CRANIAL ANATOMY OF THE LATE TRIASSIC PHYTOSAUR MACHAEROPROSOPUS, WITH THE DESCRIPTION OF A NEW SPECIES FROM WEST TEXAS

The skull anatomy of the derived phytosaur Machaeroprosopus is described for the first time on the basis of three specimens from the Upper Triassic Cooper Canyon Formation of Texas. A paranasal bone, an additional paired element of the narial region, is identified. Important new data is presented for the braincase, including the morphology of the epipterygoid and presphenoid, an anterior process of the prootic, an anteroventral process of the laterosphenoid, and a parasphenoid process. Machaeroprosopus lottorum n. sp. is characterized by four apomorphies: a supratemporal fenestra closed on the skull roof with beveled anterior rim, a comparatively short squamosal, a flat and rugose narial rim, and medially extended palatines that come close to form an ossified secondary palate. With respect to the supratemporal fenestra, the supraoccipital-parietal complex, and several features of the squamosal, Machaeroprosopus lottorum n. sp. bridges the morphological gap between species previously referred to the genera Pseudopalatus and Redondasaurus. A parsimony analysis of known species of Machaeroprosopus suggests that the development of the prenarial crest in Machaeroprosopus is a sexually dimorphic feature, and questions the validity of the genus Redondasaurus. Consequently, Redondasaurus is here considered a junior synonym of Machaeroprosopus.