Insular India

Across the latter stages of the Cretaceous and most of the Paleocene, the Indian subcontinent remained an isolated landmass as it drifted across the Tethys Ocean, forming the Indian Ocean. The process of India's separation from Madagascar first began 88 million years ago, but complete isolation only occurred towards the end of the Maastrichtian, a process that has been suggested to be the creation of the Deccan Traps. Soon after, the land mass moved northward rather quickly, until contact with Asia was established 55 million years ago. Even then, both landmasses didn't become fully united until around 35 million years ago,[1][2] and periods of isolation occurred as recently as 24 million years ago.[3]

Thus, for a period of 53 million years India has retained a degree of isolation, 11 of which it has been a complete island continent. This has allowed for its local biota to follow the typical patterns seen in islands and diversify in unique ways, much like in modern Madagascar, its sister landmass. Faunal interchanges with other landmasses, like Africa and Europe (then an archipelago of islands across the Tethys) have occurred during this period, and a considerable Asian influence can already been seen long before contact was made. This rendered India rather peculiar as not just an isolated continent but also a "stepping stone" in the dispersal of many animal and plant clades across Africa, Europe, Madagascar, Asia and possibly even Oceania. Still, several "archaic" clades managed to survive.

By the time full contact was established, a large percentage of India's indigenous fauna was outcompeted by Eurasian species. However, several groups like neobatrachians[4] and lagomorphs[5] have become widespread, becoming dominant clades across the world's faunas. The islands of the Seychelles still retain an indigenous herpetofauna, presumably an echo of the amphibian and reptile species seen in India as an island.

Cretaceous fauna

The Cretaceous fauna of India is well attested in both Coniacian and Maastrichtian aged sites such as the Lameta Formation. Generally speaking, the local dinosaurian and crocodilian fauna is almost identical to that of Madagascar, with clades like abelisaurids, titanosaurs, noasaurids and notosuchians being well represented here. A possible deviation is the presence of stegosaurs, the last remaining members of this lineage;[6] if these aren't misidentified remains of herbivorous notosuchians and sauropods, then these relics would be the only indigenous ornithischians in the entire Indo-Malagasy landmass. Another possible deviation is the presence of a troodontid,[7] a lineage more typically associated with Laurasia and thus possibly indicating interchange with Europe or even mainland Asia, but these remains are controversial and could belong either other theropods or notosuchians.

The mammalian fauna of India also bears similarities with that of Madagascar, with the gondwanathere Bharattherium, one of the most common mammals, being extremely similar to the malagasy Lavanify. The most diverse mammals in the Maastrichtian of India are eutherians, a clade normally associated with northern continents and also found in Madagascar in this epoch, which combined with their ambiguous phylogenetic positions renders them extremely important in the understanding of placental evolution. Some like Deccanolestes have been variously interpreted as euarchontans,[8] adapisoriculids,[9] or stem-afrotherians,[10] though the general consensus appears to be that they are non-placental eutherians and that there are non known Cretaceous placentals.[11] Kharmerungulatum, formerly interpreted as a stem-ungulate, is now known to be a representative of Zhelestidae, a herbivorous non-placental eutherian clade.[12] Regardless of the phylogenetics of these eutherians, they almost certainly reached India and Madagascar through either Europe, Africa or mainland Asia;[13][14] later they would propagate across Gondwana as far west as Brazil.[15]

Probably the most spectacular representative of India's Cretaceous fauna is Avashishta, a late surviving haramiyid and the last known non-mammalian synapsid.[16] Non-gondwanathere multituberculates and meridiolestidans can probably also be inferred as having lived in India during this epoch, due to the former's presence in all landmasses including Madagascar[17] and the latter being the dominant mammals in other known Gondwannan sites.

The herpetofauna of India in the Cretaceous is a mosaic of indigenous groups and forms that rafted their way from Asia. Neobatrachians are an indigenous clade and locally well represented as they are in Madagascar in the form of ranids, hylids, leptodactylids, pelobatids and discoglossids, as are madtsoiid snakes like Sanajeh and possibly Indophis and iguanian lizards, while anguids are from Laurasia.[18]

Several fish taxa are known from estuarine locales; most are marine species, but there are also forms like lepisosteids, which do also occur in Africa but are otherwise rare in gondwannan landmasses.[19] Cichlids and other forms suspected of having had an Indian gondwannan origion were most likely present.

Paleocene fauna

The fossil record of the Paleocene of India, when the continent was a fully isolated landmass, is rather sketchy and thus most inferrals about its fauna are somewhat speculative. It is known for certain that Deccanolestes and Bharattherium survived the KT event,[20] though for how further long did non-placental eutherians and gondwanatheres live in India is unknown, and by the time the landmass makes contact with Asia they are most likely extinct.

During this epoch, unambiguous placental mammals make their way into India in spite of its isolation, probably by rafting like the many placental groups in Madagascar, or perhaps brief connections with Africa and Europe (the latter still an archipelago). Hyaenodonts are an endemic African clade, first showing outside of the continent in the Paleocene of India and Europe, .[21] Glires evolved in Asia, but a lineage became isolated in India, where it gave rise to the lagomorphs.[22]

For a while it was theorised that ostriches evolved in India during this epoch, under the assumption that European ratites like Palaeotis represented recent Asiatic migrations. However, the first unambiguous ostriches are now thought to have evolved in Africa, with eogruiids having occupied their ecological niche in Asia; likewise, European ratites are now thought to be among the oldest known,[23] and probably evolved independently there, being unrelated to ostriches. Still, India probably had a thriving paleognath fauna; the volant ancestors of kiwis and elephant birds presumably flew from there to Oceania and Madagascar respectively,[24] while the mysterious Hypselornis may represent an indigenous clade.

Caecillians are an indigenous gondwannan clade, but their absence in Madagascar suggests that the Asian species have descended from African species that colonized India as it drifted north.[25]

Eocene fauna

By this time India already has an extensive placental fauna (as well as metatherians like Indodelphis), but in its isolation there are still high degrees of endemism, with some clades like anthracobunids not being found elsewhere. A study on Cambaytherium suggests that Perissodactyla might have had an insular origin in India.[26] The most notorious endemic mammals are cetaceans, which are in fact restricted to the Indian Subcontinent until the evolution of the marine "protocetids". Eocene India is also rich in bat remains, including many representatives of modern groups, though its unclear if this Indian chiropteran fauna represents an adaptive radiation or simply that bat fossils elsewhere are rare.[27]

During this time ranids, lagomorphs and hyaenodonts disperse out of India, establishing their cosmopolitan ranges.

References
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