Published in The Hindu on November 16, 2010
Drifting at a rate of 20 cm a year for 100 million years after splitting from Gondwana, the Indian subcontinent was separated geographically from the rest of the landmass till it collided with Asia 50 million years ago. Any landmass that is geographically isolated for millions of years tends to produce species that are unique and endemic to that region.However, an online paper published recently in the Proceedings of the National Academy of Sciences describes a surprising find. Despite isolation for millions of years, the Indian subcontinent had only a limited number of endemic species 50 million years ago. The 150 kg amber deposit found in the Cambay Shale Formation at Vastan village near Surat has excellently preserved specimens of nearly 100 new species of arthopods, including social insects such as ants, bees, and termites. These specimens show evolutionary relatedness to species found in Asia, Europe, Australia, and South America. Such relatedness is possible only if the Indian paleospecies had mixed with their counterparts in other countries.
If a land bridge link between the drifting Indian landmass and Africa and Asia was once proposed to explain the mammal movement, insects preserved in the amber strengthen that argument. A chain of islands or any such land connection must have existed between India and Asia before their collision. The alternative explanation of a much earlier collision can be ruled out; there is overwhelming geological evidence that India collided with Asia 50 million to 55 million years ago. The lignite deposits of Cambay, apart from preserving the amber, provide insight into the paleoenvironment of the region. The amber secreted by dipterocarpaceae — a family of hardwood trees that accounts for nearly 80 per cent of tropical canopy in South-East Asia — and fossil remains of this family provide unequivocal evidence of broad-leaved trees typical of tropical forests. Once believed to have developed 5 million-24 million years ago (Miocene) in South-East Asia, the fossil remains and amber deposits from dipterocarpaceae push back the tropical forest record to 38 million-54 million years ago (Eocene). It should come as no surprise if the Cambay Shale Formation, a fossil museum of Eocene age, overturns other notions and helps set many a record straight.