Ancient fossil beans the size of modern limes could provide new insights into the evolution of today’s diverse rainforests of Southeast Asia and Australia, say researchers at Pennsylvania State University who identified the plants.

The researchers concluded that the fossils, which are among the largest seeds in the fossil record, belong to a now-extinct legume genus that lived in Southeast Asia and was closely related to the modern Castanospermum, the so-called black bean tree, which is now found only in the coastal rainforests of northern Australia and on neighboring islands.

The study, conducted in collaboration with paleontologists from Indonesia, Canada, the United Kingdom and other parts of the United States, was recently published in International Journal of Plant Sciences.

Insights from Eocene fossils

The fossils, discovered in Indonesian Borneo, date from the Eocene, about 34 to 40 million years ago. They suggest that the ancestors of the black bean tree migrated from Asia to Australia during the tectonic plate collision that brought the land masses together and allowed an exchange of plants and animals between the continents. The collision of the Southeast Asian and Australian tectonic plates, which began about 20 million years ago and continues to this day, led to a major exchange of plants and animals. species between the land masses, the scientists said.

The finds provide the first macrofossil evidence of a plant lineage that travelled from Asia to Australia after the tectonic collision between Asia and Australia, the researchers said. The fossils are also the oldest clearly identified fossil legumes – the bean family – from the Malay Archipelago and the first fossil record ever of plants related to the black bean tree.

“These fossil seeds suggest that ancient relatives of Castanospermum migrated to Australia from Southeast Asia during the tectonic collision and later became extinct in Asia,” said Edward Spagnuolo, a doctoral student in the Department of Geosciences at Pennsylvania State University and lead author of the study.

Overcoming challenges in fossil collecting

These results challenge most of the existing direct macrofossil evidence for plant migrations, which represent lineages that reached Asia from Australia. According to the scientists, the lack of direct evidence for migration from Asia to Australia is due at least in part to a poor record of plant fossils in the Malay Archipelago, which includes the Philippines, Indonesia, East Timor, Papua New Guinea and parts of Malaysia.

“It’s really difficult to collect fossils in this part of the world,” Spagnuolo said. “Most of the surface rocks are destroyed by the heavy tropical rains or covered by vegetation, farmland and buildings, so there are hardly any places to look for fossils other than mine and quarry outcrops. There is also very little paleontological infrastructure. We are fortunate to be working with Indonesian paleontologists at the Institut Teknologi Bandung in Java who are making this work possible.”

Field discoveries and fossil analysis

An international team of researchers, including Peter Wilf, professor of geosciences at Penn State, collected the fossils in 2014 from the seams of a coal mine in South Kalimantan in Indonesian Borneo. The collection included three large beans, pollen samples and about 40 leaves. In addition to plants, the team also discovered various fossilized bird tracks, digging marks of marine life and Invertebrates and fossil turtle remains, among other fossils recently published or continuing to be studied.

The seeds are among the largest in the fossil record, apart from coconuts and some other palms. They grew in a pod that was most likely up to 3 feet long, the size of a baseball bat, and could hold up to five seeds, the scientists said.

After the fieldwork, the fossils were loaned to Pennsylvania State University, where the seeds were subjected to computed tomography. Spagnuolo and Wilf analyzed the fossil beans taxonomically, describing anatomical features useful for identification, and found that they were most similar to modern Castanospermum, for which there are no earlier fossil representatives.

“Although some of the features of these fossils are common to all legumes, there is no fossil or living legume group other than Castanospermum that has a combination of features that closely matches the fossils,” Spagnuolo said. “This gives us confidence in our identification.”

The fossil seeds were named Jantungspermum gunnellii. The genus name refers to the fossil’s heart shape—jantung means heart in Indonesian and spermum means seed in Latin. The species name is a tribute to the late Gregg Gunnell, a vertebrate paleontologist formerly of Duke University’s Lemur Center who led the field trip.

Legumes are a diverse family of flowering plants, of which there are about 20,000 species today, including many large tropical trees, the scientists said. Yet despite their abundance and diversity in modern ecosystems, these seeds are the only clear legume fossils from before the Neogene, the period between 2.6 and 23 million years ago, in the humid tropics of Southeast Asia.

“The tropics are the most diverse biome on Earth,” Wilf said. “We know very little from the fossil record about how tropical ecosystems evolved, particularly in Asia, even though the threat of extinction is rising rapidly and huge areas are lost to deforestation every year. Penn State’s paleobotany group is working on the problem in the field with colleagues in several Asian countries, and the new giant fossil beans from Borneo are a fantastic example of the potential for discovery.”

These findings confirm the presence of legumes in Southeast Asia and fill a critical gap in the fossil record, the scientists said.

“We have a large inventory of legume fossils from much of the world, but not from Southeast Asia,” said Spagnuolo. “Our work highlights the neglected paleobotanical potential of this region and the need for more fossil samples in the Malay Archipelago.”

Reference: “Giant Seeds of an Extant Australasian Legume Lineage Discovered in Eocene Borneo (South Kalimantan, Indonesia)” by Edward J. Spagnuolo, Peter Wilf, John-Paul Zonneveld, David Shaw, Aswan, Yan Rizal, Yahdi Zaim, Jonathan I. Bloch and Russell L. Ciochon, July 25, 2024, International Journal of Plant Sciences.
DOI: 10.1086/730538

The National Geographic Society, the US National Science Foundation, and Penn State supported the researchers involved in this work.