by About 70 million years ago, a small, furry, platypus-like creature shuffled along the shore of an ancient lake. This would not have been a notable incident, except for one thing: the lake was in present-day Argentina, not Australia.
The creature named Patagorhynchus pascuali, is the oldest fossil of the egg-laying group of mammals known as monotremes ever discovered in South America. The discovery could rewrite the history of where these strange early mammals evolved. Today, all five species are living monotremes – including the platypus (Ornithorhynchus anatinus), the short-beaked hedgehog (Tachyglossus aculeatus) and three species of long-billed echidnas (Zaglossus) – only found in Australia and some of the surrounding islands. So how did a platypus ancestor end up so far from Down Under?
Millions of years ago, Australia, South America, and Antarctica (as well as parts of Africa and Asia) were compressed into a supercontinent called Gondwana. This vast landmass began breaking up about 180 million years ago during the Jurassic period, but only fully separated about 66 million years ago at the end of the Cretaceous period.
Since more recent cloaca fossils have been found in South America, scientists previously speculated that the group evolved on the Australian landmass after this continent’s collapse and later migrated back to South America via a land bridge. But the fact that P.pascuali existed in Argentina before the continent’s collapse tells a different story.
“Our discovery clearly shows that monotremes evolved not only on the Australian continent, but also in other parts of southern Gondwana,” co-author of the study Fernando Novas (opens in new tab)a paleontologist at the Bernardino Rivadavia Natural Science Museum in Buenos Aires, Argentina, told Live Science in an email.
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The specimen described in the magazine communication biology (opens in new tab) on February 16, was identified by a fragment of a lower jaw containing a molar. When it comes to studying fossilized mammalian remains, “teeth give us a tremendous amount of information”, Robin Beck (opens in new tab), an evolutionary biologist at the University of Salford in the UK who was not involved in the study, told Live Science in an email. However, with monotremes, tooth identification is a bit more complicated.
“Live platypuses lack teeth,” Novas said. But another extinct platypus relative, the 30-million-year-old one Obdurodon, retained teeth in both upper and lower jaws. The P.pascuali Molars closely resembled these teeth, as well as the very small, imperfect teeth that platypuses briefly possess.
Based on its teeth and apparent habitat, P.pascuali probably had a diet similar to that of a modern platypus: mostly small aquatic invertebrates, including insect larvae and snails. The Argentine fossil bed in which it was discovered confirms this; Novas said they found insects and snail shells in the sediments in the area P.pascuali. In addition, researchers uncovered the fossilized remains of other early mammals, turtles, frogs, snakes, aquatic plants and a variety of dinosaurs.
While the discovery represents an important and interesting new piece in the monotreme evolutionary puzzle, researchers are still far from a complete picture. “There are still large gaps in the monotreme fossil record,” Beck said. For example, although no monotreme fossils have been discovered in Antarctica, ancient platypus bones likely exist deep beneath the ice, given its earlier proximity to Australia and South America.
But as one South American paleontologist put it, it’s pretty cool to know that “the Australian’s great-grandfather Ornithorhynchus was Argentinian.”
