeonucula lateralis (Terq.and Jourdy,1980) (Pl.3, Figs. 3-5)

1980 Paleonucula lateralis Terq and jourdy – Hirsch: pl. 1, fig.5

2002 Palaeonucula lateralis (Terq. & Jourdy), Abdelhamid, pl. 5, fig. 2, 3.

2014 Palaeonucula lateralis (Terq. & Jourdy), Abdel Hady and Fürsich, p. 181, fig. 6 (I, J)

2017 palaeonucula lateralis (Terq. &Jourdy), Abdelbaset etal.fig.6 D & E.

Material: - Six articulated specimens (BND422-BND426) and (BNF40) Table 14 Measurements (in mm) of palaeonucula lateralis (Terq.and Jourdy,1980)

BND422 20 28 0.71 32 BNF40 16 26 0.62 26 BND423 28 36 0.77 31 BND424 14 26 0.54 28 BND425 27 30 0.9 31 BND426 7 9 0.77 12

Descriptions Medium in size, subcircular, equivalve and inequilateral, umbones are small, prosogyrous, umbonal area convex, anterior and dorsal margin slightly convex, anteroventral margin convex, hinge line is curved and have biconvex valve. The surface of specimens is ornamented with very fine, faint threads that are difficult to be observed by naked eye and some specimens have smooth ornamentation. Remarks

The present specimens are differentiated from palaeonucula fraasi (Notling) as described by Hirsch (1980) in having subcircular outline and the specimens described by Abdelbaset have clear ornamentation, but the present material have some ornamentation.

Order Nuculidae Dall,1889 4.1.14 Family Nuculidae J. Gray,1824 4.1.14.1 Genus Palaeonucula Quenstedt,1830 Type species Palaeonucula cuneiformis (J. Sowerby, 1840)

Palaeonucula cuneiformis (J. Sowerby, 1840) (Pl. Fig.) (Pl.3, Figs. 9) 2019 paleonucula cuneiformis J, Sowerby- Fürsich: Pl. 1, Fig. 1 1940 Nucula (Palaeonucula) blanfordi sp. nov. – Cox: 16, pl. 1, figs. 15–19. Material: - One articulated specimen (BNF22)

Table 15 Measurements (in mm) of Palaeonucula cuneiformis (J. Sowerby, 1840)

BNF22 20 26 0.77 19

Descriptions Medium in size, equivalve and inequilateral, ovate in shape, moderately inflated, clearly sharp ligamental area, lunule, but have no Escutcheon, left and right side of umbo sharp Remarks The present species is differing from the specimens which is done in Madagascar (morondava basin) as described by furisch et al 2019 have wide ligamental and have no clear oblique ribs are, but the present specimens have narrow and sharp ligamental area and have prominent oblique ribs.

Plate 3 Photograph of mould and shell species

1-2. Actinostreon erucum (Defrance,1821); 3-6. palaeonucula lateralis (Terq.and Jourdy,1980); 7. Actinostreon gregareum (Sowerby,1816); 8. Modiolus bipartitus (J. sowerby,1818); 9. Palaeonucula cuneiformis (J. Sowerby, 1840); 10. Pterotrigonia Scabra (Lamarck,1819); 11. Eligmus asiaticus (Douville 1916).

4.1.15 Family Pholadidae Lamarck,1809 4.1.15.1 Subfamily Jouannetiinae Tryon,1862

4.1.15.1.1 Genus Jouannetia (Desmoulins,1828) Genus Jouannetia Desmoulins,1828 (Pl.4, Figs.7-8) Material: Two articulated specimens (BNF56 and BND 419) Table 16 Measurements (in mm) of Genus Jouannetia (Desmoulins,1828)

BNF56 46 67 0.69 45 BND419 38 51 0.75 37

Descriptions Large in size, the specimens are more or less globular, beaked, more or less equivalve and inequilateral, widely gaping anteriorly, closing posteriorly, ventral ribs, the hinge line encircled the specimens, calcite precipitate in the sample, the ribs around the ventral part is thick and on the dorsal part is thin, on the ventral part the specimens is slightly weathered and semicircular in outline, located in normal position transversing both valves. Remarks

The Genus Jouannetia, shell with or without an inwardly projecting lamina for the attachment of posterior adductor and pedal muscles. In the present available material are slightly weathered the internal features are not observed, but overall shape closely corresponds to genus Jouannetia as describe by (Desmoulins 1828).

4.1.16 Family Gryphaeidae Vyalov,1936 4.1.16.1 Subfamily Exogyrinae viyalov,1948 4.1.16.1.1 Genus Nanogyra Beurlen,1958 Type of Species Nanogyra fourtaui (Stefanini, 1925) Nanogyra fourtaui (Stefanini, 1925) (Pl. 4 Figs.7-8)

2015 Nanogyra (Palaeogyra?) fourtaui (Stefanini, 1925) – Koppka: 29, fig. 12 Material: - four articulated left and right valve (BNG566-BND568) and (BNG220) Table 17 Measurements (in mm) Nanogyra fourtaui (Stefanini, 1925)

BND566 24 18 1.33 - BND567 27 18 1.5 - BND568 24 19 1.26 - BNG220 22 15 1.47 -

Descriptions Medium in size, left valve strongly curved than right valves, anteriorly curved, umbones is coiled, the left valve have commarginal growth folds and right valve has radial ribs in folded form starting from dorsal to ventral and the left valve thin and the right valve thick. Remarks The present species is differing from liogryphaea balli which has been recorded from middle Oxfordian strata in Madagascar by having coiled umbo.

4.1.17 Family Gryphaeida Vyalov,1936 4.1.17.1 Genus Ilymatogyra Stenzel, 1971 4.1.17.1.1 Subgenus Afrogyra Malchus, 1999 Type Species Ilymatogyra (Afrogyra) africana (Lamarck, 1801)

Ilymatogyra (Afrogyra) Africana (Lamarck, 1801) (Pl 4, Fig. 5-6,9)

1999 Exogyra africana (Lamarck); Selling. Figs 9, d-g 1983 Exogyra africana (Lamarck); Lefranc in Bengtson, p. 44. 1918 Exogyra delettrei (Coquand); Greco, p. 10, pl. 2, figs 7, 8. Material: - six articulated right and left: Ilymatogyra (Afrogyra) africana (Lamarck, 1801) species (BNG252-BNG256).

Table 18 Measurements (in mm) of Ilymatogyra (Afrogyra) africana (Lamarck, 1801)

BNG252 21 18 1.16 - BNG253 21 18 1.16 - BNG254 22 20 1.1 - BNG255 23 18 1.27 -

BNG256 20 17 1.18 - BNG257 21 18 1.17 -

Descriptions Small to medium sized, oval to elongated ,partly obliquely drop like, inequivalve and inequilateral. Left valve commonly strongly convex, right valve flat or slightly convex. LV ornamented with regular, mostly scaly or smooth growth lamellae, RV covered with dense, fine growth squama and adductor muscle scar kidney-shaped Remarks The species resembles Exogyra delettrei (Coquand) sensu Greco (1918), but differs from typical E. delettrei by the stronger twisting of the umbo and narrower interspaces of the growth lamellae on the right valve.

Plate 4 Photograph of mould and shell species

1-3. Nanogyra fourtaui (Stefanini, 1925); 4-6,9. Ilymatogyra (Afrogyra) africana (Lamarck, 1801); 7- 8. Genus Jouannetia (Desmoulins,1828).

4.1.18 Family Gryphaeidae Vyalov,1936 4.1.18.1 Subfamily Exogyrinae viyalov,1948 4.1.18.1.1 Genus Nanogyra Beurlen,1958 Type species Nanogyra nana (Sowerby,1822) Nanogyra nana (Sowerby,1822) (Pl. 5, Figs.1-4) Material: Six articulated right valves (BNG174, BNG175, BNF177, BNG178, BNG611, BNG612), three articulated left valves (BND610, BNG172, BNG171) and two disarticulated right valves (BNG173, BNG176) Table 19 Measurements (in mm) of Nanogyra nana (J. Sowerby,1822)

BND610 12 11 1.09 - BND611 12 15 0.8 - BND612 17 13 1.30 - BND613 12 15 0.8 - BNG171 17 15 1.13 - BNG172 16 13 1.23 - BNG173 17 14 1.21 - BNG174 15 11 1.36 - BNG175 13 11 1.18 - BNG176 18 16 1.13 - BNG177 12 9 1.33 - BNG178 16 11 1.45 -

Descriptions Attaining small to medium size, thin shelled, inequivalve, outline suborbicular, sub trigonal to elliptical and degree of spirally variable. Umbo small, slightly convex, ornamented with spiral growth lines, umbo of left valve projected opisthogyre, more, weak furrow, the spiral growth lines continues from umbo towards ventral margin, muscular scar is on smooth part, slightly curved, inner surface of right valve is smooth, muscular scar located in half of valve height, outer surface of right valve is depressed, the shape of muscular scar is circular and the shell is thick. Remarks Large variability in Nanogyra nana was the reason that previous authors proposed several species. Nanogyra nana differ from Gryphaea and Liostrea by its small size, posteriorly directed. The present species is differing from species from species Nanogyra tramauensis is not subrectangular in shape, but the species Nanogyra nana has rounded in shape. Nanogyra (Palaeogyra) Welschi species has similar shape and size with N. nana, but possess chomata and have more pointed umbo. 4.1.19 Genus Liostrea Douville,1904 Type species Liostrea acuminate (Sowerby,1816) Liostrea acuminate (Sowerby,1816) (Pl.5 Fig.5-7) 1952. Preexogyra acuminata Sowerby; R. P. Charles & P. L. Maubeuge, Les huitres..., p. 118, PI. 5, Figs.2, 1955. Ostrea acuminata Sowerby; P. A. Gerasimov, Rukovodj’as p. 128, PI. 27. Fig. 1-3. Material: - 8 articulated left valves (BNG45-BNG57) table 20 Measurements (in mm) of Liostrea acuminate (Sowerby,1816)

BNG282 20 17 1.18 - BNG253 22 18 1.22 - BNG284 21 19 1.10 - BNG285 23 17 1.35 - BNG286 21 18 1.17 - BNG287 18 13 1.38 -

Descriptions Medium in size, ovate to crescent, maximally convex at half-way point of height, anterior margin arcuate, posterior margin with wide sinus, umbo constricted, sharp, bent backward. Ornamentation consists of growth lines and lamellae, inner surface is smooth, glossy, muscular scar in below half valve height. Muscular scar sub rounded to rounded located close to sub- posterior margin. Remarks The species Liostrea acuminate have commarginal sculpture but the species Liostrea Muliformis have radial sculpture.

4.1.19.1 Genus Exogyra say,1820 Type species Exogyra fourtaui (Stefanini,1925) Exogyra fourtaui (Stefanini,1925) (Pl.5 Figs.8-9) 2019 Exogyra fourtaui stefanin-Fursich, Pl.4, figs. 6-8. 1929 Exogyra fourtaui Stefanini – Weir: 20, pl. 1, figs. 6–10 Material: - four articulated left valves (BNG312-BNG315) Table 21 Measurements (in mm) of Exogyra fourtaui (Stefanini,1925).

BNG382 31 19 1.63 - BNG387 20 13 1.54 -

Descriptions Large to Medium in size, umbo prominent and beak bent backward, relatively narrow Exogyra with a strongly convex left valve, often much higher than long, thin shell, umbones opisthogyrate and coiled umbo, incurved, shell covered with marginal growth folds, shape of muscular scar is subcircular, sinuated close to the posterior margin and starting close to the umbo.

Remarks The specimens from Madagascar are best placed in Exogyra fourtaui Stefanini, 1925 from Somaliland. Exogyra fourtaui of Cox (1935b) from the Attock District of Pakistan can only be placed with doubts in the synonymy of this species, as the large attachment area does not reveal the characteristic shape of Exogyra fourtaui. The specimens of E. fourtaui of Diaz-Romero (1931) from a stratigraphically older horizon (Bathonian‒Callovian) of Eritrea are distinctly smaller and most likely do not belong to the species. 4.1.20 Family Gryphaeidae, vyalov ,1936 4.1.20.1.1 Genus Exogyra say,1820 Type Species Gryphaea balli (Stefanin,1925)

Gryphaea balli (Stefanin,1925) (Pl.5, Figs.10-11) 2011 Gryphaea (Bilobissa) balli (Stefanini) – Kissling et al.: 209, fig. 13H. 1935 Gryphaea balli (Stefanini) – Cox: 173, pl. 18, Figs. 1–7 Material: - four articulated left valves (BNG312-BNG315) Table 21 Measurements (in mm) of Gryphaea balli (Stefanin,1925).

BNG312 22 17 1.29 - BNG313 24 15 1.6 - BNG314 23 16 1.44 - BNG315 26 15 1.73 -

Descriptions Medium in size, left valves convex, posteriorly depressed, ventrally widened, beak moderately projecting, umbo flat, shell outline subtriangular in outline, moderately growth lines, inner surface smooth and moderately depressed near postero-ventral margin, lobe-like elongated, muscular scar ovate in shape, one specimen shows on internal surface shows encrustation. Remarks The material examined by cox (1935) sometimes exhibits a few-ill-defined radial folds on the umbo of the left valves. The present specimens do not show radial ribs, but the shall outline resembles Gryphaea balli.

Plate 5 Photograph of shell species

1-4. Nanogyra nana (Sowerby,1822);5-7. Liostrea acuminate (Sowerby,1816);8-9. Exogyra fourtaui (Stefanini,1925);10-11. Gryphaea balli (Stefanin,1925).

CHAPTER FIVE 5 PALEOECOLOGY AND PALEOENVIRONMENT Background

Macrobenthos is a powerful tool in interpreting paleoenvironments; it reflects the physical habitat, as its abundance and distribution are largely controlled by abiotic factors (i.e. physical parameters within a given environment) (Abdelhady ,2014). Bivalves are one of the most useful fossil groups in paleoecology, both for environmental reconstruction as well as for deciphering patterns and processes of evolutionary paleoecology. According to Sanders (1958) in modern environments noticed that filter-feeders numerically dominate in sand, whereas deposit-feeders dominate in muddy sediments, correlated with the clay fraction. Infauna suspension-feeders had their largest populations in well sorted fine sand, related to elevated water-energy. In terms of Salinity Bivalve occurs in fresh water has 0.0-0.5‰, brackish water 0.5-18‰ and in marine water 18-40‰ (Oertli,1963) According to Bachs and Lanier (1981) as larva of oysters developed foot, then settles to the bottom of the water column and attaches to hard substrate and the larva change to adult the larva cement itself to suitable substrate. Bivalves are rapidly dispersed during larval stage and later in life they are attached to stratum, plant debris even other shells.

According to Driscoll (1967) epifaunal species in Buzzards Bay on the east coast of North America and demonstrated that attached and sedentary epifaunal taxa are more abundant on bottoms with low silt-clay content, as stronger currents associated with coarser sediments increases the food supply and provides firm surfaces for fixation. Oschmann (1988) used the epifaunal-infaunal ratio as a standard for evaluating the suitability of the sea floor for benthic macrofauna. Here, the epifaunal-infaunal ratio is >1 in carbonates, <1 in silt, and nearly 1:1 in marl. All these features indicate that marl was a suitable substrate for macrobenthos, based on the above authors the bivalve which were found in the Blue Nile basin has more diverse in marl unit than other carbonate unit. Most bivalves are fixed to the substrate, many taxa have a wide biogeographic distribution, due to their planktotrophic larval stage (Kauffman, 1975). Bivalves are strongly facies- dependents (Hallam, 1969, 1971), this may strongly influence their biogeographic pattern (Damborenea et al., 2013). Kissling et al. (2011) found, however, that facies is not a main driver of the biogeographic pattern. Bivalve species are generally restricted to either marine,

54 brackish, or freshwater environments, although some marine and freshwater species can also tolerate brackish conditions (Remane and Schlieper, 1971).

According to Stanley (1970) the optimum burrowing temperatures for most of the examined species are in the range of 20° to 30°C, with most species burrowing at rates markedly slower than their maximum rates at temperatures below about 15°C. According to Solan et al. (2004) the movement, feeding, and respiration activities of infaunal animals modify marine sediments and Bioturbation increases the oxygen concentrations within the sediment, which, in turn, affects the biomass of the infauna, the rate of organic matter decomposition, and the recycling of nutrients essential for primary productivity.

According to Fürsich (1984) the deference between associations and assemblages are recurring and non-recurring respectively. In this study area fulfill criteria of recurring, then use associations. Bivalve Associations (BA) 5.2.1 Modiolus bipartitus Associations

This association occurs both in geligele section and Filikilik sections in which one section were far from main road (Geligele sections) and the other were found on main road (Filikilik section), both sections which consists of more marl (soft substrate) and less limestone (hard substrate) which has the lower species number than pholadomya murchisoni association. It consists of 7 samples with 11 species and 311 individual samples were collected. From this association Modiolus bipartitus (24.07%), Ilmatogyra africana (18.52%) and Nanogyra nana and Grayphaea balli (11.11% for each) were the most dominance relative abundance species.

Epifaunal bivalve’s species (63.64%) dominates over infaunal (9.09%) bivalve species, semi- infaunal (18.18%) and deep infaunal (9.09%) bivalve species. Suspension-feeders represent 81.82% of bivalves, only the species of Palaeonucula lateralis and paleonucula cuneiformis (18.18%) are deposit-feeders.

In this association all the bivalve’s species has high dominance of articulated valves that indicates no disturbance and no eddies in water currents and /or other environment constraint. lack of fragmentations, presence of some encrustations and abrasions on some samples indicates that the specimens were shows little short distance transportation and, in this association, there is ribs ornamentations (external moulds) and some growth lines (oysters). These features point to negligible reworking by storms, waves and suggest that this association records an autochthonous to parautochthonous.

According to Kidwell (1986), the dead shells act as “an island” of hard parts and provide substrate for benthic organisms (oysters, bivalves, serpulids, etc.). On the other hand, this does not exclude the presence of natural predators during the life of the marine molluscs. In fact, epifaunal fauna exhibit more taphonomic signatures than infaunal ones (Lazo, 2004; El- Hedeny, 2005, 2007; El-Sabbagh et al., 2015) and in general, encrustation characterizes low energy environments (Krautter, 1998; Wilson and Taylor, 2001; ElHedeny and El-Sabbagh, 2007). In geligele sections the encruster distributed on external and internal surface of left valves of oyster shells.

Approximately eighty two percent of the individuals were suspension-feeders which suggests that a turbulence level sufficiently high to keep food particles suspended in the water column. The dominance of epifaunal suspension-feeders in soft substrates is commonly related to anoxic conditions below the sediment-water interface, which excludes the deep and possibly also the shallow-infaunal guilds (Oschmann, 1988; Aberhan, 1992). Infauna is rare (9.09%), possibly due to the nutrient-poor carbonate regime or due to hypoxia below the sediment-water interface.

Generally, the dominance of epifaunal over infaunal and suspension feeder over deposit feeder and high dominance of articulated valves, some encrustation on some specimens of bivalves, the presence of fully soft substrate, low energy and the presence of thick bivalve shell species inferred basinal environment.

Modiolus bipartitus Association Modiolus bipartitus Association

9.09% 18.18%

9.09% 63.64%

Epifaunal Infaunal Semi-infaunal Deep-infaunal Suspension feeders Dposit feeder

Fig 6 Mode of life of M. bipartitus association Figure 7 feeding mode of M. bipartitus association

5.2.2 Pholadomya murchisoni Association

Pholadomya Murchisoni Associations has the highest species number among all identified Association which were occurs only in the Demibeza section in marly limestone. It consists of 8 samples with 16 species and 341 individuals. There are five species that have greater relative abundance, these are Ph. murchisoni (16.95%) and Ph. aubryi (11.86%), paleonucula lateralis (10.17%), Ilmatogyra africana (10.17%) and Nanogyra nana (10.17%).

This association has 7 Pholadomya species which were found in moulds form, 7 oysteriod bivalve species, 1trigoniod and 1 nuculoid species that were found with shell forms. deep Infaunal bivalves represents (43.75%), Epifaunal species (43.75%), semi-infaunal (6.25%) and shallow infauna (6.25%). suspension feeders represent (93.75) and only paleonucula lateralis species is deposit feeder (6.25%).

Approximately nighty-four percent of the individuals were suspension-feeders; which points to elevated water energy suggests the turbulence level must have been sufficiently high to keep food particles suspended in the water column. Some of the specimens of the bivalve shells are disarticulated (except moulds) and some of the specimens show some degree of abrasion and/or fragmentation) and it is from autochthonous to parautochthonous, all could reflect both a high energy and low energy environment that indicates the sections has both storm and post storm.

Suspension feeders are more abundant, this is because the result of the source of nutrients is high. Existence of both suspension feeders and deposit feeder, which indicates that food particles were concentrated in both the water column and in the sediment. The bivalves which occurs in growth positions are deep burrowers bivalves were always articulated.

The rarity of deposit-feeding bivalves can be explained as consequence of the high water- energy (storm) and of by-passing of particulate organic matter. Articulated Pholadomya species suggests that the fauna is autochthonous. many of the species which were found in this association are large in their body size, this may be indicate presence high food productivity.

In this association, there were storm generated waves which results bottom current remove bottom sediment and produce specific texture types called tempensites due to the presence of this storm the bed was fastely deposited changed to the intercalations thin marl and limestone beds.

According to Leonard-Pinel (2005) Abrasion may also lead to chipping of the edges of the shell. some of the Nanogyra nana species shows edge chipping which are flat, thin and subcircular in shape, ornamented with fine radiating ribs. Different factors (biological and/or mechanical) may contribute to the formation of edge chipping. They are highly accumulated at Demibeza section, especially in storm outcrop areas. In this association paleonucula Lateralis species shows boring which were small and rounded, that recorded dorsal to ventral margin and some specimens of external surface of this species shows lacking some characteristics features as growth lines. Short distance transportation of shells after death has a subsidiary effect on abrasion of their ornamentation.

Many Pholadomya species which were found in mould forms shows Lack of sorting, abrasion, degree of fragmentations, the presence of well-preserved ribs and articulation valves, all the points to no or only weak signatures of physical events such as currents and waves and many of these species were post-storm areas that indicates allochthonous, since it is attached to the substrate. Articulations of pholadomya bivalve species has high suggesting no reworking events. Deep burrowing suspension-feeder guild: includes deep burrowing bivalves. Therefore, the species which were found in this association ranges from autochthonous to parautochthonous.

Generally, dominance of infaunal over epifaunal and suspension over deposit feerder, according to Brett,2003 Shell beds, lags, or concentrations can be a common feature of shelf environment and in this association have many thin and fragmented shell which were affected by storms and pholadomya species that were found in post storm has no fragmentation, some abrasion and have articulated valves with beautiful rib patterns, so the environment inferred for this association is deep shelf characterized by high energy events, absence of encrustation, high rate of sedimentation and a high productivity.

Pholadomya murchisoni Pholadomya murchisoni Association Association 6.25% 6.25% 6.25%

Deep-infaunal Epifaunal semi-infaunal shallow-infaunal Suspension feeders Dposit feeder

Figure 8 Mode of life ph. murchisoni Figure 9 feeding mode of ph. murchisoni association association

Ph.murchisoni association percent Modiolus bipartitus abundance (%) association percent abundance (%)

ph.beaumontensis Exogyra fourtaui Actinostrean erucum Graphaea balli Liostrea acuminate Peterotrigonia scabra Liostrea acuminata Eligmus asiaticus Modiolu imbricatus Actinostrean…

Nanogyra nana Naongyra fourtaui Nanogyra fourtaui Ilmatogyra africana Paleonucula cuneformis Ph.socialis Modiolus bipartitus Ph.hemecardia Ph.somalinsis Ph.Murchisoni ph.aubryi Paleonucula lateralis Ph.Lirata

Ph.Murchisoni Nanogyra nana Pleonucula Lateralis 0 10 20 30 40 50 0 10 20 30

Figure 11 Percent abundance plot of M. bipartitus Figure 10 percent abundance plot of ph. murchisoni association association

Taxonomic group: B: bivalve. Mode of life: E: epifaunal; I: infaunal; ID: deep infaunal; IS: semi-infaunal; SI: shallow infaunal. Feeding mode: S: suspension-feeder; D: deposit feeder. Mobility; mobile; Sessile.

Table 22 Trophic nuclei of the bivalve associations

Associations

Taxonomy Taxonomy group life of Mode Feeding mode mobility Relative abundance (%) (%) Presence Pholadomya Murchisoni Association PalaeonuculaLateral B IS D M 10.17 60 (Terq.andJourdy,1980) Ph. murchisoni (J. Sowerby,1827) B ID S M 16.95 100 Ph. lirata (J. Sowerby,1827) B ID S M 8.47 50 Ph. aubryi (Douville,1886) B ID S M 11.86 70 Ph. beamontensis (J. sowerby,1823) B ID S M 1.69 10 Ph. somaliensis (Coxi,1935) B ID S M 1.69 10 Ph. hemicardia (Hoemer,1836) B ID S M 1.69 10 Ph. Socialis (Morris &Lycett,1854) B ID S M 1.69 10 Ilmatogyra africana (Lmarck,1801) B E S S 10.17 20 Nanogyra fourtaui (Stefanin,1925) B E S S 6.78 40 Nanogyra nana (J. Sowerby,1822) B E S S 10.17 60 Actinostreon gregareum (J. Sowerby) B E S S 1.69 10 Eligmus asiaticus (Douville,1916) B E S S 1.69 10 Pterotrigonia scbra (Lamarck,1819) B SI S S 1.69 10 Liostrea acuminate (J. Sowerby,1816) B E S S 10.17 60 Actinostrean erucum (Defrance,1821) B E S S 3.39 20 Modiolus bipartitus Association Nanogyra nana (J. Sowerby,1822) B E S S 11.11 46.15 Modiolus bipartitus (J. Sowerby,1818) B SI S B 24.07 100 Ilmatogyra africana (J. Sowerby,1822) B E S S 18.51 76.92 Liostrea acuminata (J. Sowerby,1816) B E S S 7.40 30.77 Nanogyra fourtaui (Nicolai,1950) B E S S 12.96 53.85 Grayphaea balli (Stefanin,1925) B E S S 11.11 46.15 Exogyra fourtaui (Stafanin,1925) B E S S 3.70 15.38 Palaeonucula lateralis B E D S 3.70 15.38 (Terq.and Jourdy,1980) Pholadomya murchisoni (J. Sowerby,1827) B DI S M 3.70 15.38 Paleonucula cuneformis (J. sowerby,1840) B I D M 1.85 7.69 Modiolus imbricatus (J. sowerby,1818) B SI S S 3.70 15.38

Demibeza section has intercalated coarse thick grained sediment (limestone) and fine thin grained sediment in (marl) that has both hard and soft substrate which is formed by high energy conditions and low energy conditions respectively. But the dominance of hard substrate over

61 soft substrate indicate the section were generally characterized by high energy environment that were found after post storm (fig.17). The section fastely change soft substrate to hard substrate and hard to soft substrate that indicate the section was affected by storm called tempensites (fig.16). The distribution of organic matter is strongly related to the turbulence. Under relatively high turbulence conditions (Dominance of epifaunal) available organic matter is kept in suspension.

Demibeza section comprises various bivalve abundant species dominated by Nanogyra nana and Pholadomya aubryi each have 11%, Pholadomya murchisoni and Palaeonucula lateralis each have 12%, Ilymatogyra africana, Liostrea acuminate and Nanogyra fourtaui each have 7%, Pholadomya lirata 10%, Actinostran erucum 4% and Pholadomya beaumontensis, Pholadomya somaliensis, Pholadomya hemicardia, Pholadomya socialis, Actinostrean gregareum, Eligmus rollandi and Peterotrigonia scabra each have 2% of total species.

Figure 12 Demibeza section out crop2 Figure 13 Demibeza section out crop1 (storm) (post-storm)

Demibeza section percent abundance (%)

Actinostrean erucum Liostrea acuminate perterotrigonia scabra Eligmus asiaticus Actinostrean gregareum Nanogyra nana Nanogyra fourtaui Ilmatogyra africana pholadomya socialis pholadomya hemecardia pholadomya somaliensis pholadomya aubryi Pholadomya Lirata pholadomya murchisoni Paleonucula lateralis

0 5 10 15 20 25 30

Figure 14 Demibeza section percent abundance

Geligele and Filikilik section has fine-grained sediment which is soft substrate that indicates low energy conditions and there was mostly oysters and their mode life is epifaunal bivalves that usually left valves are far more abundant than right valves, the reason is that the cemented left valves have a higher preservational potential which is attached to the substrate.

Geligele section consists of Nanogyra nana and liostrea acuminate each have 23%, Ilmatogyra africana 27%, Gryphaea balli 14%, Nanogyra fourtaui 11% and Exogyra fourtaui 2% of total species.

Filikilik section which were comprises percent abundance of Nanogyra nana 10%, Pholadomya murchisoni 13%, Modiolus bipartitus 62% and palaeonucula lateralis, paleonucula cuneformis and Modiolu imbricatus each have 5% of total species.

Fig 15 Geligele section out crop 1 Fig 16 Geligele section out crop 2

Fig 17 Filikilik section out crop 1 Fig 18 Filikilik section out crop 2

Geligele section percent Filikilik section percent abundance abundance (%)

Modiolus imbricatus Nanogyra fourtaui paleonucula cuneiformis Graphaea balli Modiolus bipartitus Liostrea acuminata pholadomya murchisoni

Ilmatogyra africana Paleonucula lateralis

Nanogyra nana Nanogyra nana

Exogyra fourtaui type of species

0 50 0 10 20 30 40 50 60 70

Figure 19 Geligele percent Figure 20 Filikilik percent abundance plot abundance plot

CHAPTER SIX 6 DISCUSSION

Introduction

Bivalvia are bilaterally symmetrical and laterally compressed aquatic mollusks and are usually elongated in an anteroposterior direction. These laterally compressed form characteristic of the great majority of bivalves renders them well adapted for burrowing in sandy or muddy substrates, a process accomplished with the aid of their extensile foot. Many burrowers ascend toward the sea floor in order to feed, withdrawing to a greater depth when this activity ceases, as when the bottom is uncovered between tides.

Their invariable and unchanging feature is a shell having two fully calcified valves which lie on the left and right sides of the body. Except for a few forms, the shell is found externally. Characteristically, the two valves are of equal convexity; except in some forms bilateral symmetry has been gone, usually due to the result of cementation of one valve to the substrate, the valves differ in size to a varying degree.

The state of preservation of shells varies according to the turbulence of the environment and rates of sedimentation.

In the Blue Nile Basin, the fossil pholadomya species are highly inflated valves, a well-develop, rounded umbonal anterior ridge bordering a deep depression on the anterior part. According to Dhondt and Jagtil,1989 the ribs characteristic is influenced by grain size of the substrate, in coarse grained sediment, ribs are fewer, stronger and more subdivided than in fine grained sediments, in the warm condition the species diversity increase than cold conditions and these warm conditions decrease the ornamentation like Ribs.

Taxonomy of the pholadomya bivalve species in Blue Nile Basin is, in general, difficult and in part confused. The difficulties result from the great variation in shell outline and number of radial ribs. In fact, tahonomic processes and weathering may strongly modify the ribbing pattern (number and character of ribs) and the shell outline. During the first half of the past century, a large number of new species and varieties were described on the basis of variations in ribbing pattern (Weaver, 1931), many of which were considered synonyms by later workers.

According to Hedeny (2006) that the animal was living with dorsal margin touching the sediment-water interface and hence the more stable, non-agitated environmental conditions facilitate the ingoing and outgoing of food-bearing water and waste materials respectively. In

66 the case of a rounded posterior margin, the less stable mobile substrate makes the animal to protrude upward to reach the water column at a higher level. Curvature of the posterior margin in this case may be filled with the bottom sediments to hide the animal shell from enemies.

The sculpture consists most commonly of one of two components or of a combination of these two. The first component is a concentric one and attributable to rhythmic changes in the rate of secretion of shelly matter along the mantle margins. The second component is a radial one and consists of elements diverging from the direction of the beak and crossing the concentric elements, although not at right angles except on one sector of the surface. Radial sculpture must result from the continuous enhanced secretion of shelly matter by particular groups of cells along the mantle margins during growth of the valve.

The mode of life bivalve is Epifaunal which are living on the surface of the ocean floor and Infauna bivalves are sessile and mobile bivalves which spend part or all of their life buried beneath the substrate. Most forms draw fresh water and food into this protected habitat through open sediment tubes or fleshy siphons, or both. Many feeds on organic debris within the sediment. A great variety of taxa have adapted to the infauna habitat, which affords natural protection from many but not all molluscan predators, and from most rigorous environmental conditions. Although most infaunal elements occupy relatively soft, unconsolidated substrates and thus are still subject to scouring by waves or currents, some have developed the ability to bore into partially or wholly lithified material or wood, attaining the ultimate protection afforded by the infaunal habit, a semi-permanent burrow, relatively indestructible, enclosing the animal throughout life.

The shape and relative convexity of infaunal bivalve shells are closely correlative with the depth and rate of burrowing, and energy expended in burrowing for all ecologic groups except the very specialized rock and wood borers. Bivalves with a rounded to broadly ovate outline are dominantly very shallow infaunal elements which burrow slowly, utilizing large quantities of energy, and once buried remain relatively stationary, in contrast Deeper- burrowing bivalves are generally less convex and more elongated, with the axis of elongation approximately coinciding with the direction of burrowing. convexity and elongation were the principal factors controlling depth of burrowing in infaunal bivalves.

Most of oyster bivalve species of right valves lie freely above the substrate and the left valve were found in attached forms in substrate. They use their feet to clear sediment from the shell surfaces (Young, 1980).

Bivalve are main biostratigraphic tool next to ammonite, this is because of ammonite were studied in which greater detail than bivalves. Based on the available taxonomic data, the bivalve fauna in Mekelle outlier Basin of Antalo limestone Formation shows the greatest similarity to the Tethyan Realm and Ethiopian bivalve province (Kiessling,2011).

Six hundred fifty-two specimens of fossil bivalves representing 21 species were collected from the Antalo limestone formation in the Blue Nile Basin (Dejen to Gohatsion section) and 54 samples of three section have been selected for a quantitative palaeoecologically analysis of the bivalve fraction of the benthic fauna.

Demibeza section comprises 16 bivalve species that consists of ph. Socialis, Ph. somaliensis, Ph. hemicardia, Ph. lirata, ph. aubryi, Ph. murchisoni, Ph. beamontensis, Pterotrigonia scabra, Actinostrean erucum, Actinostrean gregareum, Eligmus asiaticus, Nanogyra nana, Ilmatogyra africana, Nanogyra fourtaui and Liostrea acuminate. From these 16-bivalve species in which Pholadomya murchisoni, Paleonucula laterals, Nanogyra nana and Pholadomya aubryi species has high relative abundance respectively and Eligmus asiaticus, Actinostrean erucum, Actinostrean gregareum, Ph. Socialis, Ph. somaliensis, Ph. hemicardia and Ph. beamontensis has low equal relative abundance, the other species are between high and low relative abundance.

Demibeza section has intercalated coarse thick grained sediment (limestone) and fine thin grained sediment (marl) that has both hard and soft substrate which were formed by high energy conditions and low energy conditions respectively, but high energy was dominance in demibeza out crop1 (fig.17). This out crop were found in after storm (post storm). This section also consists another out crop (fig.16) that were happen by fastely changing their bed soft to hard substrate repeatedly. This out crop indicate there were storm activity, some of the species which were found in this out crop has smooth surface, due to presence of storm that has been lost their structure especially growth lines are affected by storm.

In the Blue Nile Basin the fossil Pholadomya species ornamented with coarser radial ribs which are intersected by concentric growth lines and the radial ornamentation that is prominent in the Pholadomya is either strongly or weakly developed (only in the umbonal region), Since the radials in the different species of the Pholadomya are variable in strength (weak to prominent) and in position (from umbonal region to the middle of the shell and extending in some species up to the ventral margin). Four radial ribs on average, but highly variable in number between specimens from 2 to 4 rather constant number of radial ribs on a single

68 specimen and the radial ribs straight to slightly curved and serrate near the posterior end. As it recorded in Japan, South America, western USA, Africa and western India in which the specimens are fully covered by radial ribs, but the Pholadomya species found in Blue Nile basin is not fully covered by radial ribs. The number of recorded species is highly variable depending on location, but the highest specific diversity has been recorded in the Valanginian- Aptian of the Tethyan realm, especially in the Mediterranean province. The species without radial ornament should only be included in the genus Homomya and those with radials (irrespective of its strength and position) are included in Pholadomya, but in the Blue Nile Basin no Homomya genus, since there is a rib ornamentation. This should be considered as one of the most significant morphological characters, which would allow an unambiguous distinction to be made between the two genera Pholadomya and Homomya.

Generally, this section has highest number of species than the other two sections that belongs to 6 order, 14 family ,1 subfamily, 11genus ,3 subgenus and 16 species.

Geligele sections consists of 6 bivalve species, all of they are oyster’s species, these are Nanogyra fourtaui, Graphaea balli, Liostrea acuminate, Nanogyra nana and Graphea fourtaui. Ilmatogyra africana and Liostrea acuminate has high relative abundance respectively and Nanogyra nana has low relative abundance. This section has almost soft and stable substrate, and encrustation on some specimens of oyster’s species that indicate the environment were low energy. Generally, the section has lowest numbers of species which are equal to Filikilik section in number of species, that are belongs to four family, one subfamily, six genus and six species.

Oysters (Mollusca) are cemented marine bivalves, which nevertheless are very capable of dispersion during both their Juvenile and adult stages. sea level changes directly affect the habitable space for marine animals including bivalves, with transgression expanding the space and regression reducing the space. The best preserved and diverse oyster’s fauna found in Dejen section (Geligele). In this out crop some of the oysters leaft only left or right valve and some if left and right valves of oysters attached together were the small valve are right valves and the left valves are large. Oyster shells grow by incremental deposition of calcium carbonate (mainly calcite), resulting in alternating dark and light bands corresponding to the colder and warmer seasons respectively (Kirby et al., 1998).

According to Stenzel (1971) the young oysters remained attached longer, or throughout life, developing a large attachment area and becoming less susceptible to current activity. Thus, they had no need of the coiled and thickened left valves characters tics of Gryphaea.

Filikilik section also comprises 6 both mould and oysters shell bivalve species that consists of Modiolus imbricatus, Paleonucula cuneiformis, Modiolus bipartitus, Ph. murchisoni, Paleonucula lateralis and Nanogyra nana. From this sections Ph. murchisoni has highest relative abundance and Paleonucula cuneiformis, Modiolus imbricatus and Paleonucula lateralis has equal lowest relative abundance which have soft and stable substrate that inferred basinal environment and they are belong to four order, six family, two subfamily, six genus and six number of bivalve species that have been identified and described systematically.

Most of the specimens of Blue Nile Basin preserved as shell forms and moulds showing ornament and even muscle insertion areas. Bivalves are known to be sensitive to environmental parameters and record changes in environment directly in their shells. Partially, these modifications can be observed from morphology (e.g., facies, water energy, competition for space or food availability). Benthonic mollusks, and among them especially the bivalves, have a great potential for paleobiogeographic analyses.

There are two main groups of environmental factors controlling the distribution of macrobenthos; physical (e.g., temperature, sediment type, grain size, rate of sedimentation, and water energy) and (bio)chemical (e.g., oxygen, salinity, pH value, and photosynthesis).

The taxonomic richness is results from (1) center of origin results from concentration of evolutionary appearances (2) center of accumulation in which species originate in other regions, but over time, their geographic ranges change to converge in the high diversity focus (3) center of survival or refugium, which assumes that high diversity results from the survival of formerly much more widespread highly diverse fauna (Wilson and Rosen,1998).

Hallam (1977) assumed changes in sea level to be the dominant control of diversity, extinction and endemism of Jurassic bivalves, arguing that certain other important biogeographic factors were more or less constant. high global sea level should facilitate faunal exchange between areas, reduce the degree of endemism and, due to larger shelf areas, should lead to an increase in overall species diversity (e.g. Hallam, 1977; Doyle, 1987) this is because of in the high sea level there is a light and normal temperature, generally safe condition for the development of fauna. Conversely, a low sea level leads to isolation of basins and produces endemic taxa and the fauna which are found in this sea level are no light and comfortable conditions, so the

70 animal present here is endemic, but the diversity become low, due to the presence of uncomfortable conditions.

According to Zakhera (2017) the Nanogyra nana species are apparently originated in the late Bajocian/Bathonian of the Middle East and move southwards in to Saudi Arabia and East Africa and India by the middle Bathonian/Callovian. In addition, N. nana reached northern Africa (Tunsia) and Europe by late Bathonian and also recorded in Iran and China from Bathonian-Callovian.

In the blue Basin most of bivalve species were found in mold and shell forms with the presence of two valves, but most oysters found in this basin specially geligele area has one valves which shows low degree of reworking.

A moderate nutrient supply (mesotrophic) and substrate of intermediate grain size provide the most suitable environment for diverse benthic communities, the most suitable substrate for the macrobenthos was found in the marl from which higher diversities were recorded

The distribution pattern of Jurassic bivalves cannot be explained by a single, overruling factor, but most likely is the result of a combination of several factors, the importance of which varied considerably throughout the Jurassic.

In all of these bivalve groups most, though not all, species are very inequivalve and therefore liable to be differentially moved by currents as well as waves or differentially destroyed by predators or abrasion against hard substrates

Tempestite is a storm deposit and it is rocks which show evidence of a strong storm, which has redeposited pre-existing sediments. It is very useful for aiding in paleoecological and paleogeographical interpretations. In the Blue Nile Basin (Dejen to Gohatsion section) specially in demibeza which is found Dejen section there is tempsite that is important for paleoenvironment reconstructions.

Below the Tempestite area there is pholadomya which shows radial ribs that is radiating from anterior to posterior, the pholadomya species which was found in demibeza outcrop displaying radian ribs, these patterns is important for rapidly increasing burrowing and efficiently and these ribs has functions to minimize the damage caused to the shell during active burrowing and it may serve as shell reinforcement (Seilacher 1985) against predators or sediment pressure, or to lock the bivalve within the sediment, thus avoiding exhumation by exhumation by currents.

Paleobiogeography

Paleobiogeographic studies play a significant role in studying continental drift and plate tectonics . The opening of marine corridors (i.e., Hispanic, Mozambique, and Viking) and the rifting of the Tethys north of Gondwanaland were the major paleogeographic consequences that took place during the Jurassic (Smith, 1983, 1989; Westermann, 1993; König and Jokat, 2010; Leinweber and Jokat, 2012; Porter et al., 2013).

According to Aberhan (1998) used biogeographic data of pectinoid bivalves to reconstruct the paleogeographic evolution of the Canadian terranes within the Early Jurassic.In order to utilize bivalves for issues of palaeobiodiverstiy, paleobiogeography or paleoecology the initial and essential step is a correct identification of species.

Aberhan (1998) arrived at similar plate tectonic reconstructions based on the pattern of Pliensbachian ammonite and bivalve distributions. Also, the comparative paleobiogeographic analysis of bivalves and ammonites (benthic and nektonic) in the Jurassic of Siberian palaeo- basins shows a good agreement (Meledina et al., 2005). However, Liu et al (1998) suggested that the boundaries of provinces based on ammonites and bivalves do not always coincide and they explained this by the differing mode of life of the two groups.

The results of Sha et al. (2002) for the same time slice suggested that in the case of ostreid bivalves, species such as Actinostreon gregareum and Nanogra nana were cosmopolitan, while, Eligmus rollandi (which they regarded as a potential ostreoid) was endemic. Kissling et al. (2011), in contrast, concluded that the Jurassic biogeographic patterns of corals, brachiopods and bivalves from the Ethiopian Province were identical, and suggested that physical drivers such as ocean currents or plate tectonics might have been more important than biological drivers such as environmental tolerance, life style, and larval strategies.

Most bivalves are not very mobile, many taxa such as oysters have a remarkably wide biogeographic distribution (Liu et al., 1998). This may be related to a long-lived, planktotrophic larval stage (Kauffman, 1975). The planktotrophic larval types have a higher dispersal potential (by currents).

The Jurassic period was characterized by progressive rise in sea level, an equitable climate, the westerly extension of the Tethys ocean, rifting in the central Atlantic and the starting of the splitting of the Pangea supercontinent (SellWood,1978),this continental separations with complex interplay factors, but not a single factors such as sea level changes, climate and

72 physical barriers provided a greater scope for the development of provincialism in fauna rather than cosmopolitan (Hallam,1975;1977;Liu et al,1998).

A northward shift of the Boreal/Tethyan boundary took place from Pliensbachian to Bathonian times. Hallam (1971) suggested that palaeotemperature changes were the cause, but recent studies (i.e., Clark et al., 1995; Callomon, 2003; McCann, 2008) indicated that the reason is the existence of a land barrier (Mid-North-Sea-High) that prevented cold polar waters to flow towards the South. The Bathonian is the time of the greatest spread of the Tethys realm during the Jurassic (Hallam, 1971). The boundary then moved southwards during the Callovian and reached its southernmost extension in the Oxfordian (Liu, 1995). A distinct fall in temperatures during the Middle Oxfordian (Martin-Garin et al., 2012; Alberti et al, 2012) may be connected to the break-up of Gondwana and the opening of the Transgondwana Seaway, which might have caused a stronger upwelling in the northwestern Tethys (Alberti et al., 2012) and permitted influx of polar water.

Geographically the Ethiopian Province comprises the eastern part of Gondwanaland with North Africa (Algeria, Tunisia, Libya, and Morocco), East Africa (Somalia, Kenya, Tanzania, and Ethiopia), and the Middle East (Saudi Arabia, the Levant, Iraq, Yemen, and Egypt) in addition to India and Madagascar. This province contains numerous endemic taxa and has been recognized from the Early Jurassic until the Late Cretaceous (Weir, 1925; Muir-Wood, 1935; Arkell et al., 1952; Arkell, 1956). According to Heinze (1996) similarities of bivalves between the European (North Tethys) and the Ethiopian (South Tethys) provinces are very high at the genus level and even at the species level, 35% of the Bathonian and Callovian genera of the Ethiopian province are endemic.

According to Liu et al. (1998) the Ethiopian Bivalve Province is less distinct than other bivalve provinces, and explained this by the transitional nature of the Middle East, which in the Middle Jurassic apparently was a spreading center for taxa originating in that area and migrating both westward towards North Africa and southward into India and Madagascar. According to these authors, the Ethiopian Bivalve Province can be well defined at the species level. For example, in the Bathonian and Callovian rocks of India about 25% of the bivalve species are endemic. Similarly, according to Kissling et al. (2011) the distribution patterns of corals and brachiopods are indicated of a fairly sharp boundary between the Ethiopian and European provinces running north of Jordan and Tunisia. Also, Damborenea et al. (2013) suggested that a new Ethiopian

73 unit was recognizable since the Bajocian based on the high ratio of endemic genera. This unit became indeed evident in Callovian-Kimmeridgian times.

As the endemism of the Ethiopian fauna increased from the Bathonian to the Late Jurassic, Heinze (1996), suggested to split the Ethiopian Province into two-subprovinces; the 'Ethiopian- Tethyan' subprovince to the north and the 'Ethiopian-Austral' subprovince to the South.

Mette (2004) proposed that the Bajocian faunas of Madagascar migrated from Arabia and North Africa, while the Callovian–Kimmeridgian ones are indicative of intensive migration between Madagascar and India and isolation from Africa, Arabia, and South America due to physical and/or ecological barriers (probably the Mozambique corridor) between Madagascar- India and East Africa.

The Trans-Erythraean Seaway permitted faunal exchange during the Middle Jurassic (Bathonian and Callovian), providing a direct migration route from Madagascar toward the south, before becoming fully established as the Indian Ocean in Tithonian times (Hallam, 1983; Krishna, 1994; Gardner and Campbell, 2002; Challinor and Hikuroa, 2007).

During the late Triassic Blue Nile Basin of central Ethiopia was positioned near to the western margin of the Tethys about 20 south of equator (Scotese et al,.1999). Southeastern margin belongs to the Ethiopian Bivalve Province which is only poorly defined by bivalve genera/subgenera: in particular, its northern boundary is blurred, with the Arabian region occupying an intermediate position between the Mediterranean and the Ethiopian bivalve provinces.

Figure 21 Paleogeographic reconstruction of the Middle-Late Jurassic world (Taken from Abdelhady,2014 after Scotese, 2001; Golonka, 2002) showing the position of some countries included in the biogeographic analyses. Arrows refer to marine circulation pattern (taken from Abdelhady,2014 after modification of Parrish, 1992, Arias, 2008).

CHAPTER SEVEN 7 CONCLUSION AND RECOMMENDATION Conclusion

Blue Nile basin contains large and excellent exposure of upper Jurassic rocks which is eponymous to Ethiopia bivalve province. Upper Jurassic bivalves of Blue Nile basin from Dejen to gohatsion provides first detailed understanding of bivalve species by dividing the study area in to three sections, i.e., Demibeza section, Geligele section and Filikilik section. This study provides depositional environment interpretation, making systematic paleontology (Taxonomy), paleoecological analysis, preparing bivalve species association which are living in the same environment, Paleobiogeography from literature, morphological description and knowing relative abundance of each sections and association. Pre-fieldwork, detailed fieldwork and laboratory description and analysis were the main methodology implemented to achieve the stated research problem and the objective of the present study.

Six hundred fifty-two specimens of fossil bivalves were collected from the Antalo limestone formations in the Blue Nile Basin (Dejen to Gohatsion). From this collected bivalve twenty- one bivalve species, eight order, eighteen Family, three subfamily, one superfamily, nighnten Genus and four subgenera were identified.

The study area comprises two association, these are Pholadomya murchisoni and Modilus bipartitus. Pholadomya murchisoni has the highest species number than Modiolus bipartitus association which were occur only in the demibeza section in marly limestone that consists 8 samples with 16 species and 332 individuals. The species were ranges from autochthonous to Para autochthonous, some the specimens in this association shows abrasion and fragmentation (oysters) and the others shows lack of signature of physical events with the absence of encrustation, dominance of infaunal over epifaunal and suspension feeder over deposit feeder. All of the above characteristics inferred deep shelf environment.

Modiolus bipartitus association has lower species than Pholadomya mrchisoni which were found both in Filikilik and geligele sections and it consists of 7 samples with 11 species and 305 individual samples were collected the dominance of epifaunal over infaunal and suspension feeder over deposit feeder and high dominance of articulated valves, some encrustation on some specimens of bivalves, the presence of soft substrate, low energy and the presence of thick bivalve shell species inferred open marine environment.

Recommendation

This study is the first work to conduct systematic paleontology on upper Jurassic rock down to species and generic level identifications of bivalves, in which Paleoenvironmental interpretation were carried out from both associations and paleoecological results.

Taxonomy of the bivalve order Pholadomyida is difficult and in part confused. The difficulties result from the great variation in shell outline and number of radial ribs and distortion caused by compaction makes it difficult to reconstruct the original shell outline. In fact, tahonomic processes and weathering may strongly modify the ribbing pattern (number and character of ribs) and the shell outline. Some specimens were not found in fully preserved forms and it is difficult to describe up to species level.

The study area that bivalve fossil specimens were collected were not enough to know depositional environment, the study area may also need not only bivalve specimens, but also other macro invertebrate fossil to fully reconstruct depositional environment.

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Prevalence and Associated Factors of Depression among Pregnant Mothers Who Had Intimate Partner Violence during Pregnancy Attending Antenatal Care at Gondar University Hospital Northwest Ethiopia in 2020

Background. Antenatal depression is the major obstetric problem that led to significant maternal and perinatal morbidity and mortality worldwide, especially in the third world. However, in Ethiopia this prevalence and association were not studied, as result, this study investigated the prevalence and associated factors of antenatal depression among pregnant mothers who had intimate partner violence during pregnancy. Methodology. An institution-based cross-sectional study was done among 409 pregnant mothers who had intimate partner violence during pregnancy from May to July 2019 at Gondar University Hospital. All pregnant mothers who came for ANC follow-up during the study period approached for screening of intimate partner violence during pregnancy using standard and validated screening method and instrument of the WHO multicountry study on women’s health and domestic violence to evaluate intimate partner violence, and we use EPDS for the evaluation of antenatal depression validated in Ethiopia with a cut point of 13. Result. Prevalence of depression among pregnant mothers who had any form of intimate partner violence during pregnancy was 35%: physical abuse ( AOR = 1.8 ; 95% CI: 1.19, 3.30), more than one type of abuse ( AOR = 10.18 ; 95% CI: 7.10, 16.18), poor social support ( AOR = 5.81 ; 95% CI: 1.12, 13.12), and pregnant mothers whose partner drunk for the past twelve months ( AOR = 7.16 ; 95% CI: 183, 8.00) were significantly associated with antenatal depression. Conclusion. High prevalence of antenatal depression among pregnant mothers who had intimate partner violence during pregnancy was highly associated with physical abuse, more than one type of abuse, lack of social support, and partner of pregnant mothers who is a drunk. Hence, this is important to create a screening program and prevention strategy of intimate partner violence during pregnancy at the time of antenatal follow-up to prevent and early identify its morbidity and mortality.

Assessment of renal dysfunction and associated factors among patients on Tenofovir based antiretroviral treatment at Gondar University Hospital, North West Ethiopia: Retrospective institution based cross sectional study

Cause and incidence of cancellation of elective surgeries at gondar university hospital, ethiopia.

Abstract Background: High rates of cancellation of surgical procedures are common in hospital settings which may subsequently lead to economic loss to hospital besides burden given to patients, their families and medical teams .It is well recognized that cancellation of patients from elective theatre operating lists increases cost, decreases efficiency, duplicates workload and wastes operating room time.Cancellation of elective surgical procedures also causes significantly emotional trauma to the patients as well as their families and the community in general, and its impact on hospital resources is great due to prolonged hospitalization and high cost of health care.The aim of this study was to find the causes and incidence of surgical patient cancellation at Gondar university hospital, North-west Ethiopia.Methods: prospective observational study was conducted from January 10 to April 10, 2019. Information regarding the cancellation of surgeries were collected from various sources including; the operating room daily surgical schedule, preoperative anaesthesia record sheet, primary physicians, the anaesthetist responsible for the preoperative assessment and conducting the case and by communicating patients if required. Data were checked on daily basis for completeness and were entered to Epi info and analyzed using statistical package for social sciences (SPSS) version 20 software.Result: There were 64 causes of case cancellation. The commonest reasons for cancellation were overbooking of elective surgeries (33.13%).Conclusion: Preventable causes of case cancellation were the most prominent.

Prevalence and associated factors of erectile dysfunction among men DM patients in Gondar university hospital, Gondar Ethiopia

Abstract Back ground- globally it is likely that 387 million people have DM and this number expected to increase to 592 million by the year 2035. WHO estimated the number of cases of diabetes in Ethiopia to be about 800thausadns in 2011 and expected that it would increase to about 1.8 million by the year 2030. Psychosomatic and sexual dysfunctions are one among different complication that DM patients come across during their disease course. Therefore this study aimed to estimate the prevalence and associated with erectile dysfunction among adult men DM patients in Ethiopia.Method-An institutional based cross-sectional study design was conducted among male DM patients at Gondar university referral hospital chronic illness clinic. Single population proportion sampling technique was used. 367 were selected proportionally using systematic random sampling technique. Face to face interview method was employed using a structured questionnaire for data collection. Data were analyzed descriptively and through bi-and multivariate logistic regression model.Result-The prevalence rate is 53.1%. The mean age of the respondents was 54.12(SD+16.5112) years age from 21-90 years. Majority of the respondents 158(43.1%) were Orthodox followers followed by Muslim, 113(30.8%), Protestant 55(15.0%) and Catholic 41(11.2%). the median monthly salary was 2000 Ethiopian Birr. BMI of most respondents was within the range of 18.5-24.9. ED was significantly associated with Age, Duration of DM and Alcohol.Conclusion- Finding of this study indicated that ED is a major public and self issue with its multi-factorial among male DM. The evidence from this study found that being aged, alcoholic and longer duration of DM was significantly associated with ED.Key words- Erectile dysfunction, prevalence, male DM patients, Gondar University Referral Hospital.

Data Mining Technology Enabled Anti Retroviral Therapy (ART) for HIV Positive Patients in Gondar University Hospital, Ethiopia

Determinants of under-five pneumonia at gondar university hospital, northwest ethiopia: an unmatched case-control study.

Background. Pneumonia causes about two million under-five deaths each year, accounting for nearly one in five child deaths globally. Knowing the determinants of under-five pneumonia is useful for prevention and intervention programs that are aimed to control the disease. Thus, the main aim of this study was to assess the determinants of under-five pneumonia at Gondar University Hospital, Ethiopia. Methods. An institution-based unmatched case-control study was carried out from April 1 to April 30, 2015, taking a sample size of 435 study participants (145 cases and 290 controls). The researchers used a systematic random sampling technique for selecting cases and controls. Data were entered and cleaned using Epi Info version 7 and exported to SPSS version 20 for analysis. Bivariable analysis was performed, and variables with a p value less than 0.2 were entered into multivariable logistic regression. Determinant factors were identified based on p value less than 0.05 and adjusted odds ratio with 95% confidence interval (AOR with 95% CI). Results. An increased odds of pneumonia was associated with children who had diarrhea in the past fifteen days of data collection (AOR = 6.183; 95% CI: 3.482, 10.977), children’s mothers who did not hear about how to handle domestic smoking (AOR = 5.814; 95% CI: 2.757, 12.261), and children of mothers who did not follow proper handwashing practice (AOR = 3.469; 95% CI: 1.753, 6.863). Conclusions. Being infected with diarrhea, not knowing how to handle domestic smoking, and poor compliance with proper handwashing practice were identified as determinants of pneumonia. Dedicated, coordinated, and integrated intervention needs to be taken to enhance proper handwashing practice by mothers/caregivers, improve the indoor air quality, and prevent diarrheal diseases at the community level.

Prevalence and degrees of myopia and hyperopia at Gondar University Hospital Tertiary Eye Care and Training Center, Northwest Ethiopia [Corrigendum]

In-hospital mortality among ischemic stroke patients in gondar university hospital: a retrospective cohort study.

Introduction. Ischemic stroke is the third leading cause of mortality in low-income countries and the sixth in Ethiopia. The aim of this study was to determine the rate and predictors of in-hospital mortality due to ischemic stroke in Gondar University Hospital. Methods. The study was conducted from April 1, 2017, to May 15, 2017, at Gondar University Hospital. A census using retrospective cohort study design was conducted on medical records of adult patients with the diagnosis of ischemic stroke attending the medical inpatient ward of Gondar University Hospital between November 2012 and September 2016. Cox hazard regression was used to determine the predictors of in-hospital mortality. A two-sided statistical test at 5% level of significance was used. Results. The mean (±SD) duration of hospital stay was 11.55 (10.040) days. Of the total 208 patients, 26 (12.5%) patients died in the hospital. Cox regression revealed that only a decrease in renal function, particularly elevated serum creatinine (AHR=8.848, 95% CI: 1.616-67.437), was associated with a statistically significant increase of in-hospital mortality. The symptom onset-to-admission time varied greatly among patients and ranged from 1 hour to 168 hours. Conclusion. The in-hospital mortality associated with ischemic stroke was found to be high. Mainly, elevation in serum creatinine was highly associated with poorer outcomes in terms of in-hospital mortality. Much work should be done on improving the knowledge and awareness of the community regarding ischemic stroke and stroke in general to encourage early medical seeking behavior and reduce mortality and long-term disability.

Reasons for delay in decision making and reaching health facility among obstetric fistula and pelvic organ prolapse patients in Gondar University Hospital, Northwest Ethiopia

Aims: To assess reasons for the delay in getting treatment of women with obstetric fistula and pelvic organ prolapse at Gondar University Hospital. Methods: A hospital based cross-sectional study was conducted among 384 women. Delay was evaluated by calculating symptom onset and time of arrival to get treatment at University of Gondar Hospital. Regression analysis was conducted to elicit predictors of delay for treatment. Results:  Of the total 384 participants 73(19.1%) were fistula cases and 311 (80.9%) were pelvic organ prolapse. The proportion of women who delayed for treatment of pelvic organ prolapse was 82.9%, and that of obstetric fistula was 60.9%. Women who had fear of disclosure due to social stigma (AOR=2; 1.03, 3.9), and financial problem (AOR=1.97; 1.01, 3.86) were associated with delay to seek treatment for pelvic organ prolapse. While increasing age (AOR =1.12; 1.01, 1.24)and divorce (AOR = 16.9; 1.75, 165.5) were associated with delay to seek treatment among obstetric fistula cases, Conclusions: A high proportion of women with pelvic organ prolapse and Obstetric fistula were delayed to seek treatment. Fear of disclosure due to social stigma and financial problem were the major factors that contributed for delay to seek treatment for pelvic organ prolapse. While increasing age and divorce were the predictors for delay to seek treatment for obstetrics fistula patients.

Poor outcomes associated with antithrombotic undertreatment in patients with atrial fibrillation attending Gondar University Hospital: a retrospective cohort study

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