A remarkable fossil discovery in the remote outback of South Australia has unveiled a fascinating chapter in the evolutionary history of the platypus, revealing that an ancient toothed ancestor of the modern mammal once shared prehistoric freshwater ecosystems with dolphins approximately 25 million years ago.
Paleontologists from Flinders University in Adelaide made the rare find east of the Flinders Ranges, uncovering well-preserved fossils of Obdurodon insignis, the oldest known platypus species. The discovery, described in the journal Australian Zoologist, provides unprecedented insight into a creature that inhabited the huge lakes, slow-flowing rivers, and forested lowlands of central Australia during the late Oligocene period.
"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 Dr. Aaron Camens of Flinders University, Adelaide.
The significance of this discovery lies in what it reveals about the platypus's evolutionary journey. Unlike the modern platypus, which loses vestigial teeth shortly after birth and relies on a small horny pad to chew food as an adult, Obdurodon insignis possessed well-formed molars and premolars throughout its life. The newly discovered premolar fossils demonstrate that this ancient species also had large, pointed front 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," explained study co-author Professor Trevor Worthy from the Paleontology Lab at Flinders University. Yabbies are freshwater shrimp native to Australia.
Prior to this discovery, Obdurodon insignis was known only from fragmentary remains: one-and-a-half molar teeth, a jaw fragment, and a pelvis fragment. The new fossils include a scapula that reveals the animal swam and moved much like the modern platypus, differing mainly by being slightly larger and retaining its teeth into adulthood.
The Flinders University team has conducted expeditions to this outback desert location for over two decades, meticulously studying rocks containing fossils from this ancient ecosystem. Their efforts have yielded more than a thousand fossils of non-fish vertebrate animals, though platypus fossils remain exceptionally rare—only three specimens of the toothed platypus have been collected during this extensive research period.
The fossils paint a vivid picture of a dramatically different Australia. Professor Worthy describes forests that were home to diverse communities of arboreal mammals, including koalas and numerous types of possums. In the canopy, a giant eagle called Archaehierax hunted, while sheep-sized marsupials browsed on the ground below.
The aquatic environment was equally diverse. The lakes supported many kinds of lungfish and smaller fish species. Perhaps most surprisingly, a small dolphin inhabited these freshwater ecosystems, with teeth and bones discovered at several locations where rocks expose this ancient community.
"But as the new fossils show, another mammal swam with the dolphins: an ancient, toothed platypus," Professor Worthy noted. He emphasized that while the rainforests and lakes have long vanished, platypuses have continued swimming in Australian waterways ever since.
"I have studied this lost ecosystem for many years now," said Worthy, "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."
The discovery underscores the importance of continued paleontological research in Australia's remote regions, where ancient sediments continue to yield secrets about the continent's remarkable evolutionary history. Each fossil fragment helps scientists piece together the story of how Australia's unique fauna evolved and adapted over millions of years, transforming from the lush, lake-filled landscape of the Oligocene to the diverse ecosystems we recognize today.