Over the years, a section of the Inner Mongolia region of China has coughed up heaps of remarkable fossils, ranging from feathered dinosaurs to flying reptiles.
"To say it's [a] hot spot is actually [an] understatement," says Zhe-Xi Luo, a paleontologist at the University of Chicago.
For new discoveries, everyday mysteries, and the science behind the headlines, follow NPR’s Short Wave podcast.
Many of these fossils wind up at the nearby Paleontological Museum of Liaoning, a facility that Luo and his colleague, Thomas Martin of the University of Bonn, visit each year to examine new finds. A decade ago, a specimen in a thick slab of shale caught their eye.
It was the fossilized bones and soft tissue impressions of an ancient mammal that lived some 165 million years ago. "Early fossil mammals — in fact, they are exceedingly rare," says Luo.
This animal, which the team named Megacauda sungei and have described in the journal Nature, was a prize discovery. The remains were so intact that it was possible to determine that it was likely capable of chewing food and then swallowing it, and also suckling — traits that are shared by mammals.
Luo and his colleagues conclude that these evolutionary innovations — and their presence among our ancient mammalian relatives — extend deep into the past to at least the middle Jurassic.
"It's all there in one remarkable fossil," says Luo.
A chihuahua-sized chimera
The fossil was about the size of a chihuahua, but somewhat hard to discern from the rock surrounding it.
"The fossil itself is actually very dark," explains Luo. "The bones are black, the slab [is] dark. And it's kind of difficult to have beautiful photographs."
One of his collaborators, Chang-Fu Zhou of Shandong University of Science and Technology, snapped a different kind of vivid image using a technique called X-ray fluorescence spectroscopy. "It would make this bone pop out in different colors," says Luo.
The resulting image rendered the skeleton a bright yellow and it showcased a curious combination of characteristics. The back end, which includes the pelvis, hind limbs, and tail, resembles a modern-day platypus.
These features suggest that this animal was likely semi-aquatic and foraged in the water. "The big tail is good for swimming," says Luo.
"However," he continues, "its teeth and the jaw would be very different from platypus. So the head is very much of a predator." Indeed, Luo and his colleagues found a smattering of small salamander bones inside the fossil that they suspect the animal had eaten by first cutting and shearing with its sharp teeth.
A lot to swallow
There was something else that Luo instantly noticed about the fossil. "My eyes zeroed in on these tiny hyoid bones," he recalls.
The human hyoid bone sits in the throat, half an inch or so above the Adam's apple, and supports the backside of the tongue.
The team later CT scanned the fossil, including the underside of the skull, which exposed a delicate roof of the mouth as well as a series of bone hooks and ridges.
With this combination of rarely preserved features, "we are able with great confidence to reconstruct the whole muscular skeletal machinery," says Luo.
This reconstruction revealed that the animal was likely capable of doing two things that are unique to mammals.
The first is swallowing food after chewing it. Other vertebrates tend to bite and gulp. But mammals "chew down all the food with our teeth," Luo says. "And then we swallow one bolus of food down to our esophagus at a time."
Consider, for a moment, a carnivore that chews meat, an herbivore that crunches on vegetables, and an insect-eater that chomps bugs. This diverse array of foods all get chewed up and then swallowed down a mammalian throat in a similar way.
Secondly, this fossil appears to have been capable of suckling, which embodies something even more fundamental.
"That ability to suckle mother's milk literally defined our biology," says Luo. "And that's why our group is called mammals. We're named after the mammary glands."
This is some of the oldest fossil evidence of an animal with both these key mammalian abilities.
Small but varied
"It's a fascinating specimen," says Jessica Theodor, a paleontologist at the University of Calgary who wasn't involved in the research. "The whole setup looks more like what we see in modern mammals."
Theodor would like to see some of the images in greater detail, but she says a fossil like this tracks the evolution of lactation, which she likens to a kind of dietary and hormonal conversation between mother and baby.
"The conversation is beginning very far back in the Mesozoic — in the Jurassic and Cretaceous," says Theodor. "That's amazing to me that it's this old and this fundamental."
For Luo, this fossil is the latest evidence of how new ways of feeding have helped mammals and their predecessors diversify — even back when they were the little guys on the proverbial block.
"Mammals, they're never dominant in the age of dinosaurs," he says. "But nonetheless, they hold out relatively well compared to all other vertebrates."
By the time of the Jurassic period, there were already proto-mammalian gliders, swimmers, land-dwellers, tree-dwellers and diggers. Each one was making a living in its own distinct way — all united in their shared ability to chew, swallow and suckle.
Copyright 2026 NPR
