BILLINGS - 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.

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' differences and sexual maturity to other dinosaurs, 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.

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