CONICET | Buscador de Institutos y Recursos Humanos

Macelognathus vagans was originally described as a dinosaur by Marsh from the Late Jurassic Morrison Formation of Wyoming and later Ostrom suggested crocodilian affinities. More recently, Göhlich and collaborators identified new material from Colorado as a basal crocodylomorph. However, a partial skull found in association with mandibular and postcranial remains was not described. The skull preserves most of the posterior part, including the braincase, and probably belongs to a juvenile given its dorsal curvature. Due to the small size and delicate structures within the braincase, micro CT studies were performed on this specimen. This new material reinforces the non-crocodyliform crocodylomorph affinities of Macelognathus as it bears a large otic aperture, unfused frontals and lacks ornamentation on the dorsal cranial bones. The internal struc-tures also support these affinities as this specimen has traits (i.e., heavily pneumatized and expanded basisphenoid; the presence of additional pneumatic features on the braincase; and the otoccipital-quadrate contact) not present in most basal crocodylomorphs. Furthermore, the presence of a wide supraoccipital and a cranioquadrate passage are traits shared with Almadasuchus from the early Late Jurassic of Argentina. The new anatomical information was incorporated in a phylogenetic dataset, expanding both character and taxon sampling. Macelognathus was recovered as one of the most derived noncrocodyliform crocodylomorphs, forming a clade with two other Late Jurassic taxa (Almadasuchus and the Morrison Hallopus). This derived clade is characterized by having a higher degree of suturing of the braincase, posteriorly closed otic aperture (paralleled in mesoeucrocodylians) and cursorial adaptations.