AND THEREBY HANGS A TAIL

Tyrannosaurus rex (T. rex) typifies the unusual vertebral mechanics of all theropod dinosaurs. A single support or pivot at the hips causes the tail and body to act both as counter-rotating lever arms that balance the body, but, the vertebral ligaments extending between the neural spines also creates a functioning unit simulating a bow strung by gravity. The locomotor and postural needs of this late Cretaceous dinosaur make complex demands on the vertebral morphology, and especially soft tissue supports for the “backbone”. A detailed study was made of the vertebral column of the well-preserved Field Museum T. rex specimen, Sue, (FMNH PR 2081) from the Hell Creek formation of western South Dakota. Mass distribution along this vertebral axis was calculated from data derived from a study of mass distribution of modern birds, mammals, and crocodilians. Dimensions of Interspinous ligaments that connected and supported the vertebral column against sagging were compared with static forces exerted by gravity (body mass) and dynamic forces from normal walking. The proximal half of this T. rex tail was rather stiff dorso-ventrally, but allowed substantial lateral movement. This resulted from thick intervertebral disks and the orientation of the vertebral zygopophyses. In contrast, dorsal vertebrae have very thin disks and a more restrictive zygopophyseal articulation. This suggests that the body of T. rex was stiff in all directions, and this stiffness was further increased by thoracic musculature and ribs. This arrangement served as an anchor for the flexible neck and considerable weight of the head, while allowing that tail movement that was important in changing direction. This has important consequences in posture and locomotion that should be considered in reconstruction of large theropods, and possibly smaller ones as well.