These ‘Grinning’ Triassic Reptiles Likely Died of Starvation

Sheep-sized rhynchosaurs had an unusual way of chewing plants that wore down their teeth over time

Illustration of rhynchosaur
An artist's interpretation of how a rhynchosaur might have looked Mark Witton

Between 225 and 250 million years ago, before the dinosaurs took over, a group of smaller, sheep-sized reptiles roamed across the world. These creatures, called rhynchosaurs, looked like they were permanently smirking because of the unique way they chewed plants: Instead of chomping their jaws up and down, they used a scissor-like motion to grind plants between their teeth and exposed jaw bones.

This strategy helped them break down tough vegetation and proliferate throughout the middle and late Triassic periods. But it might have also contributed to their downfall, according to a new paper published last week in the journal Palaeontology.

By the time rhynchosaurs reached old age, they had likely done so much chewing that their teeth had become completely ground down. With nothing left to bite or mash food with, elder rhynchosaurs probably starved to death, the researchers posit.

Paleontologists are interested in rhynchosaurs in part because their fossilized remains are so plentiful, making up roughly 90 percent of vertebrate specimens at some Triassic dig sites, per the New York Times’ Freda Kreier. But they’re also curious about the herbivores because their special food-grinding technique would have made them susceptible to infection, which raises questions about why they evolved this way. Since a handful of animals still eat this way today, such as certain chameleons, researchers want to learn as much as possible about this strange adaptation.

“That wear and tear is not good for you, especially if you want to keep eating,” Yara Haridy, an evolutionary biologist at the University of Chicago who was not involved in the study but reviewed it before publication, tells the Times.

Black and white scan of rhynchosaur jaw
A side view of a rhynchosaur's jaw Thitiwoot Sethapanichsakul

To better understand the Triassic reptiles, researchers studied computed tomography (CT) scans of fossilized rhynchosaur jawbones found in Devon, England. The fossils belonged to rhynchosaurs of various ages: juveniles, adults and one “particularly old animal,” says study co-author Robert Coram, a research associate at the University of Bristol who found the Devon fossils, in a statement.

From this analysis, the researchers gleaned that whenever some of the reptiles’ teeth fell out or became damaged, rhynchosaurs grew a whole new section of jaw—with new teeth—in the back of their mouths. Then, this new section moved forward so the animals could chew with these new teeth instead of the worn-out ones. As rhynchosaurs got older, their jaws got longer, and the dulled teeth and bones remained at the front of their mouths.

Eventually, though, their bodies couldn’t keep up: With ground-down teeth and no new section to replace them, older rhynchosaurs simply couldn’t eat, the research suggests. The team now wants to confirm this hunch by looking for other evidence of possible starvation among rhynchosaurs.

“It’s like elephants today—they have a fixed number of teeth that come into use from the back, and after the age of 70 or so, they’re on their last tooth, and then that’s that,” says Coram in the statement. “We don’t think the rhynchosaurs lived that long, but their plant food was so testing that their jaws simply wore out, and presumably they eventually starved to death.”

Beyond harming individual rhynchosaurs, the unusual chewing technique may have contributed to the entire species’ demise. In the early days of rhynchosaurs’ rule, the planet was covered with relatively soft ferns, which would’ve been easy for them to grind down. But around 225 million years ago, the world’s climate changed, which allowed tougher, needle-covered conifers to proliferate. If rhynchosaurs tried to continue eating the same way, it would’ve been very difficult for them to get enough nutrients to survive, per the Times.

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