Macelognathus vagans (Archosauria; Crocodylomorpha): New information on the braincase using CT-Data

LEARDI, J.M.; POL, D.; CLARK, J.M.

Buenos Aires

Jornada; XXX Jornadas Argentinas de Paleontología de Vertebrados; 2016

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 juve- nile 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 structures 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 non- crocodyliform 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.