CARRIER, D.R.*; LEE, D.V.; WALTER, R. M.: INFLUENCE OF ROTATIONAL INERTIA ON THE TURNING PERFORMANCE OF THEROPOD DINOSAURS.

Turning agility of theropod dinosaurs may have been severely limited by the large rotational inertia of their horizontal trunks and tails. Bodies with mass distributed far from the axis of rotation have much greater rotational inertia than bodies with the same mass distributed close to the axis of rotation. In this study, we increased the rotational inertia of human subjects 4.6 times, to match our estimate for theropods the size of humans, and measured the subjects’ ability to turn. To determine the torque required to execute turns, three subjects performed 45 degree jump turns on a force platform. When the rotational inertia was increased 4.6-fold, the time to push-off increased 1.8-fold and the torque impulse applied to the ground increased 3.76-fold. To determine the effect of the increased rotational inertia on maximum turning capability, five subjects performed jump turns in which they jumped vertically from a standing position and attempted to spin as far as possible before landing. This test resulted in a 4.9-fold decrease in the angle turned. We also tested the ability of three subjects to perform sharp running turns in a tight slalom course of six 90 degree turns. When the subjects ran with the 4.6-fold greater rotational inertia, the time to complete the course increased by 34 %. Hence, the results from these tests suggest that rotational inertia may have limited the turning performance of theropods. Characters such as retroverted pubes, reduced tail length, decreased body size, pneumatic vertebrae, and absence of teeth reduced rotational inertia in derived theropods and may have improved their turning agility. To reduce rotational inertia, theropods may have run with an arched back and tail, an S-curved neck, and forelimbs held backwards against the body.