Berkeley -- The gnawed remains of a 70 million-year-old victim of Tyrannosaurus rex have provided the key to how powerful the dinosaur's bite really was. Gregory M. Erickson, a graduate student in biology at the University of California at Berkeley, teamed up with engineers at Stanford University to estimate the force that created the punctures and tears in the fossilized pelvis of a hapless Triceratops discovered in Montana a few years ago. They found that the ferocious beast could exert between 1,440 and 3,011 pounds of force, greater than the crushing force of any known creature though close to the maximum force exerted by the American alligator, a dinosaur relative.
"This is like the weight of a pickup truck behind each tooth," Erickson says. The estimate is for a bite during feeding, which typically is less forceful than higher velocity snapping bites such as those used by alligators to seize prey. The new evidence refutes an argument made by some scientists that Tyrannosaurus rex was primarily a scavenger because its teeth were too weak to attack live prey. "Their teeth were as strong as those of the alligator, a predator that frequently has to deal with struggling prey," Erickson says. "From a comparative standpoint it appears that T-rex was equipped to struggle with its prey too."
While this is not proof that T-rex was primarily a predator, he says, it does show that when confronted with prey the dinosaur could have held a death grip with its powerful teeth. "We contend that if T-rex could consistently engage prey with its teeth, it could have exploited a predatory niche," Erickson says.
Erickson and his colleagues, including Dennis R. Carter, professor of biomechanical engineering and director of the Biomechanical Engineering Program at Stanford University and an expert on bone mechanics, will publish their findings in the Aug. 22 issue of the British journal Nature.
The 4 1/2-foot-long Triceratops pelvis had 58 definite bite marks and 22 probable bite marks that could only have been made by a T-rex, Erickson says. A cast of one of the punctures was an exact replica of the canine-like tooth of an adult Tyrannosaurus, and distinctive serrations like those on the cutting edges of T-rex teeth could be seen where the teeth had scraped the bone surface. Many of the bites were furrows produced by what Erickson calls "puncture and pull" biting. These are produced where the teeth penetrated the bone and ripped back through the carcass with all the force of the T-rex's 10,000 pounds. Living Komodo monitors use a similar feeding strategy to deflesh their victims. The T-rex obviously gnawed the pelvis for a while, pulverizing the vertebrae and probably severing the pelvis from the body, according to Erickson.
Erickson, who recently completed his MS at Montana State University with paleontologist Jack Horner, and amateur fossil hunter Kenneth H. Olson of Lewiston, Mon., described the specimen earlier this year in a paper in the Journal of Vertebrate Paleontology. Olson discovered the lone pelvis in 1991 in the famous Hell Creek Formation, which has yielded many T-rex and Triceratops fossils, as well as scads of fossilized duck-billed dinosaurs. The specimen now resides at the Museum of the Rockies in Bozeman. They argued that these bite marks and also some tooth marks in a duck-billed Edmontosaurus were the most definitive T-rex tooth marks to date, demonstrating that the Tyrannosaurus fed upon the most prevalent large herbivores in the area sometime between 70 and 65 million years ago.
After moving to UC Berkeley to study evolutionary morphology with Marvalee Wake, professor of integrative biology, Erickson consulted biomechanical engineer Dennis Carter about a way to estimate the force behind such impressive puncture wounds. He suggested that graduate students in a biomechanics course he teaches at Stanford should tackle the problem, so Erickson joined forces with two of them, Samuel D. Van Kirk and Jinntung Su. Consulting too with engineer William E. Caler and Stanford teaching assistant Marc E. Levenston, the team found bone that matched in both growth form and microstructure that of Triceratops -- a cow's pelvis -- and made precise measurements of the amount of force required to make an 11.5 mm (half-inch) deep puncture. The role of the tooth was played by a bronze and aluminum cast of a real Tyrannosaurus maxillary tooth, which was placed in a hydraulic loading machine to impact the cow bones. Breaking only one bronze tooth in the process, they determined that the T-rex would have exerted the most force at maximum penetration -- about 1,440 pounds for the tooth that produced the 11.5 mm deep puncture. This tooth was from the front half of the Tyrannosaurus jaw. Greater forces would have been exerted simultaneously by teeth further back in the mouth nearer the jaw hinge, however. From the data they calculated this force to be 3,011 pounds. For comparison, a human exerts a maximum force of about 175 pounds with the rear teeth, an African lion about 937 pounds, and an alligator slightly less than 3,000 pounds. Because the penetration test was performed slowly, the force estimates are less than would be expected from a really fierce bite, Erickson notes.
While the data give more weight to the argument that Tyrannosaurus was a predator, definitive proof will be hard to find, he says. Reports of T-rex bite marks on bones are rare, for one thing, though Erickson thinks they are more common than it appears. People have just not looked for them, he says. "Now that people are collecting more partial skeletons, ones that are more likely to have been exploited by carnivorous dinosaurs, they are beginning to find lots of bite marks on herbivores," he says.
"I do not believe there is enough evidence in the fossil record at this point to definitely say that T-rex was predominantly a predator or scavenger," he adds. "It's very difficult to find evidence of behavior in the fossil record."
One strategy, though, would be to look for evidence that T-rex had a stereotyped attack pattern, much the way mountain lions will attack the back of the head or neck in order to sever the spine. Such evidence might be sought in bite marks on fossil carcasses, and in healed bite marks indicating a failed T-rex attack. If these marks are found to cluster around the same area of the body it could provide definitive proof that T-rex stalked and attacked herbivore dinosaurs, Erickson says.