A flexible bone that helps mammals chew dates back to the Jurassic Period

Chew on this: Millions of years before the emergence of true mammals, an early ancestor had a tiny, saddle-shaped bone connected to the jaw that was thought to belong to mammals alone. That bone, scientists say, helps all mammals chew and swallow, and ultimately was one secret to our success, enabling the spread into various ecological niches. 

Microdocodon gracilis, a shrew-sized mammal ancestor, lived about 165 million years ago in what’s now China. Scientists led by vertebrate paleontologist Chang-Fu Zhou of the Paleontological Museum of Liaoning in Shenyang, China, examined the fossil and discovered that it included a beautifully preserved hyoid bone. That bone bears a remarkable resemblance to the shape of hyoids in modern mammals, the researchers report in the July 19 Science. 

When it comes to food, mammals have staked their claims across many types of environments. And different modern species have teeth specially adapted for their widely differing diets. Large carnivores like lions and tigers have sharp, cutting blades; some small mammals have high cusps on their teeth to help crunch insects; and others have ridge-packed teeth to help grind down plants.

But one thing that all mammals have in common is that we do chew, breaking food down into tiny pieces before swallowing it. That’s unlike, say, reptiles, which have a penchant for swallowing food whole, says vertebrate paleontologist Zhe-Xi Luo of the University of Chicago. Furthermore, mammals’ mouths, throats and tongues are also designed to be flexible and strong enough to suckle milk, a defining characteristic for the group. 

“The food transport and the swallowing of the chewed-up food is all controlled by muscle related to this highly mobile bone” called the hyoid, Luo says. 

All jawed vertebrates, from fishes to felines, have a hyoid, but in mammals this bone is uniquely mobile. Luo likens it to a child’s backyard swing, with a curved seat suspended by two chain links. “[The bone] can bend and is flexible, like a kid swinging back and forth,” he says.

Whether the evolution of that flexible bone pre-dated the rise of mammals or came later was unknown. Many mammal ancestors living during the Jurassic Period between 201 million years and 145 million years ago were already diversifying in terms of food niches and had a variety of different kinds of teeth. That suggested that the animals likely had at least some sort of transitionally mobile hyoid, Luo says. But what the bone actually looked like remained unclear.

Enter M. gracilis. The Jurassic fossil, an ancestor of mammals called a mammaliaform, represents a key stage between more primitive mammal ancestors that lived during the late Permian and Triassic periods and still retained a rigid hyoid, and the later burst of mammalian diversity that occurred during the Cenozoic Era. That period, from 65 million years ago to modern times, is sometimes called the Age of Mammals. By then, mammals had the mobile hyoid.

Crucially, the fossil is the first Jurassic mammaliaform that included a well-preserved hyoid, enabling researchers to note the distinctive shape. “Once we knew what to look for, we started to search for corroboration of similar structures in other extinct mammal lineages,” Luo says. “And we found them left and right.”

That suggests this structure was common among early mammal ancestors living during the Jurassic, and may have been an important part of mammals’ ultimate ability to diversify, the researchers say. “Mammals just chewed and swallowed themselves into evolutionary success,” Luo says. 

That these mammalian ancestors had a mobile hyoid also means that it pre-dates the evolution of a defining characteristic of mammals: the evolution of the three middle ear bones, and their separation from the jaw. Those bones, called the malleus, incus and stapes, are the secret to mammals’ sharp hearing. But it wasn’t clear whether that separation might have also led to later development of the mobile hyoid in mammals. Instead, this new fossil evidence suggests that these two adaptations were not directly related. 

That was a particularly interesting aspect of the discovery, says Simone Hoffmann, a vertebrate paleontologist at the New York Institute of Technology in Old Westbury.  

“The hyoid is used for chewing, but also for suckling milk, which is very closely tied to mammals,” says Hoffmann, who coauthored a commentary in the same issue of Science. “And [this fossil] suggests the hyoid bone may have developed way before mammals themselves. It might change our understanding of what attributes we typically think of as mammal attributes.”