Three dinosaurs that were thought to belong to very different groups are actually kids, teens and adults of the same genus, researchers say.
“I made a brand new dinosaur hall at the museum three years ago, and now I have to change it,” says Jack Horner of the Museum of the Rockies in Bozeman, Montana, an author of the study. “We’ve named twice as many dinosaurs as there probably are.” Horner and his colleagues report the findings in the October 28 PLoS ONE.
The trio falls under the umbrella of pachycephalosaurs, or thick-headed dinosaurs. They were neighbors in both place and time: All three hailed from what is now the Hell Creek rock formation in Montana and South Dakota, and all three died around 65 million years ago.
Pachycephalosaurus wyomingensis, the first discovered pachycephalosaur, was found in 1931. It had a large domed skull that some paleontologists suggested was used to butt heads in dominance displays. Stygimoloch spinifer (meaning “horned devil from the river of death”) was first described in 1983 and had a smaller, narrower dome and menacing spikes protruding from the back of its head. A newcomer called Dracorex hogwartsia (“the dragon king from Hogwarts,” for the wizard school in the popular Harry Potter books) emerged in 2006. It lives up to its name with horns dotting its flat, long snout.
These distinctive bumps and knobs were supposed to differentiate closely related genera, the taxonomic category above species. “Everybody would agree they’re all related,” Horner says. He and his colleagues now suggest the three are actually all in the same genus.
But earlier statistical studies of duck-billed dinosaurs found in western Canada had suggested some “closely related” species were actually just juveniles and adults of the same species. Horner and his team suspected the same of pachycephalosaurs.
To test this idea, the team cut open bones and skull fragments from Dracorex, Pachycephalosaurus and Stygimoloch to see how old the dinosaurs had been when they died.
“It’s real easy to tell young bone from old bone,” Horner says. Young, fast-growing bones need a lot of blood, so they are full of pores and canals to let blood vessels through. More mature bones are denser.
Bones from Pachycephalosaurus were dense and canal-free, but Stygimoloch and Dracorex both had spongy, porous bones.
The team concluded that Pachycephalosaurus was a grown-up, Stygimoloch was a teenager and Dracorex was the youngest.
“We haven’t found anything that would be called a Pachycephalosaurus that has juvenile bones, and we haven’t found anything that anyone would call a Stygimoloch or a Dracorex that has mature bones,” Horner says.
Researchers have examined the interiors of very few dinosaur bones before now, he adds. “In the past, people have been very reluctant to cut open skulls and look at their bone histology,” he says. “This will show them you can get an awful lot of information from doing that.”
The team encountered this reluctance first-hand. The only complete Dracorex skull ever found is in the Children’s Museum of Indianapolis, and Horner’s team couldn’t get access to it. All the team’s data on Dracorex is based on other bones of that species found in Hell Creek and on comparing skull shapes among the trio.
Horner’s group had skull fragments of all three dinosaurs from field work in the Hell Creek formation. But the team also wanted to use more complete skulls that belonged to private collections or museums. Those places wanted to keep the skulls intact, but Horner got permission to run CAT scans on Pachycephalosaurus and Stygimoloch skulls. All together, the team was able to examine 21 skulls and fragments.
The scans revealed that the bones that make up the left and right halves of Stygimoloch’s dome weren’t fused together, further supporting the idea that the animal is a youngster.
Horner had previously found a similar pattern in Triceratops: the telltale horns curve backward in youths and forward in grown-ups. He expects that a lot of supposedly different dinosaurs with varying skull shapes could turn out to be the same.
“Just using bone histology, we can probably reduce the number of species by a third,” he says.
Dividing dinosaurs by age can help paleontologists figure out how the creatures behaved. Horner thinks the head-butting theory, for instance, is “all a bunch of crap.” Pachycephalosaur kids and adults had wildly different skulls so that they could tell who to protect and who to mate with, he suggests.
The long spikes sported by teenagers might have been useful when the dinosaurs were trying to attract mates, for example, but less so when they got older.
“In the sub-adult phase they go through extreme expression of all these ornaments, bumps and knobs,” says Mark Goodwin of the University of California Museum of Paleontology in Berkeley, a coauthor on the paper. “But as they become adults, they disappear. They don’t have the flashiness that they need when they’re adolescents.”
The idea generally makes sense to Hans-Dieter Sues of the National Museum of Natural History in Washington, D.C., who named Stygimoloch, but he has reservations. In particular, he’s worried about the idea of horns appearing to grow and shrink over time.
“It’s unusual that they’d develop a structure like that and then in the lifetime they’d get rid of it again,” he says. “If this were a legal case I don’t think the evidence would be compelling. But it’s certainly an interesting suggestion, which can now be tested with additional fossils.”