Megacauda sungei: A Platypus-Like Jurassic Mammal That Swam 165 Million Years Ago

Megacauda sungei was a furry, otter-sized mammal that lived in what is now Inner Mongolia, China, about 165 million years ago. The semi-aquatic animal swam underwater, foraged for prey and may have lived a lifestyle similar to that of the modern platypus.

A large Jurassic mammal relative

Megacauda sungei belonged to the docodontids, an extinct group of early mammals and their extinct relatives.

Measuring about 0.5 m (1.6 ft) long and weighing 1 to 2 kg, it was approximately the same size as a modern platypus. It is also the largest known Jurassic mammal, while most of its contemporaries weighed only a few grams.

The species had a broad, flat tail, short hind legs and distinctive teeth capable of a shearing action. Researchers suggest that it may have taken shelter in burrows on land while foraging underwater for fish, salamanders and invertebrates.

Evidence of semi-aquatic behavior

“This is the earliest evidence of aquatic life in mammals,” said Dr. Thomas Martin, a paleontologist at the University of Bonn.

The fossil remains of Megacauda sungei also preserve extremely well-preserved bones from the pharynx and throat. These structures have a composition similar to those of modern therian mammals, the group that includes marsupials and placental mammals such as humans.

What the throat bones reveal about mammalian evolution

“Many animal species do not have a soft palate, only a simple hyoid apparatus with no swallowing mechanism,” Dr. Martin said.

“For example, a crocodile raises its head and makes jerky movements to slip unbitten prey into its throat. This is accomplished with the help of gravity.”

“In contrast, mammals have complex hyoid structures in their pharynx that allow them to swallow against gravity. As a result, we humans can drink water upside down through a straw while breathing.”

This ability is an important adaptation that allows mammals to feed on nutrient-rich milk and swallow it safely without it entering the nose. The researchers link this adaptation to the development of the brain and other features that eventually led to humans.

“The formation of the soft palate and the oral cavity and pharynx are directly related to mammalian evolution,” Dr. Martin said.

The new findings indicate that this anatomy was already present long before modern mammals appeared. The authors conclude that mammal-like swallowing, drinking and lactation anatomy was present in the common ancestor of docodontids and mammals.

Early mammals occupied diverse ecological niches

The discovery adds to evidence that early mammal relatives were more diverse than their generally small body sizes suggest.

“This expands the body size range of docodontans and provides further evidence that multiple docodontans exploited diverse niches with semi-aquatic and other ecotypes,” the researchers said.

Docodontans provide an early example of the evolution of semi-aquatic and other ecotypes in dinosaur-dominated Mesozoic ecosystems, long before modern mammals diversified.

The team’s study was published online in Nature on October 7, 2026.

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Y. Lee et al. A Jurassic mammal swimmer with a modified mammalian pharynx. Nature, published online October 7, 2026. doi: 10.1038/s41586-026-11107-0