Big heads and long, heavy bodies may make carnivorous dinosaurs look fierce, but they could have gotten in the way of quick turns--and led the so-called theropod dinosaurs to walk with their tails raised upward. This controversial proposal comes from a study, to be published in an upcoming issue of the Journal of Experimental Biology, based on experiments with a backpack that gives people the mass distribution of a dinosaur.
David Carrier, a comparative physiologist, had suspected that so-called rotational inertia might have been problematic for theropods. To investigate, he designed a backpack with beams projecting 1.2 meters forward and backward. Then Carrier and his crew measured the rotational inertia of a small plastic toy model of Allosaurus and scaled it up to a 90-kg human. By adding weight to the backpack, they could increase a person's rotational inertia more than 9-fold.
The first test was to turn while jumping up in the air. Five grad students recruited from the biology department found that they could twist only 20% as far as when they jumped while wearing a control backpack with weights close to their backs. Next, nine grad students ran at top speed through a flat slalom course of six 90-degree turns. Their average velocity dropped to 77% of control runs. When the students had to place their feet in particular spots--to mimic turning on rough ground--their time fell to 65%. "It makes a tremendous difference," Carrier says. "As soon as you put the pack on, you're clearly compromised."
Carrier and his colleagues propose that one way theropods enhanced their turning ability was to keep their back arched, tail raised, and forelimbs tucked back against the body. According to Carrier's calculations, this posture would have reduced rotational inertia by 50% compared to the standard posture of a horizontal trunk and tail.
Not everyone is ready for such an about-face. "It's a completely impractical way of walking," says dino-locomotion expert Don Henderson of Johns Hopkins University. The new posture, he argues, would reduce the mechanical advantage of a tail muscle that helped power the legs. Henderson also flags several features of large theropods that would have ameliorated their rotational inertia--relatively larger toes and broader feet than smaller dinosaurs--and which aren't present in grad students.