A Jurassic Swimmer Smaller Than a House Cat Offers Clues to How Mammals Learned to Swallow

Keypoints:

• A new species, Megacauda sungei, lived about 165 million years ago in what is now Inner Mongolia, China
• It measured about 20 inches and weighed up to about 4.4 pounds
• It is the largest mammaliaform known from the Jurassic
• CT scans show it swam with its front legs and wave-like tail movements
• Its throat bones point to a swallowing system important for drinking and suckling

Researchers from Shenyang Normal University, the University of Chicago and the University of Bonn describe a new fossil species in Nature. Megacauda sungei lived about 165 million years ago. The fossil came from Xiayingzi in Inner Mongolia. The name combines "large tail" with a species name honoring the Honorary Director of the Paleontological Museum of Liaoning.

The animal was small by today's standards, about 20 inches long. Even so, it is the largest mammaliaform from the Jurassic, according to Thomas Martin of the University of Bonn. Mammaliaforms are mammal relatives and ancestors. This one belongs to the docodonts, an extinct group that mostly lived in Jurassic times.

It lived in water. Three-dimensional reconstructions from computed tomography (CT) images suggest it paddled with its front legs and moved its tail in a wave-like motion. Its hind legs steered. It had a beaver-like tail and platypus-like hind limbs.

The throat is the main prize. Skull and throat bones suggest a swallowing system with features important for drinking and suckling. Martin argues that the soft palate and the shape of the mouth and throat are linked to the evolution of mammals. He also ties that adaptation to the later growth of brain complexity.

Outside experts add context. Sue Hand, a vertebrate paleontologist who was not involved, noted that suckling was once seen as a hallmark of more advanced mammals. This fossil suggests the trait was already present in the shared ancestor of docodonts and mammals at least 165 million years ago.

The authors flag limits. The fossil preserves the bony framework that supports soft tissue, not the soft palate itself. It does not document a last meal or nursing behavior. Evolution assembled mammal traits in stages, and this swimmer shows one of the earlier steps.