Speak of fossils, and dinosaurs first come to mind. A quaint fossilised leaf is one of the most recent finds throwing light on India's past. The leaf fossil is the first of Dioscorea yams from Asia and hints at a Gondwanan origin to these plants, claim scientists.

When scientists R. C. Mehrotra and Anumeha Shukla from Lucknow's Birbal Sahni Institute of Palaeosciences were leading a dig in the Eocene-era (38-56 million years ago) Gurha lignite mine in Bikaner in western Rajasthan, they obtained two well-preserved fossils of large leaves. “They are about 16 cm long,” wrote co-author Dr. Shukla in an email.

Referring to herbarium sheets available at Dehradun's Forest Research Institute, the team identified it as a species of Dioscorea, a kind of yam that grows as a herbaceous vine in the humid tropics of India and other countries. They also used the morphological features of the leaves — venation and leaf shape — to rule out other plants that look very similar to it. When they compared it to the other Dioscorea fossils obtained from Europe, Africa and America, they found it to be very distinct.

Tropical forests

The team named their new find Dioscorea eocenicus: the first ever Dioscorea fossil recorded from Asia. Currently, species of Dioscorea in India are found in the humid, tropical forests of the country. Based on this, the team infer that such tropical forests must have flourished in this part of Rajasthan during early Eocene. Other fossil plants observed in the mine also suggest this historical climate in the area, which is now dry and consists of desert vegetation.

So what caused such a drastic change in climate? As the Indian subcontinent broke away from the supercontinent Gondwanaland many millions of years ago and drifted towards the Equator, the resulting tropical weather created lush tropical forests here. As the landmass moved further north and away from the equator, dry vegetation replaced these forests, write the authors. Further, based on the location of other Dioscorea fossils across the world (from continents that used to be part of Gondwanaland) and the current pan-tropical distribution of Dioscorea species across the world, the scientists suggest in their study published in Review of Palaeobotany and Palynology that Dioscorea plants could be of Gondwanan origin. However, more fossil records would be required to confirm this, added Dr. Shukla.