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Ancient WorldsAncient Worlds

Mammals’ weird way of swallowing is at least 165 million years old

A new fossil find may help pinpoint the origins of mammals’ uber-flexible hyoid bone, which anchors the tongue and gives us our signature swallowing style.

ByKatherine J. WuNOVA NextNOVA Next
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An artist's impression of Microdocodon gracilis, an early mammal relative from the Jurassic Period with a modern mammalian hyoid. At full size, Microdocodon's body was no more than three inches long, but boasted an equally sized tail. Image Credit: April I. Neander, the University of Chicago

Remember the last time you unhinged your jaw and swallowed your prey whole?

Unless this article was clicked on by an exceptionally literate lizard, the answer is probably no: That’s not really the mammalian modus operandi. Unlike our distant reptilian relatives, we humans and the rest of our warm-blooded, milk-secreting kin have the decency to chew our food before swallowing it in measured gulps—an act of evolutionary etiquette made possible by the segmented hyoid bone nestled in our throats.

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But the hyoid matters for more than manners. As a sort of structural scaffold for the conduit between mouth and esophagus, this small-but-mighty bone shuttles food that’s already been partially broken down to the rest of the digestive tract, speeding up the release of nutrients—an important adaptation for fueling a hungry mammal on the go.

Now, a new fossil find may help pinpoint how and when this major evolutionary milestone first arose. The discovery of a 165-million-year-old shrew-sized mammal relative called Microdocodon gracilis, reported today in the journal Science, suggests the mammalian hyoid may trace its loosely anchored roots at least as far back as the late Jurassic period. It also hints that the mammal-like hyoid—and with it our lineage’s signature swallowing style—arrived early on the scene, preceding even the advent of modern mammals themselves.

“This spectacularly preserved fossil comes from a really critical time interval,” says Samantha Hopkins, a vertebrate paleontologist at the University of Oregon who was not involved in the study. “The hyoid isn’t a part of the skeleton that fossilizes a whole lot...and it’s one of the pieces we haven’t really seen much about before now.” This study, she says, “fits in with what we know about all the changes it took to become a mammal.”

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Modern mammals, such as the dog and the platypus, have segmented hyoid bones with mobile joints. The mammalian hyoid's flexibility helps humans and their relatives move their tongues and gulp down chewed food. In stark contrast, reptiles have a rigid, unsegmented, V-shaped hyoid, and often swallow chunks of food whole. Image Credit: April I. Neander, the University of Chicago

Hyoids can be found in most living vertebrates. But the version found in the mammalian throat is unlike anything else in the animal kingdom. While the reptilian hyoid resembles a pair of thin, inflexible rods joined into the shape of a V, the typical mammalian hyoid is more segmented and three-dimensional. Instead of forming a V, it looks more like two hinged horns linked into an H, with joints that allow it to recoil and flex like a beckoning finger.

This unusual configuration keeps the hyoid—and all the muscles attached to it—very mobile, enabling a kind of “powered swallowing,” says study author Bhart-Anjan Bhullar, a vertebrate paleontologist at Yale University. The bone’s agility aids mammals in effortlessly maneuvering their tongues, coordinating chewing and gulping, suckling milk from their mothers, and, in the case of humans, guzzling down soda through a straw.

In spite of its crucial function, the mammalian hyoid has been a tough adaptation to pin down. Suspended in a sleeve of throat tissue, this bone has the strange distinction of not really being attached to the rest of the skeleton. Its solo status, combined with its small, delicate structure, make the hyoid, in most cases, easily lost during fossilization.

That’s part of what made Microdocodon such a big surprise. First uncovered in the Daohugou archaeological site in northeastern China, the fossil revealed itself to study author Chang-Fu Zhou, a researcher at the Paleontological Museum of Liaoning, in a split slab of shale. The skeleton itself was mostly gone, but millennia of pristine preservation had enshrined its silhouette in rock—including the distinct outline of a familiar-looking hyoid.

“Lo and behold, we had mammal-like hyoid bones,” says study author Zhe-Xi Luo, a paleobiologist at the University of Chicago. “It never dawned on us that you could trace [this adaptation] back 165 million years.”

Soft tissue doesn’t tend to stick around in fossil form. But the presence of a bendy, hinged hyoid suggests a suite of other food-centric adaptations, such as a muscular throat to push partially processed food down the esophagus, Bhullar says. Based on Microdocodon and other fossils from the time, the hyoid probably arose in tandem with specialized, mammal-like teeth that enhanced these creatures’ capacity to chew.

The find is especially striking because of Microdocodon’s unusual place on the mammalian family tree, Luo says. Though Microdocodon is related to our own ancestors, it technically belongs to a separate lineage—the docodonts—which died out long ago. This suggests that a complex, flexible hyoid was present in the common predecessor of docodonts and the line that eventually begat modern mammals.

In other words, the genesis of the flexible hyoid dates at least as far back as the Late Jurassic, long before the ancestors of the mammal species we know and love today inherited the Earth.

Many of Microdocodon’s other features, however, were more ancient in nature. Like most living reptiles, this mammalian mini-me still had its middle ear bones anchored firmly onto its lower jaw—a feature that mammals disposed of on several different occasions throughout their evolutionary history. (As you might imagine, detaching your ear from your jaw comes with a handful of perks, chief among them increased ability to chew and hear at the same time.)

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The fossil of Microdocodon gracilis, preserved in two rock slabs, was uncovered in the Daohugou area of China, a site that dates back to about 165 million years ago. The fossil is now at the Paleontological Museum of Liaoning in China. Image Credit: Zhe-Xi Luo, the University of Chicago

Beyond portraying Microdocodon as a clear pitstop on the long road to modern mammaldom, this odd combo of modern hyoid and primitive ear is surprising in its own right, says Simone Hoffmann, a mammalian paleontologist at the New York Institute of Technology who authored a commentary on the study. Both anatomical features are derived from very similar starting material in mammalian development. It’s common for traits related in this way to evolve in synchrony. But in this case, “we’re seeing the very clear decoupling of a pattern,” she says.

This mish-mash of modern and ancient features probably made Microdocodon a sight to behold. With a body length of less than 3 inches and limb bones thinner than matchsticks, it’s thought to have resembled a very dainty shrew. And though it probably shared the Jurassic landscape with behemoth dinosaurs and other early mammals, Microdocodon may have weighed as little as 5 grams—about as much as a quarter or a crumpled sheet of paper. (That’s pretty close to the Etruscan shrew, which, at less than 2 grams on average, is the smallest known mammal by mass still around today.)

All the same, Microdocodon probably fared pretty well, noshing on insects as it skirted through the trees, Luo says.

And now, hundreds of millions of years later, this diminutive docodont is making a big splash. Armed with Microdocodon’s freshly characterized fossil, Zhou, Bhullar, Luo, and the rest of their team used their discovery as a template to examine hyoids in other early mammal lineages, all of which showed similar structures.

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Microdocodon is part of a lineage of extinct early mammal relatives called docodonts. Because Microdocodon has a mammal-like hyoid, this trait was probably present in the common ancestor of modern mammals and docodonts as well. Image Credit: April I. Neander, the University of Chicago

“The basic message here is that there was a rapid transformation [of the hyoid] that occurred [before] the docodonts,” Bhullar says. After that point, he says, “there wasn’t all that much variation in the hyoid...it was kind of a key innovation. Even as the rest of the body changed, it remained relatively static.”

Maybe that’s no shock, given the adaptations the new and improved hyoid afforded early mammals. In the wake of the mass extinction that occurred 66 million years ago, our predecessors flourished by virtue of their big brains and fast feet—two traits that require a lot of nutritional ammo.

Efficient digestion, then, may have been “instrumental in getting mammals on a path to dominance,” Bhullar says. It perhaps “even enabled the later explosion of vast diversity that’s kind of shaped the face of the living world.”

Speaking as a mammal, that isn’t such a hard truth to swallow.

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