Around 165 million years ago, in a landscape dominated by dinosaurs, a creature the size of a platypus slipped into the water in what is now Inner Mongolia to hunt. It had a broad flat tail, platypus-like hind limbs and sharp canines. Its throat, according to a paper published in Nature on Oct. 7, was already built like ours.

The animal is named Megacauda sungei, meaning "big tail," and it is the largest Jurassic mammaliaform yet known. It is not a true mammal, but a close relative from an extinct lineage called docodonts.

A Platypus-Sized Swimmer

Megacauda was about 40 to 50 centimeters long and weighed one to two kilograms, according to the University of Chicago, whose paleontologist Zhe-Xi Luo is the senior author. The fossil, found in Inner Mongolia, preserves the pelvis, hind limbs and tail, which distinguish it as a powerful swimmer. The back legs had long outer digits like a platypus, and the tail vertebrae were wide and H-shaped, supporting a broad tail outline for propulsion.

Luo's team had described a swimming docodont before. Castorocauda, reported in 2006, lived in the same niche, but the new fossil is older and more complete, said co-author April Neander, a scientific artist who worked on the CT imaging. In particular, she noted, its pelvis is more complete than any other specimen from this time.

The Throat Is the Story

The fossil's most important feature is hidden under the skull. All modern mammals, including humans, have hyoid bones, a U-shaped saddle suspended from the skull that links the mouth and throat to the esophagus and larynx. These let adult mammals swallow chewed food in small lumps, and let babies suckle milk. Most other animals lack a soft palate and swallow whole, like an alligator.

The soft tissue involved rarely fossilizes, so scientists did not know when this system arose. Megacauda preserves the delicate bony structures that suspend the palate and pharynx muscles, better than other fossils, the university said. Co-author Peishu Li, now at Ohio University, compared the anatomy with the throat muscles of the modern opossum.

Nature's summary of the paper concludes that the apparatus for suckling and swallowing was already in place in early relatives of modern mammals by the Middle Jurassic, roughly 174 to 164 million years ago.

What It Ate

The fossil also holds direct evidence of diet. It has robust canines and shearing cheek teeth suited to cutting meat, and tiny bits of vertebrate bone in its stomach. The researchers think it likely preyed on small animals such as the abundant salamanders in the same fossil beds. That makes it one of the oldest known animals adapted to swimming and hunting in water, long before platypuses, otters or minks.

My view: the stomach contents are the most persuasive part. Anatomy can suggest a lifestyle. A last meal records one.

Part of a Bigger Pattern

Megacauda belongs to a group that was far more varied than its small size implies. Docodonts are the most species-rich group of stem mammals across North America, Europe and China, Luo said, and they lived in many habitats, from tree-dwellers like Microdocodon, a tiny shrew-like animal Luo's team described in 2019, to swimmers like Megacauda. The paper describes this as an example of replicated evolution of semi-aquatic and other lifestyles in a Mesozoic world dominated by dinosaurs.

The same logic runs through a different study Blogerroom covered. A global synthesis finding that most ecosystem services rise with each added species argues that variety carries function. Megacauda shows that variety of body plans and lifestyles among mammal relatives goes back to the Jurassic.

How the Fossil Was Read

The specimen's journey is part of the story. During the difficult years of the COVID-19 pandemic, co-author Chang-Fu Zhou of Shandong University of Science and Technology carried it to several laboratories around China for CT scanning and X-ray fluorescence imaging. The large slab had to be separated into smaller pieces to be scanned, and the scans let Neander reconstruct fine detail.

Other authors include Yikun Li and Honggang Zhang of Shenyang Normal University and Thomas Martin of the University of Bonn. Funding came from the US National Science Foundation, China's Taishan Scholar Program, the German Research Foundation and Ohio University.

What to Keep in Mind

This is a single specimen, and the soft palate itself is not preserved. The inference rests on the bones that support it, compared with living mammals. I read the university release, Nature's research summary and press coverage, not the full paper, so details of the analysis should be checked there.

The finding does not mean Megacauda was a mammal. It means that the feeding machinery that defines mammals, and the ability to chew then swallow and later diversify into so many diets, was assembled before the lineage that led to modern mammals split off. As Luo put it in the university's statement, once early mammals had this apparatus, each group could develop specialized teeth and diversify into a tremendous range of foods.