Cranial osteology of Longosuchus meadei and the phylogeny and distribution of the Aetosauria

ABSTRACT

The aetosaur Longosuchus meadei is known from several specimens from the Upper Triassic (Carnian) of Texas. The structure of the single well preserved skull (TMM 31185-84B) was restudied in light of the appearance of new material of other archosaurs subsequent to Sawin's original (1947) description. In particular, an ossified orbitosphenoid and several structures associated with the nasolacrimal duct system are described and discussed for the first time.

Adaptations for herbivory in the Aetosauria include the ventral depression of the jaw joint, the edentulous beak, and the conical, unserrated teeth in later aetosaurs. The increased ossification of the braincase may serve to protect this region during food processing, and the nasolacrimal structures probably are associated with the lateral nasal sinus system widespread in archosaurs and particularly well developed in Longosuchus.

The Aetosauria are a robust monophyletic group of Suchia that can be characterized by five synapo-morphies. Aetosaurus is the most plesiomorphic aetosaur, and clades are formed by (1) Aetosauroides and Stagonolepis and (2) Longosuchus, Desmatosuchus, Typothorax, and Paratypothorax. Within the second unnamed clade, a smaller subclade consists of Typothorax and Paratypothorax.