Discovered in a thick slab of shale in Inner Mongolia, a 165-million-year-old middle Jurassic fossil named Megacauda sungei provides some of the oldest evidence of mammalian chewing, swallowing, and suckling.
Excavating Megacauda sungei in Inner Mongolia
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,” Zhe-Xi Luo, a paleontologist at the University of Chicago, said. Many of these finds 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 discoveries.
A decade ago, a specimen embedded in a thick slab of shale caught their eye. It contained the fossilized bones and soft tissue impressions of an ancient animal that lived some 165 million years ago. “Early fossil mammals — in fact, they are exceedingly rare,” Luo explained. The research team named the specimen Megacauda sungei and described its anatomy in the journal Nature.
A Chihuahua-Sized Chimera With Predator Teeth
Measuring roughly the size of a chihuahua, the fossil proved difficult to discern from its surrounding matrix. “The fossil itself is actually very dark,” Luo noted. “The bones are black, the slab [is] dark. And it’s kind of difficult to have beautiful photographs.” To bypass this visual obstruction, collaborator Chang-Fu Zhou of the Shandong University of Science and Technology utilized X-ray fluorescence spectroscopy.
The imaging technique made the skeletal remains pop out in bright yellow, exposing a striking mix of anatomical traits. The back end—including the pelvis, hind limbs, and tail—strongly resembles a modern-day platypus, suggesting the animal was semi-aquatic. However, its skull tells a different story. “Its teeth and the jaw would be very different from platypus. So the head is very much of a predator,” Luo observed. Inside the fossil cavity, researchers discovered a smattering of small salamander bones, indicating the animal sheared its prey using sharp teeth.
Bone Structures Track the Evolution of Lactation
Beyond its external morphology, microscopic and internal bone structures revealed critical evolutionary markers. “My eyes zeroed in on these tiny hyoid bones,” Luo recalled. Subsequent CT scans of the skull’s underside exposed a delicate roof of the mouth alongside a series of bone hooks and ridges. Jessica Theodor, a paleontologist at the University of Calgary who was not involved in the research, said that the specimen tracks the evolution of lactation, likening it to a dietary and hormonal conversation between mother and baby that began deep in the Mesozoic.
This skeletal machinery allowed the animal to perform two defining mammalian actions. First, it could chew food down thoroughly with its teeth and swallow a single bolus at a time, departing from the bite-and-gulp mechanics of other vertebrates. Second, the anatomical evidence points directly to the capacity for suckling. “That ability to suckle mother’s milk literally defined our biology,” Luo stated. “And that’s why our group is called mammals. We’re named after the mammary glands.”
Survival and Diversification in the Age of Dinosaurs
By the Jurassic period, proto-mammalian gliders, swimmers, land-dwellers, tree-dwellers, and diggers had already emerged. “Mammals, they’re never dominant in the age of dinosaurs,” Luo said. “But nonetheless, they hold out relatively well compared to all other vertebrates.” Each of these ancient forms forged a distinct lifestyle, unified fundamentally by the newly confirmed triad of chewing, swallowing, and suckling.
