Comparative braincase morphology of Trilophosaurus buettneri and the early evolution of the pan-archosaurian neurocranium

The relative scarcity of well-preserved fossils from the earliest history of stem lineages often limits our ability to establish robust, broad-based evolutionary patterns. This is certainly the case for the pan-radiation of archosaurs whose earliest stem taxa remain poorly understood relative to the crownward archosauriforms. Trilophosaurus buettneri is a North American Triassic stem archosaur that lies near the base of this expansive pan-radiation. We used CT to study the Trilophosaurus braincase with the goal of elucidating the early pan-archosaur neurocranium and its evolution. Results clarify several problematic characters including variable fusion of the exoccipital, absence of contralateral exoccipital contact, lack of a semilunar depression, and several others. These novel scorings fail to alter most currently accepted tree topologies, but the results are detailed. These data also include the phylogenetically earliest evidence of neurocranial pneumatization among pan-archosaurs, including a parabasisphenoid cavity that is positionally homologous to the median pharyngeal recess of crown archosaurs. These data are not without their problems, but they do allow us to hypothesize the earliest transformations in what became a much more extensive character system in crown archosaurs. Multiple autapomorphies suggest Trilophosaurus was capable of a derived behavioral repertoire, but details remain unclear. For example, Trilophosaurus bears a theropod-like elongation of the anterior semicircular canal but lacks the associated expansion of the lateral canal thought to facilitate bipedality in that group. Our data clarify plesiomorphic conditions for later archosaurian transformations while promoting the hypothesis that pan-archosaurs achieved marked structural and behavioral disparity early in their history.