Braincase and neuroanatomy of the lagerpetid Dromomeron gregorii (Archosauria, Pterosauromorpha) with comments on the early evolution of the braincase and associated soft tissues in Avemetatarsalia

Abstract

The anatomy of the braincase and associated soft tissues of the lagerpetidDromomeron gregorii (Archosauria: Avemetatarsalia) from the Late Triassic ofthe United States is here described. This corresponds to the first detaileddescription of cranial materials of Lagerpetidae, an enigmatic group of LateTriassic (c. 236–200 Million years ago) animals that are the closest known relativesof pterosaurs, the flying reptiles. The braincase of D. gregorii is characterizedby the presence of an anteriorly elongated laterosphenoid and apostparietal, features observed in stem-archosaurs but that were still unknownin early members of the avian lineage of archosaurs. Using micro-computedtomography (CT-scan data), we present digital reconstructions of the brain andendosseous labyrinth of D. gregorii. The brain of D. gregorii exhibits a floccularlobe of the cerebellum that projects within the space of the semicircular canals.The semicircular canals are relatively large when compared to other archosauromorphs,with the anterior canal exhibiting a circular shape. These features ofthe sensory structures of D. gregorii are more similar to those of pterosaursthan to those of other early avemetatarsalians. In sum, the braincase anatomyof D. gregorii shows a combination of plesiomorphic and apomorphic featuresin the phylogenetic context of Archosauria and suggests that the still poorly understood early evolution of the braincase in avemetatarsalians is complex,with a scenario of independent acquisitions and losses of character states.