'Platypus-like' fossil in China is oldest-known semiaquatic mammal-like animal.

There is a particular wonder in discovering that an animal from the age of dinosaurs was already doing something we associate with the familiar creatures of today—swimming, diving, hunting in water. For decades, the earliest mammals were imagined as small, scurrying insect-eaters, dodging the feet of dinosaurs. A new fossil from China suggests that picture was incomplete.

The fossil, found in Inner Mongolia, belongs to a creature called Megacauda sungei—"great tail"—which lived about 168 to 163 million years ago during the Jurassic period. Weighing an estimated one to two kilograms, it was roughly the size of a modern platypus and the largest of the Jurassic mammaliaforms .

Megacauda was not a true mammal, but a member of an extinct branch called Docodonta, early cousins of mammals. Its body tells a story of life in the water. It had a broad, flattened tail with widened bones, like modern platypuses and beavers, suggesting it was a capable swimmer. Its short legs had long feet and a sprawling stance that would have allowed it to move on land and in water. Its teeth were like cutting blades, made to shear live prey like salamanders, fish, and other aquatic invertebrates .

What makes the fossil particularly remarkable is what it reveals about a behavior we take for granted: nursing. The preservation of soft tissues in the throat shows that Megacauda had the anatomical equipment for suckling milk as an infant and swallowing food as an adult—features also seen in modern mammals. The fossil includes tiny bones called hyoids that support the tongue and the muscles that push food down the throat while keeping it out of the windpipe .

"This fossil has the beginning of a mammalian throat," said Rebecca German, an evolutionary morphologist not involved in the study. "That is critical to be able to swallow successfully" .

The discovery suggests that the capability to swim originated early in mammal evolution, and that the earliest mammals were more diverse in their lifestyles than the traditional picture allowed. "It really shows just how diverse these animals were through the Jurassic and Cretaceous," said Elis Newham of King's College London. "It's time to put that to bed now" .

Megacauda takes the earliest-known semiaquatic mammaliaform crown from Castorocauda, a prehistoric beaver-like creature that lived in the same region millions of years later. The research was published in Nature .

For paleontologists, the fossil is a reminder that the evolutionary story of mammals is not one of simple, linear progress from small insect-eaters to the creatures we know today. It is a story of experimentation, of creatures trying different ways of living, some of which succeeded and some of which did not. Megacauda was one of the experiments—a platypus-like creature that swam in Jurassic waters, hunted with bladed teeth, and suckled its young like a mammal, long before mammals as we know them existed.

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Sources: Australian Broadcasting Corporation, Science, Nature