The discovery of some new fossils belonging to the oldest known species of platypus, Obdurodon insignis, in the remote outback of South Australia has revealed these mysterious monotreme mammals used to have well-formed teeth and a stronger bite than their living relatives.
25 million years ago, the now-arid outback of South Australia was intersected by waterways that supported a rich diversity of animals, including ancient lungfish, flamingos and freshwater dolphins. These waterways were also home to the ancestors of living platypuses, which, today, are endemic to eastern Australia.
In a study published earlier this week in the journal Australian Zoologist, Flinders University palaeontologists Trevor Worthy, Gen Conway and Aaron Camens described the discovery of three new fossils belonging to O.insignis – a species only previously known from scant remains described in 1975.
"Platypuses are extremely rare in the fossil record and are often restricted to teeth, so it’s exciting to find new material and learn more about these unique mammals,” said Camens.
O.insignis differs from later platypuses by having well-formed teeth – molars and premolars – that researchers believe it kept well into its adult life. This is different to living platypuses, which are born with vestigial teeth that they soon lose as they mature, replacing them with small, horny pads.
The new material attributed to O.insignis includes a lower first molar, an upper second premolar, and a part of the pectoral girdle (the scapulocoracoid). Previously, this ancient platypus was only known from fragments of a jaw and a pelvis, and one and a half molar teeth.
"The new premolar for Obdurodon insignis shows this species also had large, pointed front teeth, which with its large robust molar teeth could easily have crushed animals with shells or robust exoskeletons, like yabbies [a type of crayfish],” said Worthy, lead author of the latest study.
The discovery of these well-formed teeth and a partial scapulocoracoid has painted a better picture of what O.insignismight have looked like.
And by comparing these fossils with an incredibly well-preserved skull from a younger but closely related species known as Obdurodon dicksoni (17-14 million-years-old), the researchers have gained the clearest understanding yet of these ancient animals.
“The other rare find was the discovery of a partial scapulocoracoid, or bone that supports the arm or front limb. This reveals a very similar forelimb structure to the modern platypus, indicating it could swim just as well as its modern descendant,” added Worthy. “These fossils show that 25 million years ago Obdurodon insignis was very similar to the modern platypus. It differs mainly by being slightly larger and having teeth.”
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For more than two decades, the Flinders University team has scoured the remote outback of South Australia for fossils. At the same site the new material belonging to O.insignis was found, and at other similarly aged sites nearby, researchers have found remains from many different species of fish, birds and reptiles, as well as other mammals, including freshwater dolphins.
These fossils reflect a tropical, rainforest-like environment that was pockmarked by lakes and crisscrossed by rivers.
O.insignis wasn’t a common sight in these waterways, as evidenced by the fact only a handful of its remains have ever been found – like its living descendants, it was probably reclusive, spending its days sleeping in underground burrows and only venturing out at dusk and dawn to forage for aquatic invertebrates, such as yabbies.
“An ancient, toothed platypus lived in these lakes and rivers as shown by the bones of one that settled to the floor of the lake 25 million years ago,” said Worthy. “I have studied this lost ecosystem for many years now, and it is for exquisite fossils like these that I return again and again to the desert. One never knows what erosion or one’s efforts will reveal next.”
Top image: Excavations at the fossil site in Billeroo Creek in 2017 when one of the fossils was recovered. Credit: Aaron Camens (Flinders University)
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