165 Million Years Before Otters, This Strange Predator Was Already Swimming Like One

This strange Jurassic predator may reveal how mammals evolved the ability to chew and swallow.

The Jurassic had an otter. Except it wasn’t exactly an otter. More like an otter-pus? Platy-otter?

It had the broad, flattened tail of a river swimmer, platypus-like hind legs, and a mouthful of teeth built to cut flesh. It was roughly the size of a Chihuahua, and it prowled the waterways of what is now northern China 165 million years ago, during the age of dinosaurs.

Meet Megacauda sungei, or “big tail,” a newly identified member of an extinct group of mammal relatives called docodonts. It was no ancestor of modern otters. But in a striking case of convergent evolution, its body had already acquired many of the features that make semiaquatic mammals successful today.

And the strangest part of this fossil wasn’t its tail or its teeth. It was what scientists found beneath its skull.

In a new study, an international team reports that this Jurassic predator possessed remarkably modern-looking structures involved in swallowing.

A Platypus in the Back, a Predator in the Front

The fossil comes from Inner Mongolia, from rocks roughly 163–168 million years old. At 40 to 50 centimeters long and one to two kilograms, Megacauda was unusually large for a Jurassic mammal relative.

Its elongated outer toes and mobile ankles suggest feet suited to paddling and steering. Its broad tail probably propelled it through water. Researchers suspect it hunted in water and sheltered in burrows on land, much like a platypus.

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The resemblance ended at its jaws. Megacauda had oversized canines, including an upper canine anchored by three roots, and shearing cheek teeth. Tiny vertebrate bones found in its stomach suggest it ate small animals, possibly including local salamanders.

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Another swimming docodont, Castorocauda, was described in Science in 2006. But Megacauda lived slightly earlier, and its more complete skeleton is now super valuable to paleontologists because it reveals how such swimmers moved and fed.

The Secret in Its Throat

The surprise emerged when researchers examined the fossil using CT scans and X-ray fluorescence imaging. Beneath the skull, they identified a collection of hyoid bones. In living mammals, these bones support the tongue and anchor muscles involved in swallowing.

“My eyes zeroed in on these tiny hyoid bones,” Zhe-Xi Luo, a paleontologist at the University of Chicago and senior author of the study, told NPR.

In modern mammals, this biomachinery moves chewed food down the esophagus while protecting the airway. Related structures help newborns swallow milk. By comparing these bones with those of living mammals, researchers digitally reconstructed Megacauda’s throat and found it capable of similar coordinated movements.

“The whole setup looks more like what we see in modern mammals,” Jessica Theodor, a University of Calgary paleontologist who was not involved in the research, also told NPR.

While the throat bones cannot definitively prove suckling or reveal exactly how its muscles worked, they suggest this sophisticated feeding system predated the split between docodonts and the lineage leading to modern mammals.

There was another surprise.

Its throat combined ancient features inherited from earlier mammal relatives with newer structures resembling those of modern mammals. Scientists had thought the newer structures evolved directly from the older ones. But Megacauda had both, suggesting the transformation was more complicated.

Today’s platypuses and echidnas, which lay eggs but still produce milk, feed their young differently from most other mammals. The researchers suspect their ancestors may once have possessed some of the same throat structures found in Megacauda, only to lose them later in evolution.

How Mammals Learned to Eat Differently

Fish, amphibians, and reptiles swallowed meals hundreds of millions of years earlier before Megacauda. Along the mammalian line, the transformation involved chewing food thoroughly and swallowing it in controlled portions.

Early mammal ancestors called cynodonts developed specialized teeth, stronger chewing muscles and a secondary palate: a bony partition separating the mouth from nasal passages. This helped them chew while breathing.

Chewing presented another challenge: moving those pieces safely into the esophagus without choking. This is where the flexible hyoid bones and specialized throat muscles came in.

A 2019 Science study described mammal-like hyoid bones in Microdocodon gracilis, a tiny Jurassic tree-dweller. Its jointed throat bones showed that a key part of the swallowing apparatus existed long before modern mammals. With the discovery of Megacauda, scientists now also know of the surrounding bony structures, allowing researchers to reconstruct more of how the whole system may have worked.

This machinery may have helped evolving mammal lineages exploit different foods, from insects and plants to meat, while still swallowing efficiently.

Over millions of years, mammals developed radically different teeth and diets, but retained broadly similar mechanisms for moving chewed food from the mouth to the stomach.

The study was published in the journal Nature.