Cannibalistic Dinosaurs Went Through a Lot of Teeth

If there was a tooth fairy in the Cretaceous, dinosaurs kept it busy. Unlike humans, which lose just one set of teeth over a lifetime, dinosaurs often lost tens or even hundreds of sets.

Plant-eating dinosaurs had to chew lots of tough material to sustain their large bodies, causing them to frequently replace their teeth. But researchers were surprised to discover fossil evidence recently that showed that a carnivorous dinosaur — the only known cannibal — replaced its chompers even more frequently than some herbivores. The dinosaur’s propensity for chewing on the bones of its prey might have even contributed to its rapid tooth replacement rate, scientists hypothesized. These results were published late last month in the journal PLoS One.

The research centered on several meat-eating dinosaurs, but Majungasaurus crenatissimus was really the star of carnivorous dinosaur dentition. This roughly 20-foot-long apex predator, which lived on what is now Madagascar about 70 million years ago, left behind a particularly plentiful fossil record. Several complete skeletons, hundreds of fragmentary skeletons and tens of thousands of shed teeth have been found.

“That’s pretty unheard-of,” said Michael D. D’Emic, a vertebrate paleontologist at Adelphi University in Garden City, New York, who led the study.

He and his colleagues also studied fossils of Ceratosaurus and Allosaurus, carnivores that prowled what is now the western United States about 150 million years ago.

Dr. D’Emic and his team started by cutting 21 Majungasaurus, Ceratosaurus and Allosaurus teeth into thin slices using a diamond-tipped saw. They counted fine lines in the dentin, the erstwhile living tissue of the teeth. These lines, each about a fifth the width of a human hair, reflect new layers of tooth tissue that were laid down each day, Dr. D’Emic said. “That’s how much the tooth extended in length per day.”

By counting these lines, the researchers built a mathematical model to predict a tooth’s age based on its length.

The scientists next used computed tomography scanning to image tooth-bearing jawbones of the three carnivores. They found multiple teeth on top of one another in tooth sockets, much like nestled ice cream cones. Crocodiles and sharks continually replace their teeth in the same way.

“They’re basically stacked and ready to go,” Dr. D’Emic said.

The researchers measured the lengths of roughly 140 of these stacked teeth and estimated the ages of the teeth using their mathematical model.

By taking the difference between the ages of successive stacked teeth, Dr. D’Emic and his colleagues calculated the dinosaurs’ tooth replacement rates. Allosaurus and Ceratosaurus both had rates of roughly 100 days, meaning that, on average, a socket would get a fresh tooth about every three months.

But Majungasaurus went through teeth about twice as fast, every 56 days on average, the team found.

That wasn’t predicted, Dr. D’Emic said. All of the carnivorous dinosaurs with estimates of tooth replacement rates measured to date, albeit only three genera, had rates topping 275 days. Tyrannosaurus clocked in at 777 days.

“It’s really surprising to see such elevated tooth replacement rates in a meat-eating dinosaur,” said David Evans, a vertebrate paleontologist at the Royal Ontario Museum in Toronto who was not involved in the study. “This is on the level of what we’d expect to see in bulk feeding, high-fiber herbivores.”

Majungasaurus was clearly an outlier, Dr. D’Emic and his collaborators concluded. But why?

To answer that question, the researchers turned to bones. Fossil records from Madagascar have revealed scratches and gouges on the bones of other dinosaurs that match the tooth spacing of Majungasaurus. It’s likely that Majungasaurus crenatissimus was chomping down on the bones of its prey, which included its own species.

“Their teeth were contacting bone a lot,” said Dr. D’Emic. “They were wearing their teeth out so quickly that they were having herbivore-like replacement rates.”

These results shed light on Majungasaurus’ diet and help “breathe life” into a long-extinct animal, said Dr. D’Emic. “We’re always learning things about dinosaurian biology that surprise us.”