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No Ifs, Ands or Butts About It Paleontologist Dispels Myth that Dome-Headed Dinos Used their Heads as Battering Rams By Robert Sanders, Public
Affairs 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 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 that these animals used their domes for head butting," Goodwin said. "They would have killed themselves." The dome-headed dinosaurs, classified 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 the Museum of Paleontology, and colleagues Emily Buchholtz, now at Wellesley College in Massachussettes, and Rolf Johnson of the Milwaukee Public Museum reported their findings in the June issue of the Journal of Vertebrate Paleontology. The plant-eating Pachycephalosaurs lived during the Cretaceous period, between about 130 and 65 million years ago. 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 admittedly "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 over the years. The problem, Goodwin says, is that the skulls of pachycephalosaurs are not built like the skulls of other animals that head butt. While bighorn sheep, for example, have strong necks, broad heads, spreading horns and air-filled chambers in the skull that can absorb head-to-head blows, Pachycephalosaur skulls are high and narrow, or round like a bowling ball, revealing no adaptations to minimize head-butting damage. Goodwin had long suspected that the head-butting idea was wrong, so he analyzed known fossil pachycephalosaurs to test this hypothesis, using in particular samples from the large collection in 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. It is the most complete specimen discovered to date, Goodwin said. Dating from the Upper Cretaceous period -- 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 head-butting cushion. In addition, the bone structure is highly vascular, a honeycomb structure of primary bone and vascular canals perpendicular to the dome's surface. This would tend to focus the energy into the brain -- practically guaranteeing a concussion. Furthermore, none of the examined skulls revealed evidence of remodeled bone from healed injuries. "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. "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."
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