India not the 'Garden of Eden' for modern mammals
doi:10.1038/nindia.2011.141 Published online 29 September 2011
The mystery of India's pre-historic mammals from the Cretaceous period (about 65 million years ago) has been partly unraveled by researchers. They have found new evidence in existing fossil records to refute the theory that these mammals were the ancestors of our modern placental mammals such as primates.
The team of researchers from University College London, University of Delhi, City University of New York , Stony Brook University, Indira Gandhi National Open University, Museum National d'Histoire Naturelle of Paris and the American Museum of Natural History, New York have written off the long held view that India was a 'Garden of Eden' for modern placental mammals.
Across the world, palaeontologists have always been interested in these pre-historic mammals from India. It is widely accepted that India broke off from the southern supercontinent of Gondwanaland (South America, Africa, Antarctica, Australia, New Zealand, India and Madagascar) 120 million years ago, drifted northwards as an island for about 50 million years before its collision with Asia. By the time India crashed into Asia the dinosaurs had vanished and mammals had just begun to enjoy their new dominance on Earth.
One of the hot debates in mammalian evolution is how these old mammals were related to the ones that evolved after the demise of dinosaurs and ultimately gave rise to as many as 18 modern orders, especially placental mammals such as humans, apes, elephants, rodents, cattle and whales.
Based on the fossil record, palaeontologists suggested that only a few groups of early mammals survived the mass extinction at the Cretaceous-Tertiary boundary (KTB, 65 million years ago) that decimated dinosaurs and many other animals. An explosive radiation of modern mammals followed at the beginning of the Tertiary period.
However, based on DNA of modern mammals, molecular biologists argued for a more ancient origin of many modern lineages of placental mammals in the late Cretaceous. This implies that ancestors of many modern mammalian orders lived alongside dinosaurs and several of them survived the KTB mass extinction event.
Further, due to the timing of India's collision with Asia and since it was isolated from other continents for a long period, some researchers called it the 'Garden of Eden' where many placental mammalian groups evolved and later dispersed into Asia following the collision.
"Another important reason why the Cretaceous mammalian fossil record of India attracted widespread attention from the scientific community is that it is the only landmass of the former Gondwanaland that yielded definitive eutherian mammals, the group which gave rise to placental mammals," Guntupalli V. R. Prasad, one of the key researchers and professor in the Department of Geology, Centre for Advanced Studies, University of Delhi told Nature India.
These early eutherian mammals of India, predominantly represented by the genus Deccanolestes (meaning 'the southern beast'), have been assigned to an obscure and enigmatic tree-dwelling adapisoriculids, a group also known from the early Tertiary of Europe and Africa. The morphology of their foot bones resembled the early placental group Euarchonta (represented by tree shrews, flying lemurs, and primates). This similarity led researchers to suggest that adapisoriculid mammals are euarchontans.
"The assignment of Indian Cretaceous mammals to euarchontans accords well with the hypothesis that modern placental mammals, such as primates, diversified before the Tertiary period," Prasad says.
Prasad and his colleagues made use of dental morphology, foot bone morphology and an extensive database covering modern placental mammals and their ancestral Cretaceous forms to show that the Indian Cretaceous eutherian mammals did belong to the adapisoriculid group of mammals. However, they do not represent an ancestral lineage for modern placental mammals.
"This research also indicated that adapisoriculid mammals represent a new group of mammals that split from other mammals at least 30 million years before the end of Cretaceous," he adds.
The most important question that the find poses is how these mammals reached Europe and Africa across oceans when India was physically isolated from all the continents in the Cretaceous.
"We anticipate many more surprising fossil discoveries from the former Gondwanaland, including the missing fossil history of this new group of southern mammals," Prasad says.
- Goswami, A. et al. A Radiation of arboreal basal eutherian mammals beginning in the Late Cretaceous of India. P. Natl. Acad. Sci. USA doi: 10.1073/pnas.1108723108 (2011)