Vegetarian Dinosaurs—With Fangs

The diets of dinosaurs have been told by their teeth. The blade-filled mouth of Tyrannosaurus rex speaks of ripping chunks of meat off prey or carrion. The blocky dentition of long-necked Apatosaurus or horned Triceratops tells of grinding up plants. In fact, for a long time paleontologists believed that each kind of dinosaur had only one kind of tooth.

But a newly found dinosaur, something its discoverer calls “a little, punk-sized critter,” speaks out of both sides of its fanged mouth. And the tale it tells is of teeth that may have little to do with eating.

Pegomastax africanus, described and named on Wednesday in a paper by the paleontologist Paul C. Sereno, was about the size of a house cat. While its cheeks were filled with plant-mulching teeth, its parrotlike beak was fronted by stabbing canines—and its body may have been covered with bristles like a porcupine.

“Imagine a light, fleet-footed chicken with fangs and spines,” said Sereno, who works at the University of Chicago. “That’s why I keep calling them little punks.” His analysis was published in the online journal ZooKeys.

The punks, which lived about 200 million years ago in South Africa, may resolve a longstanding debate among dinosaur researchers. What did their parent group, called heterodontosaurs, actually eat, and why did they upend the typical one-tooth-type dinosaur pattern? Sereno argues they were predominantly plant eaters, based on wear patterns of the cheek teeth, and the sharp fangs may have been used as threat displays when males competed for mates. Possibly they took occasional nips at even smaller prey.

“Contrary to what many people believe, feeding preferences represent a continuum rather than neatly defined categories. (Sorry, vegans!),” said Hans-Dieter Sues, curator of vertebrate paleontology at the Smithsonian Institution’s National Museum of Natural History, in an e-mail. “Most present-day plant-eaters will avail themselves of animal protein once in a while to meet metabolic needs that vegetation cannot provide.” He added that Sereno’s paper, which unites Pegomastax with similar dwarf dinosaurs in England and China, “will be the landmark study on this group of peculiar small dinosaurs.”

Other researchers, though, offer a note of caution. Randall B. Irmis, an assistant professor of geology and geophysics at the University of Utah who studies early dinosaurs like these, agrees that Sereno’s inferences about behavior are reasonable. But “hypotheses of ‘show and defense’ are generally not directly testable in the fossil record,” he wrote in an e-mail.

Sereno, though, pointed to two living animals that use a similar strategy. One is the peccary, a small piglike mammal found in deserts in the southern United States and Central and South America. It’s an herbivore, he said, but has long front teeth that it uses to fight. And the water deer, also called the vampire deer, has two large downward-pointing canines.

Another researcher, Laura Porro of the University of Cambridge, argued in a 2008 paper that the skull and tooth anatomy of heterodontosaurs indicated the creatures—which appeared soon after the dawn of the dinosaurs—were changing from carnivores to plant-eaters.

Sereno fit the jaws together from several fossils to make his case that they had already plumped for plants. “When the jaws close, the lower canines go into a pocket in the upper jaw without contacting the upper teeth,” he said. “That isn’t what you want if you’re a carnivore. It’s not good for ripping meat off another animal. For that, you want the teeth to cross each other in a kind of scissoring effect.”

The molarlike cheek teeth did slide past one another, he said, but they are blocky, not pointy rippers. “It had a self-sharpening effect,” he said, “and the wear patterns showed repeated grinding, like you would see as teeth tried to break up plants.” That, and the parrotlike beak, may indicate an animal that usually crunched things like seeds.

That predilection, despite the possibility of a rare bit of animal protein mixed in, may explain the end of this group. “It was really successful, and spread all over the globe between 200 million and 100 million years ago,” Sereno said. But then the heterodontosaurs were gone. “If they specialized in one kind of plant, and then that plant is gone, they are in trouble,” he said.

(Photo credits: Pegomastax drawing by Todd Marshall. Heterodontosaurus photo and sculpting by Tyler Keillor)