A SPHENODONTIAN (REPTILIA:LEPIDOSAURIA) FROM THE UPPER TRIASSIC WHITAKER (COELOPHYSIS) QUARRY, ROCK POINT FORMATION (APACHEAN: NORIAN) AT GHOST RANCH, NEW MEXICO, U.S.A

We report here the most complete sphenodontian fossil (NMMNH P-43125) known from the Upper Triassic Chinle Group in the southwestern U.S.A. The fossil was recovered during preparation of block C-8-82b from the famous Coelophysis quarry at Ghost Ranch, New Mexico. This is the first sphenodontian reported from the Rock Point Formation of the Chine Group, and the first from the Chinle represented by more than a single fragmentary dentulous element. P-43125 consists of incomplete left and right dentaries and a partial maxilla, all of which preserve multiple teeth. The dentition is both acrodont and heterodont, with as many as 15 smaller, peg-like teeth anteriorly and 4-6 larger, conical, striated posterior teeth. The numerous anterior teeth and the extremely small size of the specimen indicate that it may represent a juvenile, although it is similar in size to many Triassic sphenodontians, not all of which are juveniles. Details of skull proportions and dentition are distinct from broadly contemporaneous Triassic and Early Jurassic rhynchocephalians, including Brachyrhinodon, Clevosaurus, Diphydontosaurus, Paleollanosaurus, Pelecymala, Planocephalosaurus, Polysphenodon, and Sigmala, but P-43125 is more primitive than more derived, younger taxa as well. Many Triassic sphenodontian taxa can be identified from dentulous jaw fragments, but this specimen exhibits a mosaic of features seen in other sphenodontians, with teeth shaped like Diphydontosaurus, striated like Planocephalosaurus, and superficially similar to those of several other taxa. Future phylogenetic analyses of sphenodontians will have to account for this mosaic evolution in these relatively basal taxa.

Sphenodontians have an essentially Pangean distribution by the Norian, but the long stratigraphic range of individual genera and the difficulty of identifying specimens to species level hinders their biostratigraphic utility. This is at least the 9^th vertebrate and 7^th tetrapod taxon recovered from the Whitaker quarry, which, in addition to Coelophysis, yields coelacanthid and redfieldiid fish, the archosauromorph Vancleavea, and the crurotarsans Postosuchus, Hesperosuchus, Redondasaurus and Effigia.