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Supposed head-butting dinosaurs didn't

– Dome-headed dinosaurs widely depicted as butting heads in frenzies of male aggression almost certainly did not go head to head, according to a new study by paleontologists at the University of California, Berkeley, and the Museum of the Rockies.

The dinosaurs, known collectively as pachycephalosaurs, impressed early fossil hunters with their bowling-ball shaped heads, and a 1956 science fiction story popularized the notion that they battered one another much like big horn sheep.

 Stegoceras, a small pachycephalosaur
Pachycephalosaurs could have had colorful head ornaments, such as a horny cover, for use in communication or for sexual display. Pachycephalosaurs traditionally have been depicted with dome-shaped heads, like the life restoration (inset) from the Royal Tyrrell Museum in Drumheller, Alberta, Canada, of a Stegoceras, a small pachycephalosaur that lived 75 million years ago in what is now Montana.
 

But dinosaur diggers Mark Goodwin of UC Berkeley's Museum of Paleontology and John Horner of the University of Montana's Museum of the Rockies assembled seven fossilized skulls of several closely related species of pachycephalosaur to critically test the arguments for the hypothesis.

And in a paper appearing in the spring 2004 issue of the journal Paleobiology, they conclude that the radiating structures associated with the spongy interior of the domes, which some have argued allowed the dinosaur to absorb the power of a head-butt, are actually typical of fast-growing bone and are found only in young pachycephalosaurs, not adults.

"The porous areas of the dome are associated with vascular tissue, and some have argued that these radiating structures gave a biomechanical advantage for head butting, or perhaps were used in temperature regulation," said Goodwin, a UC Berkeley vertebrate paleontologist. "But this tissue disappears in adults, who are the ones who would have head-butted. The truth is, these structures were involved in the development of the dome and were not built for head-butting."

Goodwin and Horner not only toss out the idea of head butting, they also argue that pachycephalosaurs probably had a horny head covering or attached head ornamentation like a rooster comb, possibly brightly colored and likely used in species recognition or as a visual cue, much like their descendents today, the birds.

Fossil skull of Stegoceras
Fossil skull of Stegoceras showing the high-domed head and rear crest. (Credit: University of Alberta)
 

"We don't know what the final shape of the dome was. The dinosaur could have been a cone head," Goodwin said. "But if you look at their closet relatives, the birds, pachycephalosaurs could have had similar behavior and skin coloration."

Pachycephalosaurs were small to medium plant-eating dinosaurs - Stegoceras from 75 million years ago was around three feet tall - that walked on two legs and maintained their balance with a long tail. Later relatives, like Pachycephalosaurus from slightly younger sediments, some 68 million years ago, may have been as long as 15 feet. Fossils have been found from the western interior of North America to Central Asia and China, all dating from the Late Cretaceous, about 65 to 85 million years ago. Pachycephalosaurs lived right up to the end of the Age of Dinosaurs at the end of the Cretaceous Period, 65 million years ago, when all dinosaurs became extinct.

In 1998, Goodwin and colleagues first disputed the head-butting theory in a paper in the Journal of Vertebrate Paleontology, based on comparisons of the skulls of pachycephalosaurs and bighorn sheep and a detailed analysis of several dinosaur skulls. They also pointed out that the few known skeletons of pachycephalosaurs are slight and gracile and not suitable for absorbing the energy of a head blow. Perhaps the creatures flank-butted, like bison today, Goodwin concluded at the time, but head-to-head combat would have killed them.

To further investigate the head-butting theory, Goodwin and Horner decided to look at bone development during pachycephalosaur growth, and assembled seven dinosaur specimens housed in their respective museums, all of them from the Upper Cretaceous Hell Creek, Two Medicine and Judith River formations of Montana. Though from different species, the specimens are closely related and clearly represent a growth progression from juvenile to adult.

When they sliced through five of the fossil skulls to look at their internal structure, the youngest juveniles with nearly flat skulls showed mostly porous bone in the dome, characteristic of fast-growing trabecular bone with lots of blood vessels. The thickness of this layer decreased in older juveniles - those showing incipient bulges at the front of the skull - and in subadults with a bulge covering the entire dome. An adult pachycephalosaur sporting a high, egg-like dome had only a thin layer of spongy tissue, with most of the outer layers of the dome made of dense bone with very few pores.

"The young have a very porous dome that gets filled in as the dome develops," Goodwin said. "But pachycephalosaurs are unique animals that develop differently from other animals, and their skulls do not resemble the skulls of any mammals or animals that engage in head butting. In fact, the bone tissue in the adult pachy skull we sectioned shares characteristics with some fish bone that is also acellular, that is, has few or no cells."

Thus, the fibrous structure of the domes of young and subadult pachycephalosaurs that led some paleontologists to conclude that the domes were built to withstand head-to-head combat or regulate temperature "is actually an expression of the growth mechanism that produced the doming of the pachycephalosaurid skull during ontogeny," the researchers wrote.

The paleontologists also concluded that the different dome shapes found among pachycephalosaur fossils likely represent individuals of different ages rather than of different sexes or species.

Looking more closely at the 150 micron (six-thousandths of an inch) slices through the skulls, the two scientists noted fossilized collagen called Sharpey's fibers that typically are found in bone that has an external covering. This suggests that pachycephalosaurs sported a head covering.

"The Sharpey's fibers, which look like fine hairs in the fossil, were collagen fibers that anchored some kind of attachment to the bone, such as the comb of a rooster or the horny covering on the skulls of some species of horned-dinosaurs like Triceratops or even of Tyrannosaurus rex," Goodwin said.

The fossil growth series indicates that in young pachycephalosaurs, bony tubercles or nubbins covering the flat skull anchored scales on the head. As the animal grew, the tubercles merged together on the skull as the dome inflated, first in the front (frontal bone) and eventually the back (parietal bone), at which time the scales fused into a kind of helmet adorning the skull. Such ornamentation, comparable to the horns of African antelopes, was probably used in communication or recognition and maybe sexual display, they concluded. And like birds such as the toucan or cassowary, the ornamentation probably was brightly colored and maybe even changed color with the seasons.

Goodwin has begun to use high-resolution CT scans to look inside fossils that are too valuable to slice open, and is finding confirmation of the team's conclusions.

"By focusing on what the skulls tell us, testing the ideas in a phylogenetic and functional context, we have falsified the evidence in many other studies that they engaged in head butting," Goodwin said. "This shows we can get a lot more out of the fossil record than just the morphology."

The work was supported by Nathan Myhrvold and the Hell Creek Project, the Charlotte and Walter Kohler Charitable Trust, the Museum of the Rockies, UC Berkeley's Museum of Paleontology and UC Davis's Department of Geology.