Researchers at Penn State have uncovered ancient fossilized beans, comparable in size to today’s limes, which are among the largest seeds found in the fossil record. These findings could shed light on the evolution of the rich rainforests in Southeast Asia and Australia.
The research revealed that the fossils belong to an extinct genus of legumes that thrived in Southeast Asia and are closely related to the modern black bean tree, scientifically known as Castanospermum. Currently, this tree is found solely in the coastal rainforests of northern Australia and nearby islands. The research team included paleontologists from Indonesia, Canada, the United Kingdom, and various locations in the United States, and their discoveries were published in the International Journal of Plant Sciences.
The fossils, which were found in Indonesian Borneo, are believed to date back to the Eocene epoch, approximately 34 to 40 million years ago. Their study indicates that the ancestors of the black bean tree migrated from Asia to Australia due to tectonic plate collisions, which unified these landmasses and facilitated the exchange of various plant and animal species. The interaction between the Southeast Asian and Australian tectonic plates, which began around 20 million years ago and is ongoing, significantly influenced the plant and animal diversity between these regions, as reported by the researchers.
According to the researchers, this discovery provides the first macrofossil evidence indicating a plant lineage moving from Asia to Australia following the tectonic collision. The fossils represent the oldest confirmed legume fossils in the Malay Archipelago and are the first known fossil records related to the black bean tree.
“These fossil seeds indicate that the ancient predecessors of Castanospermum relocated to Australia from Southeast Asia during tectonic events and eventually became extinct in Asia,” explained Edward Spagnuolo, a doctoral student in Penn State’s Department of Geosciences and the lead author of the study.
This hypothesis contrasts with most existing macrofossil evidence of plant migrations, which predominantly showcases lineages moving from Australia to Asia. The researchers believe the scarcity of direct evidence for migrations from Asia to Australia may stem from the limited fossil record in the Malay Archipelago, which encompasses areas like the Philippines, Indonesia, East Timor, Papua New Guinea, and parts of Malaysia.
“Gathering fossils in this area is quite challenging,” Spagnuolo mentioned. “Due to heavy tropical rainfall, most surface rocks are eroded or obscured by vegetation, agricultural lands, and urban developments, leaving only mine and quarry sites as viable fossil hunting locations. The region also lacks robust paleontological infrastructures. We are fortunate to collaborate with Indonesian paleontologists at Institut Teknologi Bandung in Java, which enables this research.”
An international research group, including Peter Wilf, a professor of geosciences at Penn State, collected the fossils in 2014 from coal mine seams in South Kalimantan, Indonesian Borneo.
During their collection efforts, the team secured three large beans, several pollen specimens, and around 40 leaves. They also unearthed various fossilized bird footprints, traces of marine invertebrates, and remnants of fossilized turtles, among other organisms, some of which have recently been published or are still under analysis.
The seeds are among the largest recorded in the fossil record, excluding those from coconuts and some other palm varieties. The research indicates that they likely grew inside pods that could grow up to 3 feet long, similar to a baseball bat, accommodating as many as five seeds, according to the scientists.
Post-field research involved lending the fossils to Penn State, where CT scans were utilized to examine the seeds. Spagnuolo and Wilf analyzed the fossilized beans in terms of taxonomy, identifying anatomical features significant for classification, and found that they were closest in resemblance to modern Castanospermum, which previously had no fossil representatives.
“While some traits of these fossils are widespread across the legume family, there is no other fossil or living legume group besides Castanospermum that combines features similar to those found in the fossils,” Spagnuolo noted. “This provides us with confidence in our classification.”
These fossilized seeds have been named Jantungspermum gunnellii. The genus name is derived from the heart shape of the fossil—’jantung’ means heart in Indonesian and ‘spermum’ means seed in Latin. The species name honors the late Gregg Gunnell, a vertebrate paleontologist who previously worked at the Duke University Lemur Center and led the field excursion.
Legumes constitute a diverse array of flowering plants, with about 20,000 current species, including many sizeable tropical trees. Despite their prevalence in modern ecosystems, these seeds represent the only confirmed legume fossils from before the Neogene period (which lasted from 2.6 million to 23 million years ago) in Southeast Asia’s wet tropics.
“The tropics host the most varied biosphere on our planet,” Wilf stated. “Our understanding of how tropical ecosystems evolved—particularly in Asia—remains scant, especially as extinction threats amplify and significant deforestation occurs yearly. The Penn State paleobotany team is actively addressing this issue with partners in several Asian countries, with the new giant fossil beans from Borneo exemplifying the potential for such discoveries.”
These findings reaffirm the presence of legumes in Southeast Asia and bridge an important gap in the fossil record, the scientists affirmed.
“We possess an extensive fossil record for legumes in many parts of the world, but Southeast Asia is a notable exception,” Spagnuolo emphasized. “Our research underscores the often-overlooked potential for paleobotanical discoveries in this area and highlights the necessity for increased fossil sampling in the Malay Archipelago.”
Collaborators also included John-Paul Zonneveld, a professor at the University of Alberta, Canada; David Shaw, a stratigraphic palynologist with Biostratigraphic Associates in the UK; Aswan and Yan Rizal from Institut Teknologi Bandung, Indonesia; Yahdi Zaim from Institut Teknologi Sumatera, Indonesia; Jonathan Bloch, curator of vertebrate paleontology at the Florida Museum of Natural History; and Russell Ciochon, professor emeritus at the University of Iowa.
Support for this research was provided by the National Geographic Society, the U.S. National Science Foundation, and Penn State.
