CRANIAL OSTEOLOGY OF LONGOSUCHUS-MEADEI AND THE PHYLOGENY AND DISTRIBUTION OF THE AETOSAURIA

The aetosaur Longosuchus meadei is known from several specimens from t he Upper Triassic (Carnian) of Texas. The structure of the single well preserved skull (TMM 31185-84B) was restudied in light of the appeara nce of new material of other archosaurs subsequent to Sawin's original (1947) description. In particular, an ossified orbitosphenoid and sev eral structures associated with the nasolacrimal duct system are descr ibed and discussed for the first time. Adaptations for herbivory in th e Aetosauria include the ventral depression of the jaw joint, the eden tulous beak, and the conical, unserrated teeth in later aetosaurs. The increased ossification of the braincase may serve to protect this reg ion during food processing, and the nasolacrimal structures probably a re associated with the lateral nasal sinus system widespread in archos aurs and particularly well developed in Longosuchus. The Aetosauria ar e a robust monophyletic group of Suchia that can be characterized by f ive synapomorphies. Aetosaurus is the most plesiomorphic aetosaur, and clades are formed by (1) Aetosauroides and Stagonolepis and (2) Longo suchus, Desmatosuchus, Typothorax, and Paratypothorax. Within the seco nd unnamed clade, a smaller subclade consists of Typothorax and Paraty pothorax.