Although Tyrannosaurus rex's foot-long, serrated teeth soak up almost all attention, more sophisticated dinosaur dentistry fans should be looking to the humble triceratops, whose teeth are so complex that even their fossils are still self-wearing, even as the animal that used them is long-extinct.
Researchers from biology, paleontology and mechanical engineering collaborated on a paper, published Friday in the journal Science Advances, that found evidence that the teeth on the triceratops are more complex than those found in reptiles or mammals today. Their teeth were self-wearing—by using them, triceratops wore them into chewing shape—and surprisingly enough, the fossils of those teeth still are capable of getting worn back into functional chompers.
"If you took those [fossil] teeth out today and put them in a dinosaur or in an analogous animal today, they will work," Gregory Erickson, a paleobiologist and co-author of the study, told me. "They will self-wear back to functionality."
Reptiles today have teeth for seizing and crushing, but the triceratops had teeth for chewing. It may seem like a small thing, especially for mammals, who have now cornered the market on mastication. But check that mammalian privilege: Chewing allows animals to "orally process" plants and turn them into smaller, more digestible pieces, allowing the chewer to get sustenance much more efficiently. It also allows the chewer to mince up plant matter that other reptiles can't.
"The animals that do this—the hadrosaurs, the ceratopsians, the sauropods—the were some of the most successful dinosaurs," Erickson said over the phone, "so we think part of their success is that they were able to exploit different kinds of plant matter that their competitors couldn't."
He explained that the most complex teeth found today belong to horses and bison, comprising four layers of tissue. Triceratops, he found, had five layers of tissue, and had hollows running down the side, like "fighting knives," Erickson said. Triceratops teeth don't just grind food, they slice it too.
"As you go across the slicing face, different layers of the teeth are exposed in different combinations," he said. "Those combinations make it possible for the teeth to self wear and shape into fuller-like structure in triceratops, like a blade with a recessed middle to it."
"Mechanically that's a wonderful thing to do, because it's like fighting knives and swords with a fuller down the side as well," Erickson added. "It cuts down on the friction while they're cutting through something and also helps them release from the plants with each chewing stroke so they can cut very efficiently. So that's what's pretty cool about triceratops."
Interestingly, the fossils that Erickson studied still retained this ability to self-wear.
"I started cutting up dinosaur teeth and starting finding other tissues there, but when I got to the polishing stage, to look at them under the microscope, I realized I couldn't make them flat again," he said. "I started polishing them—which mimics an abrasive pad—and they would start self-wearing back to their functional morphology. And I realized the properties must still be there… in these 66-million-year-old teeth! People think of fossils as turned to stone but what we're finding is that a lot of these teeth still preserve enough of their original mineral content that they preserve these properties."
The project wasn't just for paleobiologists. Erickson worked with engineers who study tribology, the science of how materials interact while in motion. They ran diamond tips on the teeth to study how the teeth wear down, and also developed computational models to replicate the teeth at work. Erickson said the group told him that the teeth were "the most complex thing they'd ever seen," and that it required them to make a new computational algorithm for three-dimensional wear.
"This is something they don't really do in industry, but instead of looking at it as something that hasn't been done before, they looked at it as a challenge," he said. "I was fortunate they were intrigued."
It also means that the results have implications beyond biology as well.
"One of the cool things that's come out of this is that we've followed this up with engineering papers and presentations to introduce the three-dimensional wear modeling that they've developed, letting the industry know that it's possible to do these incredible three-dimensional models," Erickson said. "It's the rare case of paleontology introducing something that can be applied in industry."