Ancient platypus cousin lived alongside dinosaurs

Fossils show monotremes were present in South America 10 million years earlier than previous research suggested. An ancient platypus relative found in Argentina calls into question when and where this weird egg-laying clade of mammals evolved.

The only monotremes to survive into the modern day are echidnas and platypuses which are endemic to Australia and neighbouring islands.

But fossil evidence shows that the mammalian oddballs lived in prehistoric South America as well.

More than 30 years ago, the first evidence of monotremes outside Australasia was found when a tooth from an ancient platypus, Monotrematum sudamericanum,was found in Patagonia, Argentina. That animal’s fossils were found to be 62 million years old.

The newly discovered ancient platypus cousin, named Patagorhynchus pascuali, lived 70 million years ago in the Late Cretaceous, when dinosaurs still roamed the Earth. The find is described in a paper published in Communications Biology.

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Patagorhynchus pascuali illustration by Gabriel Lio.

“Occurrence of basal monotremes in the Early Cretaceous of Australia has led to the consensus that this clade originated on that continent, arriving later to South America,” the authors write. “Here we report on the discovery of a Late Cretaceous monotreme from southern Argentina, demonstrating that monotremes were present in circumpolar regions by the end of the Mesozoic, and that their distinctive anatomical features were probably present in these ancient forms as well.”

At the time, Argentina and the rest of South America was still part of the supercontinent Gondwana. India, Australia, Arabia, Madagascar and Antarctica had split from South America and Africa about 140 million years ago.

Patagorhynchus is known from a second lower molar fossil. Other fossils found around the ancient monotreme were remains of early mammals, frogs, turtles, snakes, aquatic plants, freshwater snails, birds and larvae. In addition, there were dinosaurs including fossils of long-necked theropods and two-legged theropods.

Judging by the size of the tooth, the authors suggest that it was probably intermediate in size between Monotrematum (roughly the same size as modern platypuses) and another genus of ancient platypus cousin, Obdurodon, which could reach nearly a metre long.

Patagorhynchus pascuali skull reconstruction. Credit: Santiago Miner (Laboratorio de Anatomía Comparada y Evolución de los Vertebrados)

Large male platypuses today rarely reach 60 centimetres.

The ecological niche and behaviour of Patagorhynchus was likely similar to living platypuses, the authors say.

In the past, it was assumed that monotremes had only dispersed from their origin in what is now Australia after the extinction of the non-avian dinosaurs 66 million years ago.

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“The discovery of Patagorhynchus pascuali clearly demonstrates that the monotremes had already attained a wide paleogeographic distribution, stretching across southern South America, Australia, and Antarctica, the later one as a connecting pathway (but fossil monotremes are still unknown from this landmass),” the authors conclude.

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