Web Extra Tuesday, June 15, 2004

Domed dinos made love not war

In the 1950s, paleontologists hypothesized that dinosaurs with skulls shaped like bowling balls butted heads, much like sheep or other modern horned animals might. Called "battering rams" by one researcher, dome-headed dinosaurs may have head-butted to compete for territory or mates, an idea that was further popularized in a science-fiction story at the time. But a recent reassessment of some of these fossils revealed something quite different: that the spherical skulls could be divided by age instead of gender, calling into question these animals' presumed habits.

Pachycephalosaurs may or may not have butted heads with each other, depending on what their skeletons say.

The rounded heads, radiating bone structures and the spongy bone composition in the skulls of animals called pachycephalosaurs fueled speculation about their lifestyle. However, in an analysis of a group of skulls collected in Montana containing different species of pachycephalosaurs, Mark Goodwin of the University of California Museum of Paleontology in Berkeley and Jack Horner of the Museum of the Rockies at Montana State University in Bozeman showed that younger pachycephalosaurs had the bulging crania with spongy inner skulls and radiating bone structures, but adults did not.

"We have a growth series," Goodwin says. "We're able to show that they're gone — the structures are absent in adults." Using high-resolution scans of thin sections, the researchers also can show bone sutures and where the adults' brains made contact with the skulls — but no cushioning cavity that is generally present in modern head-butting animals.

Instead, write Goodwin and Horner in the spring issue of Paleobiology, the older specimens from the community had a thick layer of tiny mineralized collagen fibers, called Sharpey's fibers. The fibers tend to anchor skin coverings or other kinds of integument of modern animals, such as the beaks of toucans. So rather than butting heads, the authors hypothesize, pachycephalosaurs had head structures with coverings that were used for other kinds of displays, identifying them to their own species mates or reflecting light through attached horn material (to potential mates in presumably attractive ways) to signal their sexual identity or maturity.

The lightness of the vertebrae and the rest of the adults' skeletons, Goodwin says, also indicate that the creatures may not have been able to survive an impact from a head butt. "Bone is a dynamic tissue and remodels itself in response to stress," he says. "We didn't see any evidence of increased remodeling" in the skulls or hind legs of several skeletons previously examined that would indicate a response to impacts.

"I'm not absolutely convinced," says Peter Galton, a paleobiologist at the University of Bridgeport in Connecticut, who helped to establish the pachycephalosaurs' "battering ram" image in his work in the 1970s. Galton says that the work is useful, particularly in showing that the radiating bone structures are actually age-related characteristics. Nevertheless, he says, "there are a lot of other features used to argue for head bouncing or flank butting. Those are not really addressed."

Galton says that other characteristics "don't make sense if they are just a display structure," such as shortening of the head case. He also points out that the back of the skulls of pachycephalosaurid dinosaurs would serve as "a large plate for the attachment of enlarged neck muscles," and that the vertebrae and hip joints are well-placed to withstand impacts, in both males and females.

Nonetheless, Galton says, a range of behaviors may have been possible, including the use of dome and horns for display only. And, he says, collisions "head-on may be a little extreme."

Naomi Lubick

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