The historic biogeography of India: isolation or contact?
Geophysical maps depicting continental movement have consistently shown India, as it moved northward, to be located far out in the Tethys Sea. India split off from the African east coast about 148 m.y.a. From that time onward, according to almost all geophysical accounts, India was isolated from all of other continents until the early Miocene when it made contact with Eurasia. But the biological data, both fossil and Recent, indicate that this concept cannot be correct. If India had really existed as an isolated, oceanic continent for about 100 m.y., it should have developed a peculiar biota with many endemic genera and families in its terrestrial and shallow marine habitats. But there are virtually no remains of organisms indicating that India was isolated for any substantial time (millions of years). Instead, we find that almost all Indian taxa were possessed in common with other continents. As time went on, the northern relationships became stronger and the southern ones weaker. Most of the recent geophysical accounts show India not making contact with Eurasia until the early Miocene, but fossil materials show that this event must have taken place by the early Eocene. It has been postulated that, as India moved northward, it created a biogeographic barrier that separated marine fish populations and resulted in the east-west provinces that are now apparent in the Indian Ocean. At the same time, the barrier effect was supposed to have resulted in the formation of sister species that are now located far apart. Information currently available indicates that most living, tropical marine species are probably not over 3 m.y. old. Consequently, the northward movement of India, which took place primarily between 148 and 50 m.y.a., could have no bearing on the relationships of modern species.