The skull of an early Late Triassic aetosaur and the evolution of the stagonolepidid archosaurian reptiles

摘要

Disarticulated bones of several individuals recovered from the Late Triassic fluvial and lacustrine deposits at Krasiejów, Poland, are here described, allowing the restoration of the skull structure of a new aetosaurian archosaur: Stagonolepis olenkae sp. nov. The Krasiejów deposits probably correspond in age to the Lehrberg Beds (late Carnian) of Baden-Württemberg, Germany. The stratigraphical position of the new taxon combined with other available evidence is used to propose a model of aetosaurian evolution. The proposed phylogenetic position of Aetosaurus ferratus (Norian, Germany) as the basal aetosaurid is refuted and this species is instead proposed to be the most derived member of the Stagonolepis–Aetosaurus evolutionary lineage. Gradual change in several morphological characters can be observed from Stagonolepis robertsoni, through the new species from Krasiejów, to the stratigraphically youngest Aetosaurus ferratus. These changes include a decrease in the number of teeth and a decrease in the convexity of the ventral profile of the maxilla. The anterior elongation of the maxilla is associated with the expansion of the anterior tip of the maxilla towards the naris. In S. robertsoni and S. olenkae, the maxilla extends to middle of the naris, whereas in Aetosaurus, it reaches the anterior half of the naris.