Guwahati: A single fossil leaf buried for nearly 24 million years in Assam’s Makum Coalfield has given scientists a new clue to why a plant lineage found today only in the peat swamps of Southeast Asia disappeared from the Indian subcontinent.
The fossil, discovered in the late Oligocene deposits of the Makum Coalfield in Tinsukia district, has been identified as a new fossil species, Tetrameristophyllum makumensis. Researchers say it closely resembles the living Tetramerista glabra, a plant now restricted to freshwater peat-swamp forests of western Malesia.
The discovery, reported in the 2026 Geological Journal, pushes the known history of the Tetrameristaceae family much farther west than its present-day distribution and suggests that the separation of these plants from the Indian subcontinent happened sometime after the late Oligocene, roughly 24–23 million years ago.
For Harshita Bhatia, lead author of the study with Gaurav Srivastava of the Birbal Sahni Institute of Palaeosciences, the fossil is more than an evolutionary curiosity. It offers a window into a much older Assam—and a warning about what happens when specialised habitats disappear.
“Our new study provides a glimpse into such a story from the late Oligocene, around 24–23 million years ago. We discovered a fossil leaf from the Makum Coalfield in Assam that represents a Tetramerista-like plant. Today, Tetramerista is restricted to freshwater peat-swamp forests of Southeast Asia. Our findings show that this plant group once occurred much farther west, in the Indian subcontinent, when suitable wetland habitats were present here.”
Assam once had the habitat the plant needed
The fossil came from the Tikak Parbat Formation at Tirap colliery in the Makum Coalfield. The researchers date it to the late Oligocene, when the area was part of a landscape of wetlands, forests, river systems and deltaic environments. The geological record indicates a warm and humid setting with high rainfall.
The fossil leaf itself is not an exact modern Tetramerista. The researchers have therefore placed it in a new genus and species, Tetrameristophyllum makumensis, based on its distinctive combination of leaf characteristics and its close morphological affinity with Tetramerista glabra.
The important point is where the plant was found.
Today, Tetramerista survives in permanently wet freshwater peat-swamp forests of western Malesia. Its closest relatives occupy highly specialised tropical habitats elsewhere: Pelliciera occurs in Neotropical mangroves, while Pentamerista is found in lowland savannas of northern South America.
The fossil shows that the geographical gap separating these plants today was not always there.
Climate alone does not explain the disappearance
The researchers tested a question that goes to the heart of the discovery: did changing climate drive Tetramerista out of India?
Their answer is: probably not by itself.
The team compared 18 climatic variables across 151 modern localities of the three living genera. The analysis found substantial overlap in their broad climatic conditions. In other words, the plants may be geographically separated today, but they are not necessarily separated by radically different climates.
Bhatia said:
“Interestingly, our climate analysis shows that living members of this plant family occupy broadly similar climatic conditions. This suggests that climate alone may not explain their present-day geographic separation. Instead, the loss and reorganisation of specialised wetland habitats may have played an important role in shaping where these plants could survive.”
That shifts the focus from climate to habitat.
The study suggests that changes in hydrology and landscape associated with tectonic and geomorphological changes, including the India-Asia convergence and the development of the Himalayan and Asian monsoon systems, may have altered the freshwater swamp environments on which Tetramerista depended. Increasing rainfall seasonality could also have reduced the stability of permanently waterlogged peat-swamp habitats.
The fossil therefore points to a familiar ecological story—but one unfolding on a geological timescale.
When the wetland disappeared, so did the plant
The researchers argue that the disappearance of Tetramerista from India was most plausibly linked to the loss and reorganisation of freshwater peat-swamp forests, rather than climate change alone.
Bhatia puts the lesson more directly:
“The fossil record therefore offers a striking perspective on a process that remains relevant today: when specialised habitats disappear, the species that depend on them may disappear from those regions as well.”
That conclusion gives the Makum fossil a significance beyond palaeobotany.
Freshwater peat-swamp forests remain among the most threatened ecosystems in Southeast Asia. The paper notes that more than half have been lost or severely degraded through logging, drainage and conversion to plantations. Tetramerista glabra, which survives in these habitats today, is therefore still tied to an ecosystem under pressure.
The Assam fossil offers a long-term record of what habitat loss can mean for a specialised plant: not necessarily immediate extinction everywhere, but regional disappearance followed by a striking geographic gap that can persist for millions of years.
A fossil leaf with a modern message
The Makum discovery also adds an unusual dimension to Assam’s fossil record.
The ancient landscape was not simply a tropical forest. Geological evidence points to a mosaic of freshwater, terrestrial and coastal environments existing close together. The researchers suggest that such environmental diversity could have supported plants with highly specialised ecological requirements.
Over millions of years, however, the landscape changed.
The plant survived elsewhere where suitable peat-swamp habitat persisted. In India, the habitat it depended on appears to have disappeared or been reorganised.
That makes the small fossil leaf from a coalfield in Upper Assam an unlikely messenger from deep time.
Its message is not simply that a plant once grew here.
It is that species can outlive dramatic changes in climate—but not necessarily the disappearance of the particular habitat to which they are adapted.
And that is a lesson that, the researchers say, remains highly relevant today.