In the dinosaur world, paleontologists are pretty protective of the species they've named.

So Jack Horner said he's not making any friends with his newest theory that many species of dinosaurs may actually be juvenile or subadults of the same species. In his conservative estimate, the theory could mean the loss of up to a third of classified dinosaur species.

"There are skeptics out there, as usual, which is good," said Horner, paleontology curator at the Museum of the Rockies in Bozeman. "But we're in the business where skeptics have to have evidence. We have a lot of evidence for our hypothesis."

It's not the first time Horner has ruffled the staid halls of museums. He was the first to document that dinosaurs cared for their young after uncovering a nesting site near Choteau, and he popularized the idea that Tyrannosaurus rex was more scavenger than killer.

Dome-headed science

Horner recently published the research he co-authored with Mark Goodwin of the University of California Berkeley in the online journal PloS One. The paper focused on dome-headed dinosaurs that Horner and Goodwin theorize went through some wildly different head hardware at different stages of their lives.

"I have to admit it seems pretty weird that they go through such a drastic change," Horner said. "But we're seeing it across a wide range. Basically, all the fancy dinosaurs are doing this."

The reason for the changing ornamentation was to signal species difference to other dinosaurs and sexual maturity of dinosaurs in a social setting, much like feathers on birds, the researchers posit.

In examining the skulls of Pachycephalosaurus wyomingensis, Stygimoloch spinifer and Dracorex hogwartsia, including the use of CT scans that provided microscopic analysis of slices of bone, Horner and Goodwin decided they were all the same species at different stages of life. The Stygimoloch, a species discovered by UC Berkeley paleontologists in Montana in 1973, was a subadult Pachycephalosaur, they decided.

"We had pretty good ideas this was going on but the CAT scan really nailed it," Horner said. "Before the CAT scan we could only guess at it, or you could drop it on the floor."

The Dracorex, based on morphological analysis since it was not available for a scan, was ruled a juvenile Pachycephalosaur. And the Pachycephalosaur was categorized as the full-grown dinosaur.

"The strength of it is there are multiple lines of evidence," Goodwin said.

The scientists say the change in bone configuration on the dinosaurs' head is a matter of redistribution, not bone loss.

"Bone is a very dynamic tissue," Goodwin said, noting that if it weren't, broken arms would not mend.

Bone skeptics

Paleontologist Bob Bakker, who discovered the Dracorex in South Dakota in 2006, was quoted in a 2008 blog article saying the men's theory didn't hold up to scrutiny. But that was before their research paper was complete. Bakker couldn't be reached for comment on the new research.

"These dinosaurs aren't doing anything more drastic than modern mammals," Horner said, noting that deer and elk grow and lose their antlers.

Previously, their research also found that triceratops' horns change over their lifetime after the discovery of juveniles. In juvenile triceratops, the horns grew backward. In adults, they grew forward.

The evidence came from 11 years of digging in the Hell Creek formation near Fort Peck Reservoir and collecting all of the fossil evidence they possibly could find. In total the collection contains about 60 triceratops skulls, many of them juvenile.

"People have the tendency to look at the difference between things rather than the similarities," Horner said.

When Horner examined the collection of Hell Creek fossils with such a critical eye, he said the number of species dropped by almost one-half.

"People have to re-evaluate things looking at growth," he said.

"It also stresses the importance of field work," Goodwin said. "When you have these collections, then we get these assemblages and can see the trend here."

Contact Brett French at french@billingsgazette.com or at 657-1387.