Dinosaur fossil records from India and their palaeobiogeographic implications: an overview

The complex palaeogeographic history of India involving a gradual transition from Gondwana to Laurasia with an intervening phase of prolonged physical isolation, and the biotic signatures of this complex history as preserved in India’s Mesozoic fossil record are of much current interest and continue to be debated. Seen in this context, the fossil record of dinosaurs from India provides a unique opportunity to study their diversity and palaeobiogeographic distribution in time and space. The Indian fossil record, as currently documented, is patchy and restricted mainly to three intervals of the Mesozoic era: Late Triassic, Early/Middle Jurassic and Late Cretaceous. The Late Triassic–Jurassic record, representing a Pangean setting, is known primarily from the Gondwana formations of Pranhita–Godavari (P–G) Valley in the southern Indian state of Andhra Pradesh, although sporadic Jurassic occurrences are also known from Kutch (Gujarat) and Rajasthan. The earliest Late Triassic dinosaur fauna of India comes from the rhynchosaur–dominated Lower Maleri Formation of Carnian age. Known from fragmentary and isolated specimens, the Late Triassic dinosaur fauna is currently represented by the sole species Alwalkeria maleriensis, which is possibly a basal saurischian with uncertain relationships. A slightly younger dinosaur fauna from the archosaur–dominated Upper Maleri Formation of late Norian–earliest Rhaetian age consists of a more diverse assemblage including the two named basal sauropodomorphs (Nambalia roychowdhurii and Jaklapallisaurus asymmetrica). In contrast to the Late Triassic, the Early Jurassic record of Indian dinosaurs described from the Upper Dharmaram and Lower Kota formations of P–G Valley, is far more abundant, diverse and based on more nearly complete material that is currently referred to four named taxa of stem sauropodomorphs or basal sauropods (Lamplughsaura dharmaramensis, Pradhania gracilis, Kotasaurus yamanpalliensis, Barapasaurus tagorei) plus an ornithischian (Ankylosauria). Kotasaurus, one of the earliest known sauropods, is more primitive than Barapasaurus and shared numerous plesiomorphic characteristics with prosauropods.  Together, the Late Triassic and Early Jurassic sauropods dinosaurs of India document the early radiation of this group. Amongst the other important records of Jurassic dinosaurs in India is the oldest known camarasauromorph sauropod whose identification is based on a metacarpal, a first pedal paw and a fibula from the Middle Jurassic (Bajocian) strata of Khadir Island, Kutch. Fragmentary postcranial skeletal material of an unidentified Middle Jurassic dinosaurs is also known from Kuar Bet (Patcham Island) in the Rann of Kutch and the Jumara area of Kutch Mainland.Post–Gondwana, the Late Cretaceous dinosaurs of India occur in a different geodynamic setting in which the Indian Plate, as traditionally considered, was a northward drifting island continent in the middle of the Indian Ocean. Apart from the solitary record of a Cenomanian–Turonian sauropod from Nimar Sandstone, Cretaceous dinosaurs from India are documented mainly by skeletal remains and eggs/eggshells from the Maastrichtian infratrappean (=Lameta Formation) and intertrappean deposits in the Deccan Volcanic Province of eastern, western and central peninsular India, and from broadly coeval Kallamedu Formation of Cauvery Basin, southern India. Skeletal remains of the Lameta dinosaurs belong to two major groups, titanosaur sauropods and abelisaurid theropods, plus a possible ankylosaur, whereas the Cauvery records include fragmentary titanosaur bones and a solitary tooth of a troodontid theropod. Apart from bones and teeth, a number of dinosaur egg–bearing nesting sites are also known to occur in the Lameta Formation of east–central and western India, extending for more than 1,000 km across the states of Madhya Pradesh, Gujarat and Maharashtra. Close phylogenetic relations of the Lameta titanosaurs and theropods with corresponding taxa from the Maastrichtian of Madagascar (Vahiny, Majungasaurus) and the rare occurrence of Laurasian elements such as a troodontid, pose interesting palaeobiogeographic problems in the context of India’s supposed oceanic isolation, especially after its separation from Madagascar at ~ 88 Ma.