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Mangroves on the Deccan

~ by Prachi Thatte (WWF-India)

Home to several species of mammals and birds, the Deccan plateau has a mix of dry and moist deciduous forests. These forests are adapted to moderate rainfall during monsoon and a long dry season characteristic of central India. But this has not always been the case. The geological and climatic history of earth is marked by many periods of changes and even flips. A recent study adds to the evidence that the Deccan plateau once supported dense rainforests and a seaway that passed through the heart of central India.

During the age of dinosaurs, around 200 million years ago, South America, Africa, India, Australia and Antarctica together formed a supercontinent, the Gondwana. Due to plate tectonic movements, the Indian subcontinent gradually separated from rest of the land masses, started moving north and eventually collided with Eurasia, forming the mighty Himalayas. Before the collision, the subcontinent went through a long isolated journey along different latitudes and experienced different climates. This story of movement and dynamic climate is stored in the fossils found deep under the surface of the soil.

The breakup of Gondwana, 200 million years ago, and motion of continents to their present-day positions. Source: Bones, scales, leaves and even pollen and cysts get preserved between sedimentary deposits creating a rich historical library over years. Fossils stored at different depths and the associated soil/rock can give a glimpse of the geological and biological history of a region. A recent study, carried out by scientists from Birbal Sahni Institute of Palaeosciences (Lucknow) and Indian Institute of Technology (Roorkee), looked at such a piece of history from Ashti village in Yeotmal district of Maharashtra. The researchers bored an 80 meters deep narrow shaft and excavated the soil and rock sample. Around 60 m below the ground surface they found fossils- pollen, spores and cysts around 60-70 million years old.

These fossils were found below the layer of lava that came from the Deccan volcanic flow. So the fossils belong to a time before the volcanic event that happened around 66 million years ago. The fossils were identified as spores from different species of aquatic ferns and pollen from a couple of species of palm. The authors also discovered two new species of pollen- Dipterocarpuspollenites cretacea and Retiacolpites pigafettaensis. Although these species with jawbreaker names are now extinct, the existing relatives of these are found in low-lying rain forests and thrive in highly humid conditions. This suggests that their extinct relatives might also have preferred similar conditions. The authors also found high abundance of Nypa palm pollen. Nypa is a coastal palm that requires highly humid conditions and is currently only found in Sundarbans in India. Based on all this fossil evidence, the authors conclude that, before the Deccan volcanism, while the Indian subcontinent was still floating, the area around Yavatmal was a low-lying coastal area with high humidity due to persistent rainfall.

Representative light microscopic photographs of Nypa pollen (scale bars= 10 uM).

During the period to which these fossils belong, the sea level was ~100m higher than today. Parts of several continents were flooded with marine water and some also had seaways cutting across them. Paleontologist Ashok Sahni, known for his work on dinosaur fossils and nesting sites in central India, had proposed the presence of such a seaway, a trans-Deccan strait, in central India. In his 1983 research paper, he had suggested invasion of marine water into central India through the Narmada rift zone in the west and the Godavari rift zone in the east.

Post 1983, several researchers found fossils of mangrove plants and microscopic marine organisms in the central and western parts of central India. In the present study, authors found a large number of Nypa palm pollen, a brackish mangrove palm, along with cysts of marine planktons that proliferate in low-saline conditions. These fossils, along with mangrove fern and plant fossils found in Chhindwara in Madhya Pradesh, Nawargaon in Wardha basin, Maharashtra and Rajahmundri close to Andhra Pradesh coast, as a part of other studies, confirm the invasion of marine water along the Godavari rift zone, supporting the possibility of a trans-Deccan strait.

The fossils found in this study contribute towards understanding the climatic and geological changes the Indian sub-continent went through in the past, during its long solitary march after splitting from Gondwana. During ~100 million years of drifting, the sub-continent experienced changing climate and evolution of several species of plants and animals, some of which got preserved as fossils. From the first dinosaur fossil found in Jabalpur in 1828, the subcontinent has yielded a stream of important finds, from primitive plants to large reptiles. These have helped in unveiling the dynamic past environments and landscapes. Such studies provide crucial evidence to understand the earth’s evolution and the periods when it underwent significant climate change. For an interesting interactive website that shows how the earth and the continents looked along with fossil evidences across geologic time go here. Original paper: Vandana Prasad, Anjum Farooqui, Srikanta Murthy, Omprakash S. Sarate and Sunil Bajpai (2018) Palynological assemblage from the Deccan Volcanic Province, central India: Insights into early history of angiosperms and the terminal Cretaceous paleogeography of peninsular India. Cretaceous Research. Volume 86, Pages 186-198.

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