Late Cretaceous continental vertebrate fossil record from India: palaeobiogeographical insights

Abstract

Geophysical data suggested a minimum of 35 Ma physical isolation for the Indian plate from the time of its separation from Madagascar around 88 Ma ago to its final collision with Asia in the Early-Middle Eocene (55-50 Ma ago). Such an extended period of segregation of any landmass is expected to result in genetic isolation of pre-existing populations leading to the development of endemic biota. Therefore, continental Late Cretaceous biota of India hold the key to our understanding of effects of long isolation and northward drift of the Indian plate over different latitudinal belts. Focused palaeontological research in the last one and half decades on the Deccan volcano-sedimentary sequences (infra- and intertrappean beds) has resulted in the recovery of diverse assemblages of vertebrate, invertebrate, and plant fossils. The Deccan volcano-sedimentary sequences were dated Late Cretaceous-Early Palaeocene in age based on vertebrate, ostracod, planktonic foraminiferal, palynofloral and geochronological data. Critical evaluation of the biota from these strata brings out a complex biogeographical picture. The Late Cretaceous biota of India include some taxa of Gondwanan affinities (leptodactylid, hylid and ranoid frogs, madtsoiid and nigerophiid snakes, pelomedusoid turtles, mesosuchian crocodiles, abelisaurid dinosaurs, and gondwanathere mammals), Gondwanan relicts (haramiyidan mammals), certain taxa of Laurasian affinities (pelobatid and Gobiatinae frogs, anguimorph lizards, eutherian mammals, charophytes), and ostracods of predominantly endemic nature. Since India was once part of the former Gondwanaland, the presence of Gondwanan taxa in the Late Cretaceous of India is not anomalous from a biogeographic point of view. These taxa might have been derived from Gondwanan stocks that boarded the Indian plate prior to its break-up from Africa or might represent immigrants from South America that reached the Indo-Madagascar block via Antarctica and the Kerguelen Plateau/Gunnerus ridge not later than 80 Ma. However, the presence of Laurasian non-marine taxa in the northward drifting Indian plate defies palaeogeographical data showing a wide body of marine water (Tethys) separating India from Asia. In the light of latest palaeontological, stratigraphic, geochemical and geophysical data from the northern margin of India, one cannot rule out dispersals from the northern landmasses across the Kohistan and Dras island-arcs and Trans-Himalayan magmatic arc. Other biogeographical models, such as "Out-of-India Dispersals" for many vertebrate, invertebrate, and plant groups, also deserve a close examination. At present, limited quantitative fossil data is available to test these biogeographical models.