Ancient fossil seeds the size of limes, some of the largest in the fossil record, could provide new insights into the evolution of Southeast Asian and Australian rainforests. Penn State researchers identified the fossils as belonging to an extinct genus of legumes closely related to modern Castanospermum, the black bean tree, now found only in the coastal rainforests of northern Australia and nearby islands.
Tectonic Shifts Linked to Early Plant Migration
In a report published in the International Journal of Plant Sciences, an international team of researchers, including paleontologists from Indonesia, Canada, the United Kingdom, and the United States, explained that the fossils found in Kalimantan, Indonesia, date back to the Eocene Epoch, about 34 to 40 million years ago.
According to phys.org, these findings suggest that the ancestors of the black bean tree migrated from Asia to Australia due to tectonic plate collisions that allowed the exchange of plant and animal species between continents. This discovery is also the first macrofossil evidence of plant migration from Asia to Australia following the plate collision, the oldest legume found in the Malay Archipelago, and the first plant fossil related to the black bean tree.
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Challenging Prevailing Migration Theories
Lead author Edward Spagnuolo from Penn State's Department of Geosciences explained that the fossil seeds indicate that ancient relatives of Castanospermum migrated to Australia from Southeast Asia during tectonic collisions and later went extinct in Asia. This hypothesis contrasts with most macrofossil evidence, which typically suggests migration in the opposite direction.
Scientists noted that the lack of evidence for migration from Asia to Australia is partly due to the poor fossil record in the Malay Archipelago. Spagnuolo added that collecting fossils in this region is extremely challenging because surface rocks are often damaged by tropical rain or covered by vegetation and development.
Collaborative Effort
The research team also revealed that paleontological infrastructure in the region is very limited. They were fortunate to have a partnership with Indonesian paleontologists at the Bandung Institute of Technology, who supported this research.
In 2014, the international team, including Peter Wilf from Penn State, collected fossils from a coal mine in South Kalimantan. The collection included large seeds, pollen samples, leaves, and various fossilized bird tracks and turtle remains.
These seeds are among the largest in the fossil record and likely grew in pods that could reach up to 3 feet in length. After being collected, the fossils were loaned to Penn State for CT scanning and further analysis, which revealed similarities with modern Castanospermum.
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Naming and Significance of the Discovery
The fossil was named Jantungspermum gunnellii, with the genus name referring to the heart-shaped form of the fossil and the species name honoring Gregg Gunnell, a vertebrate paleontologist who led the field expedition.
Legumes, with around 20,000 species today, are an incredibly diverse family of flowering plants. However, this fossil is the only identified legume from Southeast Asia dating back to before the Neogene period.
Wilf added that although tropical regions are the most diverse biomes, the fossil record of tropical ecosystem evolution, especially in Asia, remains very sparse. This discovery underscores the need for more fossil sampling in the Malay Archipelago to fill significant gaps in the global fossil record.
