Who would have thought that a pea-sized fossil could reveal the story of ancient plant migration? Well, stay tuned.
Researchers at Pennsylvania State University have used – you guessed it – giant fossil seeds to unlock evolutionary mysteries about the rainforests of Southeast Asia and Australia.
Revealing the identity of the mystery bean
These aren't just seeds: they're ancient fossils the size of limes, making them the largest seeds ever found in the fossil record.
Scientists discovered that the fossils belong to the modern-day Castanospermum, an extinct genus of legume closely related to the black bean plant, which is now found only in the coastal rainforests of northern Australia and nearby islands.
“Some of these fossil features are common to legumes in general, but other than Castanospermum, there is no other group of legumes, fossil or modern, that has a combination of features that match the fossils so well,” said Edward Spagnuolo, a doctoral student in Penn State's Department of Geosciences and lead author of the study.
Discoveries about plant migration
The trail of discovery began on the Indonesian island of Borneo, where a team including Peter Wilf, a professor of geological sciences at Pennsylvania State University, unearthed fossils dating back to the Eocene epoch, 34 to 40 million years ago.
These fossils suggest that ancestors of the black bean plant migrated from Asia to Australia during a plate collision that facilitated the exchange of plant and animal species between continents.
Giant bean fossils of the newly discovered extinct legume Jantungspermum gunnellii were discovered on the Indonesian island of Borneo. Courtesy of Edward Spagnuolo
“These fossil seeds suggest that an ancient relative of the linden tree migrated from Southeast Asia to Australia during a tectonic collision and then became extinct in Asia,” Spagnuolo added.
This groundbreaking discovery provides the first macrofossil evidence of a plant lineage migrating from Asia to Australia following a tectonic shift between the two regions.
Additionally, these seeds are the oldest definite legume fossils found in the Malay Archipelago and the first fossil record of a plant related to the black bean plant.
Overcoming the challenges of fossil collecting
Digging for fossils in this area is an incredible feat. “It's really hard to collect fossils in this area,” Spagnuolo said.
“Most of the Earth's surface rocks have been destroyed by tropical rains or covered by vegetation, farmland or buildings, so there are few places to look for fossils other than in outcrops in mines and quarries.”
However, the team found a unique partner in Indonesian paleontologists at the Bandung Institute of Technology on the island of Java, who made this research possible.
A treasure trove of discoveries
An international team of researchers collected the fossils from a coal mine in South Kalimantan, Indonesia's part of Borneo, in 2014.
Their collection includes three large beans, pollen samples, about 40 leaves, bird tracks, marine invertebrate imprints, and other fascinating fossils, including a turtle fossil.
These seeds, some of the largest in the fossil record, can reach up to three feet in length and may have grown in pods housing up to five seeds.
After field work, the fossil was loaned to Pennsylvania State University, where, through CT scan imaging and taxonomic analysis, Spagnolo and Wilf found that the fossil most closely resembled a previously unidentified modern species of Castanospermum.
The seeds have been named Jantungspermum gunnellii in honor of the late vertebrate paleontologist Greg Gunnellii, formerly of the Duke University Lemur Center.
The story of legumes and plant migration
The legumes are a diverse family of flowering plants, currently numbering around 20,000 species, including many large tropical trees.
However, despite their abundance in modern ecosystems, these seeds are the only legume fossils identified from pre-Neogene periods in the humid tropics of Southeast Asia, dating back 2.6 to 23 million years ago.
“The tropics are the most diverse biome on Earth,” says Wilf, “and despite the rapidly increasing risk of extinction and the loss of vast areas to deforestation each year, the fossil record tells us very little about how tropical ecosystems evolved, especially in Asia.”
These discoveries filled an important gap in the fossil record and confirmed the presence of legumes in Southeast Asia.
“Our study highlights the overlooked paleobotanical potential of this region and the need for more fossil samples in the Malay Archipelago,” Spagnuolo stressed.
The full study was published in the International Journal of Plant Sciences.
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