The early evolution of vestibular morpohology in Archosauria

BRONZATI,M.; EVERS, S.W.; CABREIRA, S.F.; CHOINIERE, J.; DOLLMAN, K.; EZCURRA, M.; PAULINA CARABAJAL, A.; RADEMACHER, V.; DA SILVA, L.; STOCKER, M.R.; LANGER, M.C.; WITMER, L.; BENSON, R.B.J.; NESBITT, S.J.

Praga

Congreso; International Congress of Vertebrate Morphology; 2019

ICVM

During the first stages of the Triassic (ca. 245 Ma), Archosauria diverged into two major lineages, that including crocodilians and the one that includes birds. Compared to their disparate living members, earlier members of these lineages shared many traits, but nevertheless span a wide range of ecologies and document important shifts in locomotorstyle, from primitive sprawling to crocodilian-like semi-erect and bird-like erect limb orientations, and to flight in pterosaurs. To chronicle the divergence of these groups, we focused on the labyrinth given that the living members of these lineages have highly disparate anatomies. The bony labyrinth, within the braincase, is part of sensory systemsassociated with balance coordination and gaze stabilization during locomotion. Hence, its morphology potentially provides a wealth of ecological information for extinct taxa. We compiled a dataset of reconstructed 3D-models of the endosseous labyrinths of 27 extinctarchosauromorphs (Early Triassic ? Early Cretaceous; 252 ? 112 Ma) accessed via microcomputed tomography of the braincase. Variation was analyzed by applying geometric morphometrics methods to 3Dvirtual models. The results of a principal component analysis show small overlap between the morphospace associated to labyrinth morphology of Triassic pseudosuchians (crocodile-line) and avemetatarsalians (birdline). These data suggest that differences in labyrinth morphology between the lineages were already present at their divergence, and the separation was maintained through time. Overall, avemetatarsalians have generally higher than wider labyrinths, matching the proportionsof the occipital region of their skull. On the other hand, pseudosuchians generally have labyrinths with an inverse relation between height and width, also mostly matching the architecture of their skull. The differences in labyrinth morphology seem thus related to skull structure of the animals rather than to aspects of their ecology.