Two new archosaur reptiles from the late Triassic of Texas
Atanassov, Momchil Nikolaev
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Two new basal archosaurs, Pteromimus longicollis and Procoelosaurus brevicollis are described from the Tecovas Formation (late Camian) and the Cooper Canyon Formation (early Norian) of the Dockum Group of West Texas. Pteromimus is based on an incomplete skull and associated and isolated skull and vertebral elements. The skull is high, with narrow snout, large orbit and naris, pneumatized maxilla and premaxilla, large inflated frontal, short parietal, plate-like supraoccipital, and reduced post-temporal openings. The dentition is thecodont and consists of conical teeth with fluted apices. The premaxillary teeth are long and slightly recurved, and the maxillary teeth are short and more laterally compressed. The vertebrae of both Pteromimus and Procoelos aurus £ire with procoelous and hollow centra, with large cavities for the interspinal ligament, well-developed epipophyses, and long and low neural arches carrying long neural processes v«th spine tables. Pteromimus is distinguished by low neural processes with weakly developed spine tables and ornamentation, while Procoelosaurus is characterized by higher neural processes that expand into wide spine tables with distinctive sculpturing. The cervical vertebrae of Pteromimus are elongate, dorsoventrally compressed, and bear unusual rib articulations consisting of a large posterior depression and a small anterior process. Procoelosaurus is represented by an association of vertebrae, pelvis, hindlimb and skull elements, and by a large number of isolated vertebrae. The maxilla and dentary of Procoelosaurus bear numerous teeth with thecodont implantation and crowns that are recurved, laterally compressed, and serrated. Procoelosaurus has at least three sacral vertebrae, large plate-like ischium, and ilium with a low blade, reduced preacetabular process, and a long, tapering postacetabular process. The femur appears similar to those of pterosaurs with anteriorly bowed shaft, well developed condyles, and an intumed head. The ankle and foot of Procoelosaurus are similar to those of early pterosaurs with an advanced mesotarsal tarsus, hooked fifth metatarsal, and closely appressed metatarsals I-IV subequal in length and indicating a plantigrade pes. A phylogenetic analysis of Archosauromorpha indicates that the two new reptiles and pterosaurs are omithodiran archosaurs and form a monophyletic clade that is the sister-group to Scleromochlus + Dinosauromorpha. Pteromimus is the sistergroup of pterosaurs, and Procoelosaurus is the sister-group to the clade Pteromimus + Pterosauria.