A RIGHT-HANDED TYRANNOSAURUS REX?: LATERALITY IN TYRANNOSAURID DINOSAURS

Lateralization is common in many tetrapods and is best known to us from “handedness” in humans. It reflects specialization of the cerebral hemispheres to undertake specific cognitive functions and to process input from the contralateral eyes. Paleopathologies provide circumstantial evidence of laterality in the skeleton of the Late Cretaceous Tyrannosaurus rex known as “Stan.” Stan has a number of healed wounds, likely inflicted by other tyrannosaurids, almost all (except a few dentalites) unilateral, on the left side of its skeleton. These include numerous healed dentalites on the skull (supraocciptal crest, jugal), lower jaw (angular) and a broken and healed rib, as well as fused cervical vertebrae after a traumatic injury. These injuries were cumulative over several events and did not kill the tyrannosaur. The location of the injuries on the left side is circumstantial evidence of laterality because: (1) in agonistic encounters, the tyrannosaur would have likely turned its “weak” side toward the opponent, so that injuries would be inflicted on that side; and (2) given that the right hemisphere processes visuospatial input critical to rapid responses, as in escape and attack, the tyrannosaur placing the opponent on the left side can be expected (as is seen in some extant amphibians, reptiles and birds). These lines of reasoning support Stan as a “right handed” T. rex in which the weak side of the body was the left side. However, similar evidence for laterality in many other T. rex skeletons is not compelling because they do not display multiple trauma-related pathologies. For example, Sue has unilateral pathologies, but these are mostly due to disease and non-combat injury. Nevertheless, like Stan, Trix has multiple pathologies due to trauma, mostly healed dentalites concentrated on the left side of the skull and lower jaw. Whether or not other dinosaur skeletons show evidence of laterality should be sought in similar circumstantial evidence—unilateral, multiple healed injuries incurred during agonistic encounters. Laterality is adaptive and is found in many birds (the closest living relatives of theropod dinosaurs), so it is not surprising to find evidence of it in T. rex.