Indian Cretaceous Terrestrial Vertebrates: Cosmopolitanism and Endemism in a Geodynamic Plate Tectonic Framework

Abstract

The Indian stratigraphic record with well documented Mesozoic and Early Tertiary terrestrial biotas is now adequate to shed light on the geodynamic chronology of the subcontinent during the rift, drift and collision phases. The record provides one of the best examples of the response of a biota that is undergoing considerable latitudinal displacement, dispersal, origination, evolution and extinction.

This paper restricts itself to terrestrial biotic events related to the drift of India from the Middle and Late Cretaceous to the end of the Early Eocene, between approximately 90 and 50 Ma with the main focus centering on the Deccan Trap associated sedimentary sequences (DTASS). Affinities of the marine biota to that of Madagascar are known on the basis of Cretaceous ammonites but landmass contiguity is best established on the basis of large terrestrial vertebrates that require stable corridors for dispersal. In this context, the report of gigantic sauropod dinosaurs along with putative angiosperm remains in a mangrove ecosystem setting in the Nimar Formation is of great interest as similar environments have been reported from Egypt during the Cenomanian-Turonian.

For palaeobiogeographical implications, the DTASS is the most interesting and informative as this highly fossiliferous sequence has yielded several important faunal (mammals, dinosaurs and ostracodes) and floral (pollen, charophyte, and diatom) assemblages. The sudamericid gondwanathere mammals clearly have an affinity with similar forms known from Madagascar and South America implying a strong connection possibly extending to Africa. The other mammals of the Deccanolestes group are presently hard to characterize in terms of their closest relatives but have been nested with the Asian proteutherian clade. South American-Madagascan affinities are also borne out by the common presence of large carnivorous abelisaurid dinosaurs, Rajasaurus narmadensis, the most completely known Indian Maastrichtian abelisaurid which is strikingly similar to Majungatholus from the coeval Maevarano Formation of Madagascar. The distribution of sauropod titanosaurids is more cosmopolitan (South America, India and Southern Europe, and North America) and is matched by the similarity in titanosaurid oospecies wherever described. The ostracode assemblage have recently been well studied and suggest some degree of endemism at the species level, much more than was previously suspected with only a few forms of Central and East Asiatic affinity. The pollen assemblages from the sedimentary sequences associated with the Deccan Traps are dominated by Aquilapollenites, a characteristic marker for Euramerican Maastrichtian palynostratigraphy. Kurmademys and Sanchuchemys are two pleurodiran bothremyidid turtles from the Cretaceous of Tamil Nadu and the Jogeshwari Intertrappeans of Mumbai respectively, which together with other contemporary pelomedusids indicate a great biodiversity of the group. The frogs, on the other hand, need to be thoroughly re-studied in the context of their origins and affinities as recent data has shown a remarkable disjunct distribution between the recent family Sooglossidae from the Seychelles and Nasikabatrachidae from the Western Ghat regions of India. This implies a common ancestry in the late Mesozoic that predates the separation of the Seychelles from western India at about 65 Ma. Fossil snakes from this time are much generalized and probably represent basal lineages. Two lineages of freshwater fish, the Lepisosteidae and the Osteoglossidae are fairly cosmopolitan in their global distribution. The ray Igdabatis is now known from the Maastrichtian of Spain, Niger and India.

Several evolutionary originations as indicated by their oldest stratigraphic record, took place when the Indian landmass was a drifting island subcontinent. The best examples are the oldest global occurrence of Maastrichtian freshwater diatoms, and of grasses represented at least by five taxa of living subclades. The oldest and most diversified Asian record of bats occurs in the Lower Eocene (about 52 Ma) and the origination of whales and sea cows at about the same time is now a classic example of macro-evolution in a penecontemporaneous collision phase.

More work needs to be undertaken in those cases where the geophysical constraints do not match the conclusions based on palaeobiogeographic affinities. At least two factors may account for the perceived discrepancies: (a) poor sampling in intervening landmasses of coeval time; and (b) differing ecological constraints controlling the dispersal of different-sized forms and the incompleteness of the stratigraphic record.