The dry thorn forests of southeast Gujarat were rich rainforests nearly 55 million years ago. The recent discovery of a wood fossil, whose closest living relatives are rainforest trees, adds proof to this theory.
Plant fossils can help palaeobotanists reconstruct ancient vegetation. Scientists have discovered numerous such fossils from the 55 million-year-old Vastan lignite mine in Gujarat’s Surat district. In an excavation here, palaeontologists at Lucknow’s Birbal Sahni Institute of Palaeosciences, too, came across a small wood fossil.
“It was brown in colour, around 8 cm long and 5 cm wide,” says R. C. Mehrotra, one of the co-authors of the study published in Palaeoworld.
When the team observed its fine wood slivers under a microscope, they noted several minuscule wood cells and rays (pale streaks on a tree trunk that run parallel to the circular tree rings). These closely resembled the wood of plants in the mahogany family (Meliaceae). The characteristic long rays of the fossil helped narrow the search further: they were similar to those of living evergreen trees belonging to the genus Chisocheton, which are found in parts of India, China, south Asia and Australia.
The scientists realised their find was very similar to a wood fossil Chisochetonoxylon bengalensis (named for its similarity with Chisocheton tree wood) discovered in West Bengal in 1979.
However, the new fossil’s vessels (a type of plant cell) were smaller and rays arranged in fewer layers. They christened their find Chisochetonoxylon vastanensis.
Vastan has also yielded other evergreen tree fossils in the past, the closest living relatives (based on morphology) of which include Aglaia and Calophyllum trees found in India's evergreen forests. Hence, the Vastan area – where tropical thorn forests now dominate – was a “luxurious” evergreen forest around 55 million years ago, write the authors. The reason lies in continental drift: as the Indian subcontinent broke away from the supercontinent Gondwanaland and drifted near the Equator, the resulting tropical weather created lush rainforests here, they add. As the landmass moved further north and away from the equator, drier vegetation replaced these forests.
Researcher Anusree A.S. Pillai of Norway’s Oslo University, who has studied ancient vegetation changes in India from tiny plant fossils, agrees. “These studies put together do point to the existence of evergreen forests in the region during this time,” she wrote in an email to The Hindu.