Scientists have identified a new species of mammal ancestor from a 165-million-year-old fossil found in China, revealing an animal that swam with a broad, flattened tail and swallowed its food in ways seen in modern mammals. The findings, published in the journal Nature, suggest that some early relatives of mammals had already evolved modern feeding mechanics millions of years before true mammals appeared.

The species, named Megacauda sungei, lived during the Jurassic period and belonged to an extinct group called docodonts – close relatives of mammals in the Mesozoic Era. Researchers from the University of Chicago studied the exceptionally well-preserved fossil and found strong hind limbs, a flattened tail and specialised throat structures. The animal measured around 40 to 50 centimetres in length and weighed between one and two kilograms, and its features point to a semi-aquatic lifestyle much like modern semi-aquatic mammals.

A throat built for chewing

The most important discovery lies beneath the skull: delicate hyoid bones that supported the muscles used in swallowing. In modern mammals, including humans, these bones form a U-shaped structure connected to the skull through smaller bony segments, allowing animals to chew food and swallow it in smaller portions rather than gulping large pieces or whole prey as animals like alligators do. The same structures also help young mammals suckle. Megacauda carried similar arrangements, providing some of the earliest evidence of a feeding mechanism that later became central to mammalian evolution. The study’s senior author, Zhe-Xi Luo, professor of organismal biology and anatomy at the University of Chicago, called it an incredibly important evolutionary innovation – once early mammals had this apparatus, each group could develop specialised teeth and diversify into a tremendous range of food resources, from carnivores and insectivores to herbivores and omnivores. Co-author Peishu Li compared the animal’s throat anatomy with the throat muscles of the modern opossum.

A hunter of Jurassic waters

The fossil also showed how the animal moved and fed. Its hind limbs had long outer digits similar to those of a platypus, while the tail contained wide, H-shaped vertebrae that would have propelled it through water. The skull held strong canine teeth and sharp cheek teeth for cutting and chewing meat, and tiny vertebrate bones preserved in the stomach suggest it fed on small animals, possibly salamanders whose fossils were found in the same deposits. Scientists believe Megacauda hunted in water while also spending time on land. The discovery follows a 2006 find by Luo’s team of another swimming docodont, Castorocauda – but the new fossil is older and preserves far more of the skeleton. The specimen, from China’s Inner Mongolia region, was studied with CT scanning and X-ray fluorescence; during the COVID-19 pandemic, co-author Chang-Fu Zhou of the Shandong University of Science and Technology carried it between laboratories for examination.