Fossil From Inner Mongolia Shows 165-Million-Year-Old Mammal Could Chew and Suckle, Study Says

The specimen, which the team named Megacauda sungei, was described by Zhe-Xi Luo, a paleontologist at the University of Chicago, and his colleagues including Thomas Martin of the University of Bonn. Luo told NPR News that early fossil mammals are "exceedingly rare," and that the shale beds of Inner Mongolia, which have yielded feathered dinosaurs and flying reptiles, are more than a hot spot.

The fossil had been held at the Paleontological Museum of Liaoning, where Luo and Martin examine new finds each year. About a decade ago, the specimen caught their eye. Its remains were intact enough for the team to determine that the animal was likely capable of chewing food before swallowing it, and of suckling. Luo said the evidence for both behaviors sits "all there in one remarkable fossil."

The animal was about the size of a chihuahua and was difficult to see in the surrounding rock. Luo said the fossil itself is very dark, with black bones in a dark slab, making conventional photography unhelpful. Chang-Fu Zhou of Shandong University of Science and Technology, a collaborator, produced a clearer image using X-ray fluorescence spectroscopy, which rendered the skeleton in bright yellow.

That image revealed a mix of characteristics. The back end of the animal, including the pelvis, hind limbs and tail, resembles a modern-day platypus, features the team interprets as signs that it was semi-aquatic and foraged in water; Luo said the large tail would have been useful for swimming. The teeth and jaw, by contrast, differ sharply from those of a platypus, and Luo described the head as that of a predator. The researchers found a scattering of small salamander bones inside the fossil, which they suspect the animal ate after cutting and shearing with its sharp teeth.

Luo said his attention was drawn immediately to tiny hyoid bones. In humans, the hyoid sits in the throat, about half an inch above the Adam's apple, and supports the back of the tongue. A CT scan of the fossil, including the underside of the skull, exposed a delicate roof of the mouth along with bone hooks and ridges. Luo said that combination of rarely preserved features allowed the team to reconstruct the muscular and skeletal machinery with great confidence.

That reconstruction indicated the animal could do two things unique to mammals. The first is swallowing food after chewing it; other vertebrates tend to bite and gulp, while mammals chew food with their teeth and then swallow a single bolus at a time, whether the diet is meat, plants or insects. The second is suckling. Luo said the ability to suckle mother's milk "literally defined our biology," and is the reason the group is named for the mammary glands. The fossil is some of the oldest evidence of an animal with both abilities.

Jessica Theodor, a paleontologist at the University of Calgary who was not involved in the research, called it "a fascinating specimen" and said the whole arrangement looks more like what is seen in modern mammals. She said she would like to examine some of the images in greater detail, but that a fossil like this traces the evolution of lactation, which she likened to a dietary and hormonal conversation between mother and baby. Theodor said that conversation began very far back in the Mesozoic, in the Jurassic and Cretaceous, and that she found it remarkable the behavior is this old and this fundamental.

For Luo, the find adds to evidence that new ways of feeding helped mammals and their predecessors diversify, even while they remained minor players. Mammals, he said, were never dominant in the age of dinosaurs.

Editor's Summary

A study in Nature describes Megacauda sungei, a chihuahua-sized mammal that lived about 165 million years ago in what is now Inner Mongolia, China, and whose fossil preserves evidence of chewing, swallowing and suckling. The researchers say the find pushes both mammalian feeding traits back to at least the middle Jurassic. An outside paleontologist called the specimen fascinating and said it traces the evolution of lactation deep into the Mesozoic.