Megacauda, a 165-million-year-old swimmer, already had a throat built for milk

A half-meter, two-kilogram animal that hunted in Jurassic lakes 165 million years ago already had a soft palate, the feature that lets mammals swallow and breathe at the same time, according to a study published in Nature by researchers from Shenyang, Chicago and Bonn. The species is named Megacauda sungei.

The University of Bonn's release leads with the animal as a small "top predator" and presents it as the oldest sign of mammals living in water. Its throat may be the more lasting result, and the swimming claim has a predecessor.

The throat is the new part

Many animals have no soft palate and only a simple hyoid bone in the throat. Crocodiles tip their heads back and let gravity carry food down, Bonn paleontologist Thomas Martin explained.

Mammals have a soft palate that separates the mouth from the pharynx and a more complex hyoid that lets them swallow even against gravity. That is what allows a newborn to suckle milk without it going up the nose, and Martin links it directly to the evolution of mammals. In Megacauda, that arrangement is already in place long before modern mammals appeared, the team says.

The fossil, found in Xiayingzi in Inner Mongolia, belongs to the docodonts, an extinct group of early mammaliaforms. CT reconstructions suggest it paddled with its front legs, waved its flattened tail and steered with its hind legs. Its canines have three roots, which the researchers called a surprise.

A Jurassic swimmer was found twenty years ago

The "earliest aquatic mammal" title has been claimed before. In 2006, a team co-led by Zhe-Xi Luo, then at the Carnegie Museum of Natural History, described Castorocauda lutrasimilis, a beaver-tailed docodont about 164 million years old, also from Inner Mongolia. The museum called it the earliest-known mammal with features for swimming and the largest-known Jurassic mammal, at an estimated 500 to 800 grams.

Luo, now at the University of Chicago, is a co-author of the new paper. The Bonn release does not mention Castorocauda, so it is not clear from the announcement how the two animals compare on age or on the evidence for swimming.

What is clear is the size gap. At around two kilograms, Megacauda is several times heavier than the 2006 estimate for Castorocauda, and Martin calls it the largest mammaliaform from the Jurassic. Most mammal relatives of the time weighed only a few grams, according to the release.

Named for a museum director

The specimen was bought years ago by the Paleontological Museum of Liaoning in Shenyang along with other finds, and its importance only became clear in a joint examination. The species name honours the museum's honorary director, Sun Ge, and Megacauda means "large tail". Martin said the classification took years of teamwork.