With its head lowered and legs pounding, Tyrannosaurus rex gave the stars of Jurassic Park a run for their lives in a speeding jeep. Now a new biomechanical model suggests that a 6000-kilogram Tyrannosaurus could not have packed enough muscle into its legs to hustle faster than about 40 km/h. Although the finding doesn't change ideas about Tyrannosaurus's hunting ability--in its own time--paleontologists say the study sets a new standard for biomechanical analysis of an extinct organism.
John Hutchinson, a postdoc at Stanford University, and Mariano Garcia, now at BorgWarner Automotive in Ithaca, New York, designed a simple model of tyrannosaur leg bones to figure out how much muscle would be required to balance forces and keep the dinosaur on its feet. To test the model, the researchers studied the closest living relatives of dinosaurs: reptiles and birds. Hutchinson dissected a chicken and an alligator and weighed their muscles. The model correctly predicted that a chicken would have more than enough leg muscle to sprint, while an alligator couldn't run on two legs.
Hutchinson then studied Tyrannosaurus bones and ran the model. It suggested that in order to run, a tyrannosaur would have needed to carry 86% of its body mass as extensor muscles in its legs, the authors report in the 28 February issue of Nature. Hutchinson and Garcia estimate that the fastest a tyrannosaur could travel was 40 km/h. "Even that is pushing the limits of credulity with very generous assumptions," Hutchinson says. But even without sprinting, a tyrannosaur would still have been able to hunt, Hutchinson and other paleontologists say. Large prey such as duckbilled dinosaurs and Triceratops would have been limited by the same factors and probably couldn't have run fast either.
Why does speed matter? Once an upper limit is established, Don Henderson of the University of Calgary in Alberta notes, paleontologists can put a cap on ecological questions such as how much territory a tyrannosaur could patrol in a day. Hutchinson says the technique of calculating minimum muscle mass could be used to answer other questions, such as whether sauropods or pterosaurs could walk bipedally. "This is one of the most sophisticated studies on dinosaur locomotion ever," says Greg Erickson of Florida State University, Tallahassee.