Pachycephalosaurus, the delicately built, 15-foot-long, dome-headed dinosaur that lived alongside Tyrannosaurus and Triceratops, is a staple of children’s books and popular culture—one usually depicted as a benign plant eater. But the discovery of a new skull with the most complete jaw and set of teeth yet found for the species dumfounded scientists when it was revealed at the Society of Vertebrate Paleontology meeting in Albuquerque, New Mexico, earlier this week.

This juvenile Pachycephalosaurus, like all known specimens of its kind, had broad, leaf-shaped teeth toward the back of its jaw, suited to shredding rough plant matter, fruits, and seeds.

But in the front portion of its jaw—a part of the species never found fossilized before—this specimen bared sharp, triangular, blade-like teeth that look more like those seen in carnivores such as Tyrannosaurus and Velociraptor. It is unclear if the species had these teeth temporarily during its youth, or if they were a permanent fixture for the dinosaur.

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The fact that Pachycephalosaurus had these teeth during at least some stage of its development is fascinating, comments Steve Brusatte, a paleontologist at the University of Edinburgh in the U.K., who attended the talk in Albuquerque.

“I’ve studied [carnivorous] theropods for 15 years, and I’m pretty sure if you handed me a tooth like that, I would say that’s a theropod tooth,” he says. “It had the combination of a beak with these very sharp, steak knife-like serrated teeth … They must have been eating some kind of meat. Why else would you have steak knives at the front of your mouth?”

Dinosaur development

The discovery was presented by Mark Goodwin of the University of California Museum of Paleontology in Berkeley, who, alongside David Evans of the Royal Ontario Museum in Toronto, Canada, has unearthed and studied many new Pachycephalosaurus specimens in recent years.

Goodwin’s work last caused controversy in 2009 when he presented evidence—now widely accepted—that two species of dinosaur found in the Hell Creek Formation of the western U.S. were not, in fact, species in their own right, but actually juvenile forms of Pachycephalosaurus. These creatures, previously named Stygimoloch and Dracorex, appeared markedly different from adults. They were covered with bumps and horns as part of their complex head ornamentation, but did not yet possess the large domes of a full-grown Pachycephalosaurus, hence the previous confusion.

View Images An illustration shows what the juvenile Pachycephalosaurus might have looked like in life. Its head exhibits pronounced cranial ornaments, bumps, and nodes along the entire back surface of its flat skull. Illustration by Kari Scannella

The remarkably complete new skull of a Dracorex-like juvenile was discovered in eastern Montana and donated to the Royal Ontario Museum. It dates back 66 to 68 million years ago, in the late Cretaceous, shortly before the asteroid impact that ultimately exterminated the non-avian dinosaurs. It is one of 71 mostly fragmentary fossils that Goodwin and Evans are studying to better understand how Pachycephalosaurus developed as it grew from youth to adulthood.

At the paleontology meeting in Albuquerque, Goodwin also revealed another finding from the pair’s research: that the style and complexity of the head ornamentation of Pachycephalosaurus appears not only to have changed during an individual’s maturation, but also during the two million years of evolution recorded in the rocks of the Hell Creek Formation. This adds further complexity to the picture first put forward in 2009.

Diet du jour

Finding teeth that “look, to all intents and purposes, like theropod teeth” in the new skull was a great surprise, Goodwin says. He speculates that these animals may have been opportunistic eaters and at least partly carnivorous, perhaps changing their diet seasonally as many bears do today.

Maybe Pachycephalosaurus filled a general omnivore role, suggests Brusatte, “eating bushes and ferns, but also some small mammals, frogs, salamanders, lizards, and maybe even small dinosaurs.”

Confirming the dinosaur’s dietary preferences will require more concrete evidence of exactly what it ate, and there are several ways the scientists might go about finding that out. One way would be to do an analysis of the ratio of carbon isotopes in this pachycephalosaur’s tooth enamel. This chemical signature can provide information about the composition of an animal’s diet—as can a study of the minute pits and scratches on the surface of the teeth. Another method would be to look at bite marks on other fossil bones from the Hell Creek Formation, to see if any of them match the shape and size of the newly found teeth.

This research could have broader implications as well. Philip Currie, a paleontologist at the University of Alberta in Canada, says he is keen to reexamine numerous puzzling ‘theropod’ teeth found isolated in rocks of the same age over many years. In light of the new discovery, he wonders if these mystery teeth may in fact belong to Pachycephalosaurus.

Evans agrees: “If these teeth were found isolated from a jaw—and doubtless many have been—they could easily be mistaken for the teeth of small carnivorous dinosaurs.”

“It is especially crazy that these teeth at the front of the jaw may also have been reinforced at least partially by a beak,” Currie says. “We have always been somewhat mystified by what these animals were eating, but I think the teeth at the back of the jaws clearly show it’s an herbivore. The question is why it would need carnivore-like teeth at the front?”

Most scientists like to place dinosaurs into neat categories, says Danny Anduza of the University of California Museum of Paleontology, who has himself helped excavate several Pachycephalosaurus fossils. “What makes this study so exciting is that the authors use new evidence to challenge some of those assumptions.”

He says the work also highlights the importance of continued fieldwork. “Even after all these years of collecting, the discovery of just one new specimen can change the way we look at a dinosaur group,” Anduza says. “I think that's pretty inspiring. It's a reminder to get out into the field as often as possible, and to always look over the next hill.”