Discovery of mulberry fossil suggests northwest India had a humid climate 56 million years ago

Scientists have unearthed the first fossil record of mulberry in Asia at the Gurha lignite mines near Bikaner in western Rajasthan, a find which they say points to the existence of a hot and humid climate in the north -western India 56 million years ago.

Although the mulberry (More genus) is now widely cultivated in India and Asia, no fossils of the tree have yet been found on the continent, according to a team of scientists from HBN Garhwal University in Uttarakhand, the University of Calcutta and Sidho Kanho Birsa University at Purulia in West Bengal. They report in the newspaper Review of Paleobotany and Palynology that the presence of the fossil suggests that the mulberry was an important component of tropical and subtropical evergreen forests growing in a hot and humid climate of the region during the Eocene period, i.e. 56 to 39 million years ago years.

According to them, the mulberry then declined from the region, which is now dry with desert vegetation, probably due to the drastic climate and latitudinal change of the region, related to the formation of the Himalayas and the seasonality of rainfall since the Eocene.

Most mulberry trees are now found in the “Old World tropics”, particularly in Asia and the Indo-Pacific islands.

Mahasin Ali Khan, assistant professor in the department of botany at Sidho-Kanho-Birsha University and one of the authors of the paper, said mongabay india that “to date, no fossil More has been reported in Asia.

ancient climate

This lack of fossil evidence limits scientists’ understanding of the diversification and evolution of More in Asia, says Khan. Remains of leaves and fruits of More have only been reported from the previous Paleocene epoch dating to 66–56 million years ago; and the last 33–23 million year old Oligocene sediments of the United States. Hence the discovery by the Indian team of More the early Eocene leaf remains of India “are remarkable” and constitute the first recognition of this genus of mulberry from the Cenozoic (66 million years ago to the present) sediments of Asia,” says their report.

Previous fossil studies also indicate a hot and humid tropical to subtropical climate in western Rajasthan, in contrast to the current dry desert conditions, the report said.

The equatorial position of the Indian subcontinent during the Eocene period was “ideal” for the growth of tropical evergreen forests, while fossils recovered from the Eocene period in lignite mines in western Rajasthan also indicate the presence of tropical rainforests in western India, the report says.

Morus alba or white mulberry. Photo credit: Lazaregagnidze/ Wikimedia Commons (CC BY-SA 4.0)

In India, the genus is no longer found in western Rajasthan where the fossil was discovered, and the authors possibly attribute its extinction from the region to “drastic climate change” as well as latitudinal movement in the region. as a result of the collision of the Indian and Eurasian Plates, uplift of the Himalayas and the Tibetan Plateau, and the evolution, strengthening and long-term fluctuations of monsoon conditions.

The scientists hope the new study “provides a launch pad” for further detailed studies of the newly collected materials, providing a clear picture of their implications.

Eocene plant fossils

Scientists are particularly interested in the Eocene period, characterized by warm temperatures, for studies of the evolution, diversity and dispersal within and between continents of modern plant and vertebrate species. While the authors report that the discovery of the fossil in the region “provides unambiguous evidence” that mulberry trees were well adapted to the climatic conditions of the Eocene period, not all paleobotanists agree.

Rakesh Mehrotra, president of the Paleobotanical Society and scientist at the Birbal Sahni Institute of Paleosciences, Lucknow, says he disagrees with the study’s findings as mulberry “is a temperate genus that the found naturally in central China and cultivated in many countries”. countries, including India.

“Its presence in the early Eocene of India is doubtful because the fossil flora of this period was typically tropical in nature,” Mehrotra said. Fossils recorded from the early Eocene of western India, including the Gurha mine, support his theory, Mehrotra added.

He points out that the lower Eocene is characterized by a warmer phase, even at high latitudes. Carbon dioxide levels were also higher, ranging from 1,000 parts per million to 2,000 parts per million due to increased volcanic activity.

In 2018, Mehrotra and his colleagues published in Review of Paleobotany and Palynology that “the climatic dynamics of the Indian subcontinent and the biotic exchanges between neighboring continents can be traced by studying the Eocene fossil assemblages which are well preserved in rock records”. The fossil record of Early Eocene sites is important for its potential contribution to our understanding of climate-biota interactions, he said.

Equatorial forest

In the western part of the Indian subcontinent, large deposits of lignite are known in Gujarat (Kutch and Cambay basins) and in Rajasthan (Barmer and Bikaner-Nagaur basins). Based on the analysis of the closest living relatives of plant and animal remains in the lignite deposits of these regions, “it was concluded that a highly diverse tropical evergreen forest was present in most basins of western India,” Mehrotra said.

The equatorial position of the Indian subcontinent during the early Eocene also supports this theory, Mehrotra said.

The lignite mines of Gujarat and Rajasthan have proven to be a rich treasure trove of Eocene fossils, indicating the existence of dense tropical forests in today’s arid region and equatorial climatic conditions in the subcontinent. Indian 55 million years ago.

In 2019, Mehrotra’s team reported a rare fossil record of a fruit of Mallotus mollisimus, a plant of the euphorbia family and whose fruits are too soft to be kept, from the Gurha mines of Rajasthan. The fossil indicates the existence of rainforests in the area and the origin of the plant from Gondwanaland.

Similarly, a fossil wood found in the early Eocene Vastan lignite mine near Surat district in Gujarat, by Mehrotra’s team, shows a strong resemblance to the modern genus Chisocheton of the known mahogany family. under the name of Meliaceae. These plant fossils are the best source for reconstructing the past environment of any region, and the locality probably had a “luxurious and highly diverse evergreen rainforest” in contrast to the thorny rainforest of present-day 2018 report by Mehrotra and colleagues in paleoworld said.

“This highly diverse Early Eocene equatorial forest, which once covered a significant portion of the Indian subcontinent, is now restricted to fringes known as the Western Ghats in southern India, attesting to climate change” , he added.

This article first appeared on Mongabay.