The Sad Story Behind a Prehistoric Reptile’s ‘Grin’

Before the dawn of the dinosaurs, the world belonged to reptiles called rhynchosaurs.

They were the size of pigs and thrived all over the planet 225 million to 245 million years ago during the mid- and late Triassic Period.

Despite being the most successful plant-eater of their day, “nobody has ever heard of them,” said Michael Benton, a paleontologist at the University of Bristol in England. They may get overshadowed by the earliest dinosaurs, he suggests, although they make up 90 percent of vertebrate fossils in some Triassic digs.

Scientists have often attributed their wild success to their unique way of chewing, in which they used a scissoring motion to grind tough plants between a row of teeth and bare bone.

This highly unusual way of eating may have doomed rhynchosaurs as they reached old age. A paper published last week in the journal Palaeontology by Dr. Benton and his colleaguessuggests that rhynchosaurs may eventually have ground down their teeth to nothing as they aged. Unable to bite their food, these old rhynchosaurs may have starved to death, Dr. Benton said.

Dr. Benton started working on rhynchosaur teeth while earning his Ph.D. in the 1980s. While examining their fossils, Dr. Benton noticed that the reptiles appeared to be grinding food by pushing teeth up against their jaws — a surprising technique because grinding food up against jawbones opens up animals to the risk of infection. The development of their jaws also gave adult rhynchosaurs a slope to their mouths — fixing them in a permanent “grin.”

While a few animals chew this way today — including some chameleons — scientists “don’t really understand how the heck they do it,” said Yara Haridy, an evolutionary biologist at the University of Chicago, who was not involved in the paper but reviewed it for the journal. The use of this technique is especially strange for an animal like a rhynchosaur that fed nearly exclusively on tough plants, which would have damaged their teeth over time.

“That wear and tear is not good for you, especially if you want to keep eating,” Dr. Haridy said.

Reptiles today deal with the toll of eating tough food by sprouting new teeth to replace damaged ones. But rhynchosaurs didn’t appear to do this, at least not in the same way.

To find out how the animals may have dealt with the dangers of feeding time, Dr. Benton and his colleagues used X-rays to peer into the jaws of rhynchosaurs found at a site in Devon, England. Among these fossils was the jaw of a large rhynchosaur that was probably very old when it died.

The X-rays revealed that, rather than just replacing teeth that fell out, rhynchosaurs continuously grew out their jaws. The new section of jaw would start near the back of the reptiles’ mouths, complete with new teeth. The new teeth and jaw would then shuffle forward like a conveyor belt until it took the space of the worn teeth near the front.

Yet the old and damaged area of the jaw didn’t disappear. By continuously growing out their jaws and pushing old sections forward, rhynchosaurs ended up developing a sloping, grin-like curve to their mouths. This permanent smile grew more pronounced the older they got.

But the jaw replacement didn’t work forever. A look at the old rhynchosaur revealed that it had lost nearly all its teeth by the time it died. What teeth were left were worn down nearly to the bone, which would have put the animal in a “desperate” situation, Dr. Benton said.

The find hints that rhynchosaurs that survived long enough to get old may have eventually starved to death — similar to how elephants today run out of teeth (and options) as they age. But proving this hypothesis will require searching for more evidence of starvation in rhynchosaur fossils, Dr. Haridy said.

The weird chewing tactic may also have doomed the reptiles in the long run. While the scissoring technique worked well when the world was filled with ferns, that may have changed after the climate shifted near the end of rhynchosaurs’ tenure on Earth around 225 million years ago, Dr. Benton said. The sudden dry conditions led conifers to take over as the dominant plant. And while early plant-eating dinosaurs could handle this extra-tough food, it seems that rhynchosaurs could not.

“You’ve got to be quite a specialist to be able to get a lot of nutrition from conifer needles,” Dr. Benton said. Rhynchosaurs, it seems, would “rather not have to eat a Christmas tree.”