EOCENE VERTEBRATES FROM LIGNITE MINES OF GUJARAT (WESTERN INDIA) WITH EMPHASIS ON MAMMALS

Abstract

The early Eocene terrestrial fossil record of India, particularly for the 52-55 Ma interval, is crucially important in understanding the dispersal patterns of many of the modern mammalian orders, particularly in the context of India-Asia collision and the Out-of- India hypothesis. Until recently, there was virtually no record of terrestrial vertebrates from the Indian early Eocene. Investigations carried out as part of the present doctoral work have led to the recent discovery of a significant early Eocene terrestrial vertebrate fauna from the Cambay Shale deposit in the Vastan Lignite Mine, District Surat, Gujarat, western India. This fauna, approximately 53.5 m.y. old, is remarkably diverse and consists of an admixture of terrestrial and aquatic vertebrates (fishes, amphibians, lizards, snakes, birds, turtles, and most importantly, mammals). Mammals are the most diverse group (with as many as 25 taxa already recovered) and include members of at least ten placental mammalian orders: perissodactyls, artiodactyls, creodonts, condylarths, primates, insectivores, apatotherians, proteutherians, chiropterans, rodents. In addition, marsupials have also been discovered for the first time from the Indian Eocene. Most of these taxa are new at family or genus level, and constitute the oldest known Cenozoic records in South Asia. The collection comprises a large number of jaws (mainly dentaries, but also a few maxillae), hundreds of isolated teeth and postcranial remains including long bones, in some cases associated. Special emphasis is placed in this dissertation on certain groups that appeared virtually simultaneously across the northern continents during the intense warming interval (Paleocene-Eocene Thermal Maximum, PETM) around 55 Ma, i.e. artiodactyls, perissodactyls, primates and hyaenodontid creodonts. Perissodactyls are the dominant group in the Vastan fauna (both taxonomically and numerically) and include a new family Cambaytheriidae, characterized by remarkably bunodont teeth without any loph development. Tapirs represent the second perissodactyl group in the assemblage known from well-preserved maxilla placed in a new taxon. Primates are represented by least 4 taxa, two of which are possible omomyids and the third, an adapiform, probably pertains to an as yet unnamed family. Artiodactyls are represented by a single species belonging to a new genus Gujaratia. The remainder of the described mammals- creodonts and condylarths- are also new. On the whole, the Vastan mammals are holarctic, endemic at family or genus level, and share a number of primitive characters with allied taxa from the northern continents. The discovery of this remarkable fauna raises an important question as to their biogeographic origins. Two possible, but poorly tested, hypotheses are i) that they originated in the Indian subcontinent and ii) that they indicate dispersal from the northern continents, followed by renewed isolation, around the time of initiation of India-Asia collision. The latter possibility is tentatively favoured here considering the slightly younger age of the Vastan mammals relative to their oldest holarctic relatives. Apart from the Vastan fauna, a second, slightly younger (late early Eocene), vertebrate fauna was recovered from Naredi Formation at the Panandhro Lignite Mine, District Kutch, Gujarat. Unlike theVastan fauna, this assemblage is predominantly marine and comprises fishes, snakes, crocodiles, turtles and rare marine mammals. Mammals form the most important component of the Panandhro fauna, and are represented mainly by archaic cetaceans (primitive whales) belonging to the family Remingtonocetidae, characterized by narrow, long-snouted taxa. The recovered material comprises fragmentary skulls and jaws and a few isolated teeth. If the early Eocene age of the whale-yielding strata is correct, then these specimens are possibly amongst the oldest known remains of fossil whales. These finds reinforce the importance of the Kutch Eocene in understanding the origin and early radiation of whales. Summing up, the Vastan Lignite Mine has opened up a long-awaited source of early Eocene terrestrial mammals in the Indian subcontinent. Work so far done clearly shows that this fauna is potentially important in understanding the pattern of events (including the India-Asia contact) that led to the diversification and dispersal of a number of modern mammalian orders. However, much work remains to be done to fully uncover the taxonomic diversity, and to understand the phylogenetic relations of the Vastan mammals based on increased anatomical coverage. In addition, it is important to search for mammals in the older intervals, particularly in the latest Paleocene to earliest Eocene deposits in India.