An amphidontid symmetrodont from the Early Jurassic Kota Formation, India

Transcription

1 <oolo&'cal Journal of the Linnean Society (I 985), 85: With 2 figures An amphidontid symmetrodont from the Early Jurassic Kota Formation, India P. YADAGIRI Manoranjan Building, Mukaramjahi Road, Hyderabad , India Keceiued April 1983, reviwi and anepled for ~uhl~ca~ion January 1985 A new aymmetrodont genus and species from the Lower Jurassic Kota Formation is defined and a brief description is given. The new species is compared with the amphidontid symmetrodonts from America and Manchuria. It is tentatively included in the family Amphidontidae. KEY WORDS:- Symmetrodonta - Amphidontidae - new taxon - molar tooth. CONTENTS Introduction Geological setting Systematicdescription Nakunodon gen. nov Nakunodon paikasiensis sp. nov Discussion Acknowledgements Refcrcnces INTRODUCTION The first Mesozoic mammal was discovered in the Stonesfield Slate (Middle Jurassic) in about 1764, but its significance was not recognized until 1828 when the specimen was presented to the York Museum (see Owen, 1871: 15). The fossil record of early mammals is now fairly well known from both the Upper Triassic and the Upper Jurassic. The record from the Lower and Middle Jurassic was sparse till the discovery of symmetrodonts from the Kota Formation of Pranhita-Godavari valley, India (Datta, Yadagiri & Rao, 1978) and the Bathonian Forest Marble Formation, England (Freeman, 1979). Three new genera and species, viz. Kotatherium haldanei (Datta, 198 I), Trishulotherium kotaensis and Indotherium pranhitai (Yadagiri, 1984) of symmetrodonts belonging to the family Keuhneotheriidae have been described from the Kota Formation. Nakunodon paikasiensis gen. et sp. nov. is a fourth genus, characterized by a smaller metastyle and stylocone and a larger paracone making a monocuspid surface. The significance of this new form of amphidontid symmetrodont is evaluated /85/ f07 $03.00/ The Linnean Society of London

2 412 P. YADAGIRI Jurassic and Cretaceous mammals were small creatures, the size of modern shrews and rats. In the last two decades, these tiny fossils have gained prominence owing to their presumed importance in the evolutionary history of mammals. New methods of recovery of these mammalian microfossils have been employed which resulted in recognizing new localities, new forms and a diversified fauna, but still many gaps remain to be filled. The taxa are often known only from fragmentary jaws and isolated teeth and the reconstruction of complete skeletons has not been achieved for most of the forms. The new mammalian fauna came to light with the discovery of skeletal remains in the Lower Jurassic Kota Formation, Pranhita-Godavari valley, India (Datta et al., 1978). Three new genera of symmetrodonts have already been described, Kutatherium haldanei from the collections made from a quarry section near Yamanpalli village in Andhra Pradesh, India (Datta, 1981), and Trishulutherium kutaensis and Indutherium pranhitai from a new locality, about 10 km north of the Yamanpalli quarry (Yadagiri, 1984). Symmetrodonts were shrew-sized mammals, known only from jaw fragments and teeth. Their lower molars are characterized by a triangular, imperfectly symmetrical cusp arrangement. The central external cusp (protoconid) is the highest and there are smaller anterior (paraconid) and posterior (metaconid) internal cusps. They are grouped into three families; Kuehneotheriidae, Amphidontidae and Spalacotheriidae (Cassiliano & Clemens, 1979). In the Amphidontidae, the lower molars are functionally monocuspid, the paraconid and metaconid being little more than breaks in the contour of the slope of the crown. The family includes two genera, viz. Amphidun superstes (Simpson, 1929) and Manchurudun simplicidens (Yabe & Shikama, 1938). The type of Manchurudun (a fragmentary lower jaw and part of the scapula) was lost during the Second World War and the only source of information is the paper by Yabe & Shikama (1938), whereas Gobiundon infnitus, a presumed amphidontid symmetrodont from Khooboor, Mongolia is regarded as a numen nudum (Cassiliano & Clemens, 1979). Thus this new amphidontid skeletal material from the Kota Formation makes a significant contribution to our knowledge of the amphidontid symmetrodonts. An isolated, almost complete, upper molar tooth was recovered from the new locality about 10 km north of Yamanpalli quarry. It is recognized as belonging to a new genus of Amphidontidae, viz. Nakunodon paikasiensis gen. et sp. nov. GEOLOGICAL SETTING The Upper Gondwana sequence in the Pranhita-Godavari valley was originally considered to consist of three groups, namely Maleri, Kota and Chikiala (King, 1881). This has subsequently been modified by Kutty (1969), who showed that the Kota Group consisted of three different formations: Dharmaram, Kota and Gangapur. The Kota Formation includes sandstones, clays and a prominent limestone zone. It overlies the Dharmaram Formation which has yielded prosauropodian dinosaurs. It is overlain by the Gangapur Formation with an angular unconformity. The limestone bed is richly fossiliferous and has yielded different types of

3 AN EARLY JURASSIC AMPHIDONTID 41 3 Figure 1. Nakunodon paikasiensis gen, et sp. nov.; holotype in lingual (A) and buccal (B) views. Stereophotographs. The scale bar represents 0.5 mm.

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5 AN EARLY JURASSIC AMPHIDONTID 415 fishes (Yadagiri & Prasad, 1977, Yadagiri, Prasad & Satsangi, 1980) and a pterosaur (Jain, 1974). The clay beds immediately underlying the limestones have yielded a variety of sauropod dinosaurs (Yadagiri et al., 1980) and early mammals. The Kota Formation is assigned to the Early Jurassic (Liassic) age based on piscean fauna (Yadagiri & Prasad, 1977). However, the ostracod fauna indicated that the age may extend up to Middle Jurassic (Govindan, 1975). The marly clays were soaked in water overnight and sieved through selected meshes and dried. The microvertebrates were recovered from the dried-up samples. In addition to skeletal parts of mammals, a number of ostracod shells, charophyta and scales and skeletal parts of fishes were collected. SYSTEMATIC DESCRIPTION fnfra Class Pantotheria (Marsh, 1880) Order Symmetrodonta (Simpson, 1925) Family Amphidontidae (Simpson, 1925). Nakunodon gen. nov. Type species: Nakunodon paikasiensis sp. nov. Diagnosis: as for the only species. Etymology: The paracone forms a monocuspid surface with a curvature like a nail (nakuna in Sanskrit) hence the name. Nakunodon paikasiensis sp. nov. (Figs 1, 2) Material: The holotype, right upper molar only; G.S.I. Type No. SR/PAL/12. Deposited in the Regional Palaeontological Laboratories, G.S.I. Hyderabad, India. Type locality: 6 km east of Asafabad Railway Station; Adilibad district, Andhra Pradesh, India. Diagnosis: A symmetrodont upper molar with a small metastyle and stylocone; paracone makes monocuspid surface; heavy cingulum around the crown; roots robust but short. Etymology: The species is named after the locality, Paikasigudem. Description: The specimen is excellently preserved except for a small portion near the metacone which is chipped. The crown is made up mostly of paracone (Figs 1 and 2) which is triangular in shape and measures 0.4 mm in length and 0.5 mm in width. In occlusal view, the crown is of monodont type similar to Amphidon. The metacone is a smaller protrusion while the parastyle is slightly larger than the metacone. The protocone is curved, convex on the labial side, its apex is pointed. The lateral surface shows breaks and forms ridges. The roots are robust and comparatively short. The length of the anterior root is 0.5 mm and its width is Figure 2. Nakunodon paikasicnsis gen. et sp. nov. in distal (A), mesial (B) and occlusal (C) views. Stereophotographs. The scale bar represents 0.5 mm.

6 416 P. YADAGIRI 0.25 mm. The length of the second root is 0.25 mm and its width is 0.15 mm. The cingulum is strongly developed around the crown. DISCUSSION The family Amphidontidae of symmetrodonts is represented by two inadequately known genera, Amphidon and Manchurodon. Amphidon was diagnosed as having monocuspid lower molars in which the paraconid and metaconid were merely breaks in the contour of the slope of the crown (Simpson, 1929). Subsequently, after additional preparation of the type and only specimen, it became clear that the teeth are heavily worn (Crompton & Jenkins, 1968). To what extent the small size of the accessory cusps are products of attrition remains unknown. The holotype of Manchurodon is an unnumbered mandible with nearly complete, monocuspid teeth and a fragmentary scapula, possibly of the same individual (Cassiliano & Clemens, 1979). The specimen is considered to have been lost in war. The rock unit Husin Series in which the specimen was found is considered to be Jurassic on the basis of plant fossils. However, on the basis of the vertebrate assemblage and in particular the mammalian specimen, Endotherium, the sediments are now thought to be of Early Cretaceous age (Chow, 1953). In view of the above considerations, it may be stated that the Family Amphidontidae was not properly established because of inadequate information from two known genera. The specimen of Nakunodon is fairly well preserved and is a significant addition to the study of the characters of the Family Amphidontidae. It is recorded from the well established stratigraphic horizon of Lower Jurassic age. Nakunodon is found in association with other symmetrodonts of the family Kuehneotheriidae, viz. Kotatherium, Trishulotherium and Indotherium. In Nukunodun, the paracone is wider than tall with a marked vertical ridge in the middle, while in Amphidon it is more slender and becomes pointed at the apex. On the sides of the crown of Amphidon, two breaks are seen; in Nakunodon the slope is gradual. The cingulum in Nakunodon is heavy with a straight margin, while in Amphidon the cingulum is drawn up. The roots in Nakundon are robust and are in contact with each other. Yabe & Shikama (1938) stated that the molars in Manchurodon were functionally monocuspid and noted the presence of a posterior cuspule metaconid but not a paraconid. The presence of a metaconid in Manchurodon separates it from the new genus Nakunodon in which only paracone is present. In view of these differences, the Kota amphidontid is considered to represent a new genus. Nakunodon can alternatively be interpreted as a member of a separate lineage. The unique character is the absence of a metacone and a protocone. If reduction of these cusps occurred during evolution from a kuehneotheriid ancestor, this was a reversal of the general trend among mammals; the addition of cusps to the single cusp of the teeth of their primitive reptilian ancestors. Dental simplification, as interpreted here for this new genus of amphidontid, is a phenomenon that has been recorded in the evolutionary history of Cenozoic mammals. Thus, it can be stated that in Lower Jurassic times in India, there

7 AN EARLY JURASSIC AMPHIDONTID 417 existed together kuehneotheriids with a special dental pattern and the flukunodon type showing a trend to the simple dentition. ACKNOWLEDGEMENTS I express my sincere thanks to the Deputy Director General, Geological Survey of India, Southern Region for facilities to carry out research work. I am grateful to Shri B. R. Jagannatha Rao, Director, Regional Palaeontological Laboratory, Geological Survey of India and Dr. K. Satyanarayana, Reader, Osmania University for their guidance and valuable suggestions. I would like to express my sincere gratitude to Prof. A. W. Crompton, Harvard University, U.S.A., for providing scanning electron microscope photographs and guidance. REFERENCES CASSILIANO, M. L. & CLEMENS, W. A,, Symmetrodonta. In 2. Lillegraven, Kielan Jawarowaka & W. A. Clemens (Eds), Mesozoic Mammals: Berkeley: University of California Press. CHOW, M. M., The significance of the finds of Mesozoic fossil mammalia in the north east provinces. Acta Palaeontologica Sinica, 1: CROMPTON, A. W. & JENKINS, F. A. Jr, Molar occlusion in Late Triassic mammals. Biological Reuiew, 43: DATTA, P. M., The first Jurassic mammal from India. <oological Journal of the Linnean Society, 73: DATTA, P. M. YADAGIRI, P. & RAO, B. R. J., Discovery of early Jurassic micromammals from Upper Gondwana sequence of Pranhita-Godavari valley, India. Journal of the Geological Society of India, 19: FREEMAN, E. F., A middle Jurassic mammal bed from Oxfordshire. Palaeontology, 22: GOVINDAN, A., Jurassic freshwater Ostracods from the Kota Limestone, India. Palaeontology, 18: JAIN, S. L., Jurassic Pterosaur from India. Journal of the Geological Society of India, 15: KING, W., The geology of the Pranhita Godavari valley. Memoir of the Geological Suruey of India: KUTTY, T. S., Some contributions to the stratigraphy of the Upper Gondwana formations of the Pranhita Godavari Valley, Central India. Journal of the Geological Society of India, 10: OWEN, R., Monograph of the fossil Mammalia of the Mesozic formations. Palaeontographical Society Monograph, 24, 115 P. SIMPSON, G. G., American Mesozoic Mammalia. Peabody Museum (Yale University) Mem., Vol. 3, part 1. YABE, H. & SHIKAMA, T., A new Jurassic mammalia from south Manchuria. Imperial Academj of Tokyo Proceedings, 14: YADAGIRI, P., Observations on Kota Formation of Pranhita Godavari valley South India. Geological Survey of India, Mix. Publications, 45: YADAGIRI, P., New Symmetrodonts from Kota Formation (Early Jurassic), India. Journal of the Geological Society of India, 25: YADAGIRI, P. & PRASAD, J. N., On the discovery of new Phfflidophorus fishes from the Kota Formation, Adilabad district, Andhra Pradesh. Journal of the Geological Sociely of India, 18: YADAGIRI, P., PRASAD, K. N. & SATSANGI, P., 1980a. The sauropod dinosaur from Kota Formation of Pranhita-Godavari valley, India. Proceedings of International Gondwana Symposium I: YADAGIRI, P., SATSANGI, P. P. & PRASAD, K. N., 1980b. Study of Piscean fauna from Adilabad district, Andhra Pradesh, GSI. Palaeontologia Indica, new series, 45: 1-31.