NEWS RELEASE, 09/14/98
Common picture of head-butting in dome-headed
dinosaurs is wrong, says UC Berkeley paleontologist
By Robert Sanders, Public Affairs
BERKELEY -- We've all seen the picture. Two dome-headed dinosaurs, heads lowered, charging one another like rams in rut.
It's a cute theory, says University of California, Berkeley, paleontologist Mark Goodwin. But it's wrong.
"Based on new fossils that include the bones from the top of the head, it seems unlikely these animals used their domes for head butting," Goodwin said. "They would have killed themselves."
The dome-headed dinosaurs, lumped together as pachycephalosaurs, may have pushed one another with their domed heads, he said, or even butted one another's sides like bison - a behavior known as flank butting. But the idea that they butted heads like bighorn sheep is mistaken.
"It's time to kill the myth," he said.
Goodwin, principal scientist at UC Berkeley's Museum of Paleontology, and colleagues Emily Buchholtz, now at Wellesley College, Mass., and Rolf Johnson of the Milwaukee Public Museum, reported their findings in the June issue of the Journal of Vertebrate Paleontology.
Pachycephalosaurs lived during the Cretaceous period, between about 130 and 65 million years ago, and tended to be small to medium-sized plant eaters. Ranging from three to 15 feet long, they walked on two legs and had a long, stiff tail for balance.
The myth about head-butting apparently arose from an admitted "very wild surmise" by well-known American Museum of Natural History paleontologist Edwin "Ned" Colbert in 1955, sparked by a superficial resemblance between the skulls of pachycephalosaurs and bighorn sheep. The idea took on a life of its own and generated many dramatic depictions in lay books on dinosaurs. Many paleontologists accepted the theory as well.
The problem with this scenario, Goodwin says, is that the skulls of pachycephalosaurs are not really built like the skulls of other animals that head butt. Bighorn sheep, for example, have a broad head, spreading horns and air-filled (pneumatic) chambers in the skull to absorb head-to-head blows; a strong neck; and horns that get entangled so that contact becomes a feat of strength with pushing and shoving. The well-developed horns also serve to correct for any misalignment during head-butting, and reduce the chance of flank-butting by getting entangled and tying up the animals.
Pachycephalosaur skulls display a range of characteristics, but none seem adapted to head-butting with a minimum of damage to the combating parties. For example, in many pachycephalosaurs the dome is high and narrow, or round like a bowling ball, making it likely two animals making head to head contact would cause serious injury or death.
Goodwin had long suspected that the head-butting idea was wrong, so he set out to analyze known fossil pachycephalosaurs to test this hypothesis, using in particular samples from the large collection in UC Berkeley's Museum of Paleontology. Unfortunately, most North American pachycephalosaurs are known only from their skulls and nothing more.
One crucial fossil he studied was a new skull of Stygimoloch spinifer from the Hell Creek Formation of North Dakota, found in 1987 by field crews from the Milwaukee Public Museum. It is the most complete specimen discovered to date, Goodwin said.
Dating from the Upper Cretaceous - approximately 65 million years ago - the specimen includes not only the bony dome but also three to four large horns along the rear edge of the dome, which is also ornamented with clusters of bony knobs.
This skull showed definitively that the horns point backward, making them unsuitable for fighting. More likely they were used in display, to protect the neck, or for flank butting, Goodwin said.
A fossil skull of another pachycephalosaur, Stegoceras, had no air spaces that could have
provided a cushion against damage during head butting. In addition, the bone structure is highly vascular, as shown by a honeycomb structure of primary bone and vascular canals perpendicular to the surface of the dome. This would tend to focus the energy into the brain - practically guaranteeing a concussion.
Furthermore, there was no evidence of remodeled bone from any healed injuries on any of the skulls examined.
"The bones behind the skull are nothing like what we see in bighorn sheep," Goodwin said, "so the analogy that they used their heads in that manner is not a very good one. Just because an animal looks like it might have engaged in some kind of behavior doesn't mean it did."
Based on microscopic examination of fossilized bone tissue from the dome of a Stegoceras, the authors concluded that the dome probably grew throughout the animal's lifetime, rapidly at first, and later slowing down as it thickened with age. In fact, many specimens thought to be from a different genus of flat-headed pachycephalosaur could well represent juvenile members of the thick-domed species.
In further work, Goodwin and paleontologist Jack Horner of the Museum of the Rockies have conducted a microscopic analysis of thin sections of the skulls of six specimens representing at least three different genera of North American pachycephalosaurs, and will report their findings at a meeting of the Society of Vertebrate Paleontology later this month in Snowbird, Utah.
"If you look at the skulls of pachycephalosaurs under high magnification, they don't have the structure to act as good shock absorbers," Goodwin said." It doesn't appear to offer a biomechanical advantage, and that's what we're going to look at next."
The research was funded by the Museum of Paleontology at UC Berkeley.
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