Evrim Yazgin
Cosmos science journalist
Fossil beans about the size of limes have been found in Indonesian Borneo. The ancient seeds could help explain the evolution of today’s rainforests in Southeast Asia and Australia.
The fossils were found in rock layers dating to 34 to 40 million years ago during the Eocene period (56–34 million years ago). They are described in a paper published in the International Journal of Plant Sciences.
Researchers collected 47 plant fossils from the Tanjung Formation in South Kalimantan. Among the fossils were 43 fossilised leaves and 3 large seeds up to 7.2 centimetres in length.
The seeds belong to an extinct legume which the authors name Jantungspermum gunnellii. The extinct legume’s beans would have grown in pods which may have grown to nearly 1m long.
Their structure closely resembles that of Castanospermum, the Australian black bean tree, also called Moreton Bay chestnut, native to the rainforests of Queensland, northeast New South Wales, and southwest Pacific islands Vanuatu and New Caledonia.
The collision of the Australian tectonic plate (Sahul) with Southeast Asia (Sunda) occurred about 20 million years ago and had a major impact on the vegetation of both regions, the authors say.
“These fossil seeds suggest that the ancient relatives of Castanospermum migrated into Australia from Southeast Asia during the tectonic collision event and later went extinct in Asia,” explains lead author Edward Spagnuolo, from Pennsylvania State University in the US.
In contrast, most fossil evidence suggests the movement of plant species from Australia into Asia, making this legume’s legacy unusual.
However, the authors say that this may also be due to the lack of fossil evidence found in Southeast Asia.
“It’s really hard to collect fossils in this part of the world,” Spagnuolo says. “Most surface rocks are destroyed by the heavy tropical rainfall or covered by vegetation, agriculture and buildings, so there are few places to look for fossils other than mine and quarry exposures. There is also very little paleontological infrastructure. We’re fortunate to have a partnership with Indonesian palaeontologists at Institut Teknologi Bandung in Java, who make this work possible.”
“The tropics are the most diverse biome on Earth,” says Penn State professor Peter Wilf. “We know very little from the fossil record about how tropical ecosystems evolved, especially in Asia, even as extinction risks are rising quickly, and we lose vast areas every year to deforestation. The Penn State palaeobotany group is working on this problem in the field with colleagues in several Asian countries, and the new giant fossil beans from Borneo are a fantastic example of the discovery potential.”
“We have a great legume fossil record for much of the world but not Southeast Asia,” Spagnuolo adds. “Our work highlights the neglected palaeobotanical potential of this region and the need for more fossil sampling in the Malay Archipelago.”
