It's the iconic dinosaur battle, seared into every kid's imagination from picture books and cartoons: Tyrannosaurus rex lunges, mouth agape, and Triceratops parries with its horns and bony neck frill. This scene probably did unfold in North American forests 65 million years ago, but new research suggests Triceratops also used its headgear in fights against its own species.
Paleontologists have proposed this idea before. It makes sense, given that other animals with horns or antlers, such as deer, use them against their own kind in battles for dominance or mating rights. The new study, published Wednesday in the journal PLoS ONE, documented wounds on Triceratops fossils, backing the idea up with hard data for the first time.
"Most previous studies have looked at one or two individual specimens," said lead author Andrew Farke of the Raymond M. Alf Museum of Paleontology in Claremont, California. "Our study is the first one to look into this in depth. The purpose is to move beyond the speculation and put some hard numbers on the biology."
Piecing together the behavior of extinct species is one of the trickier aspects of paleontology. Drawing on behavior of living animals, deducing capabilities from bone structure and biomechanical computer simulations, studying the paleoenvironment, inferring behavior from the poses of fossilized skeletons and extrapolating relationships from the proximity of fossils to one another still leaves a lot to the imagination. But in some cases, as with the Triceratops skulls, an animal's behavior leaves a discernible trace behind on the fossils.
The researchers examined skull specimens of two dinosaurs, Triceratops and Centrosaurus. Triceratops lived between 68 million and 65 million years ago, and Centrosaurus last walked the earth 75 million years ago. They're relatives, but their heads differed greatly. Triceratops had one big horn above each eye and one small one on its nose. Centrosaurus had a big nasal horn and one small bump over each eye.
Analysis of more than 400 data points revealed that Triceratops had many more wounds on one part of its frill than Centrosaurus did. Predatory attacks cannot explain the difference, the scientists argue, as both dinosaurs lived in similar habitats and faced similar predators (mainly T. rex and its cousins). And if disease were to blame, the damage would not have been so localized.
The most likely explanation, they say: Triceratops was banging heads with its own species in a way that Centrosaurus was not.
"The overall pattern says these injuries were caused by the horns of other animals," Farke said. Further, according to the authors, the frill damage is consistent with models of combat in Triceratops.
The researchers are not arguing that combat was the only purpose of Triceratops' horns and frill, however.
"I like to use the analogy of a Swiss Army knife," Farke said. "They could have been used for a variety of purposes, such as defense, combat, and display."
The ancestors of Triceratops had thin, enlarged frills and no oversized horns. This suggests the frill originally had a signaling function, say the authors, as well as a role in the attachment of jaw muscles. After big brow horns evolved, combat likely followed, spurring the frill to evolve to be thicker and tougher.
The new study shows how much scientists can learn by scrutinizing fossils for evidence of old wounds and illnesses.
"Studies like these really open the door to using paleopathology as an interpretive tool," said co-author Ewan Wolff of the University of Wisconsin-Madison's School of Veterinary Medicine. "You can learn a lot about the biology and behavior of ancient animals."
And it's a reminder that big stories are often hidden in tiny details. The frill wounds didn't exactly jump off the bone — identifying them was painstaking work.
"They're little ridges in the bone, where the bone has healed," Wolff said. "Usually things that are fairly dramatic get studied. And the things that are a bit more subtle get missed."
Elizabeth Rega, associate professor of anatomy at Western University of Health Sciences in Pomona, California, hailed the study for its large data set and proper use of statistical inference.
"There's less arm-waving in this paper than in many other paleopathology studies," said Rega, who was not involved in the research. "The authors have done a good piece of work. It's an important paper for Triceratops studies. It's an important paper for paleopathology."
Rega said the study also generated further questions. "Why do the cheekbones show the same injury rates? And why are their featured horn injuries oriented up and down, not front to back?"
Mark Goodwin, assistant director of the Museum of Paleontology at the University of California, Berkeley, was not entirely convinced by the authors' arguments.
"What they showed in their paper is evidence of differential pathologies, not evidence of combat," he said. "As far as I can tell, there's no correlative evidence between combat and lesions on the dinosaur skulls."
Goodwin suggested that the difference in size and shape of the frill bones of Triceratops and Centrosaurus could also explain the lack of similar lesions.
"This can play a role in the kinds of pathologies that occur and affect their cause, whether from injury or infection or close encounters," he said. "The hard skin or keratin covering these bones may have been different, too."
Goodwin agreed that Triceratops headgear could have been used in combat, but in his view the evidence points more strongly to a display and visual-communication function.
For one thing, he said, the horns and other ornaments were on the face, where other Triceratops could see them easily. Also, Triceratops horns and frills changed substantially as the animals aged. The horns of juveniles, for example, curved backward, while those of adults curved toward the snout. In a previous study, Goodwin suggested this switch in horn orientation likely advertised the attainment of sexual maturity.