This surprising mammal fossil is giving scientists something to chew on

A fossil roughly the size of a Chihuahua is offering scientists an unusually detailed glimpse of mammalian life during the middle Jurassic, about 165 million years ago. The animal, named Megacauda sungei, was preserved in a thick slab of shale discovered in the Inner Mongolia region of China, an area renowned for producing exceptional fossils.

The specimen attracted attention because both its bones and impressions of soft tissue survived. Early fossil mammals are exceedingly rare, and this degree of preservation allowed researchers to infer aspects of the animal’s behavior as well as its structure. Their analysis suggests that Megacauda could chew food, swallow it, and suckle its young—three traits associated with mammals.

Those findings push evidence for these evolutionary innovations deep into the past, at least to the middle Jurassic. Rather than revealing an animal that neatly resembles a modern species, however, the fossil presents an intriguing mixture of characteristics. Its pelvis, hind limbs, and tail resemble those of a modern platypus, while its sharp teeth suggest that it was a predator.

Researchers first noticed the specimen about a decade ago at the Paleontological Museum of Liaoning, where many fossils from the region are kept. Zhe-Xi Luo, a paleontologist at the University of Chicago, and Thomas Martin of the University of Bonn regularly visit the museum to examine new discoveries. The team has described the fossil in the journal Nature.

The fossil’s appearance initially made it difficult to study. Its bones are very dark, as is the surrounding rock, so ordinary photographs cannot easily distinguish the skeleton from the shale. Chang-Fu Zhou of Shandong University of Science and Technology addressed that problem with X-ray fluorescence spectroscopy, a technique that can highlight the chemical differences between materials.

The resulting image made the skeleton appear bright yellow and brought its unusual anatomy into sharper view. In that sense, the specimen is valuable for two reasons: its preservation records a combination of mammalian traits, while modern imaging methods make those traits visible. Together, the fossil and the analysis suggest that important features of mammalian biology were already present among ancient relatives long before mammals became familiar in their modern forms.