Australia and South America weren’t always so separate. At one time, many millions of years ago, these two continents were connected, along with others, in the southern supercontinent of Gondwana.
Gondwana was a warm, forested place — a perfect home for tree frogs. In fact, the supercontinent fostered the common ancestor of the Australian pelodryadid frogs and the South American phyllomedusid frogs that are still seen, stuck to leaves and branches, today. But when did this common tree frog ancestor appear, and when did it diverge into these two separate frog families?
A new study published in the Journal of Vertebrate Paleontology provides some answers. Identifying a new species of Australian tree frog from the Early Eocene around 55 million years ago, the study suggests that the Australian and South American tree frog lineages split apart at around that time, too, or even earlier than that. All in all, the study rethinks the timeline of tree frog evolution and suggests that these lineages are a lot older than traditionally thought.
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The Australian-South American Frog Separation
To be sure, the new study isn’t the first to investigate the divergence of the Australian and South American tree frogs. By following the genetic transformations of these two families over time, prior molecular studies have shown, for instance, that the pelodryadids and the phyllomedusids split apart approximately 33 million years ago.
The fossil record has supported that timing in the past, with the earliest Australian tree frog fossils previously appearing in Australia after 33 million years ago, in the Late Oligocene and the Early Miocene epochs about 26 and 23 million years ago. But the new species undermines this timeline, suggesting that the separation of Australian and South American tree frogs actually occurred around 55 million years ago at a minimum, or at least around 22 million years earlier than previously thought.
The new species, named Litoria tylerantiqua, is now the earliest identified species of Australian tree frog there is. Represented by fossilized pelvic bones from the Murgon fossil site in Queensland, Australia, the pelodryadid species is approximately 55 million years old, or about 30 million years older than any other Australian tree frog found in the fossil record.
“While molecular studies are important for understanding the evolutionary relationships of different groups of animals, these studies should be calibrated using knowledge from the fossil record,” said Roy Farman, the lead study author and a lecturer at the University of New South Wales, according to a press release. “In this case, the fossil record provides a more accurate time for separation of the southern world’s tree frogs.”
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Tree Frogs, Fossilized and Picked
Three fossilized ilia (pelvic bones) represent a new species of Australian tree frog, Litoria tylerantiqua. (Image Credit: UNSW Sydney/Roy Farman)
To confirm the species’ identification as a pelodryadid, the study authors compared the pelvises of the new species with the pelvises of pickled frogs, including Australian and South American tree frogs, from museums in Australia. “We were able to determine from the shape of the fossil ilia — one of three bones that make up each side of the pelvis — that this new Murgon species of frog is more closely related to the Australian tree frogs (pelodryadids) than the South American tree frogs (phyllomedusids),” Farman said in the release.
Though the structure of the pelvic bones from the new species was relatively simple to study, the structure of the bones from the pickled species was not.
“We had a real problem at the start of this study,” Farman added in the release, because the bones of the pickled specimens were “invisible,” being obscured by the specimens’ soft tissues.
“Museums understandably want to ensure these often unique or rare pickled specimens remain intact for molecular studies because DNA can be obtained from their soft tissues,” Farman said in the release. “This meant that instead of [skeletonizing] these specimens, we needed instead to make CT scans of them, enabling us to create 3D models of their otherwise invisible skeletons.”
The new species is named after the Australian herpetologist Michael Tyler, who was famous for studying the fossil record of Australian frogs and toads.
“It is only fitting to name Australia’s earliest tree frog in [honor] of a man who was a giant in Australian frog research,” Farman said in the release.
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Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
