Ontogenetic Changes in Endocranial Anatomy in Gorgosaurus libratus (Theropoda: Tyrannosauridae) Provide Insight Into the Evolution of the Tyrannosauroid Endocranium

Abstract

Over the past two decades, increased accessibility to computed tomography (CT) scanners has greatly facilitated documentation of the endocranium in numerous extinct theropod taxa. However, most of these studies have focused on the morphology of mature individuals, thus changes or variation through ontogeny of the endocranium in theropods remains largely unknown. The current study sheds light on the endocranial anatomy of the eutyrannosaurian tyrannosauroid, Gorgosaurus libratus, in both an ontogenetic and evolutionary context. Based on CT scans of six Gorgosaurus braincases, including those of two recently discovered juvenile individuals, we virtually reconstruct and describe the endocranial morphology for a growth series of G. libratus. Despite considerable changes in skull architecture, relatively few ontogenetic changes occurred in the endocranium of Gorgosaurus. These changes include a subtle increase in the length of the hindbrain region of the endocast and increased inflation of the tympanic sinus diverticula in adults relative to juveniles. Among the most significant ontogenetic changes is a decrease in the distinctiveness of the brain morphology in endocasts as Gorgosaurus mature. The endocasts of juvenile Gorgosaurus exhibit better defined cerebral hemispheres, optic lobes, and cerebella than those of larger and more mature individuals. This suggests a closer correspondence between the endocast and the brain in juvenile tyrannosaurids, indicating the endocast of juvenile individuals provides a more accurate representation of the structure of the brain and its regions relative to the endocast of more mature individuals. The brain of Gorgosaurus displays a mix of basal archosaurian traits and more derived coelurosaurian traits. More primitive archosaurian features of the Gorgosaurus brain include large olfactory bulbs and tracts, a posteroventrally oriented long axis of the cerebrum, and posteriorly positioned optic lobes, whereas derived features include prominent hindbrain flexure, a somewhat enlarged cerebrum, and a cerebellum that at least partially separates the left and right optic lobes. An understanding of the evolutionary acquisition of such derived features leading to the avian brain may be further elucidated via the study of the endocasts of juvenile individuals (more reflective of the structure/organization of various brain regions) of earlier-diverging theropods (e.g., Allosauroidea, Megalosauroidea, and Coelophysoidea).