Revision of the Archosauromorph Reptile Trilophosaurus, With a Description of the First Skull of Trilophosaurus Jacobsi, From the Upper Triassic Chinle Group, West Texas, Usa

The first abundant, well-preserved fossils of the unusual archosauromorph reptile Trilophosaurus jacobsi Murry are from an Upper Triassic bonebed in the lower-most Trujillo Formation of the Chinle Group in Borden County, Texas. A nearly complete left side of the skull and incomplete but articulated mandible of a juvenile individual demonstrate that Trilophosaurus jacobsi Murry is referable to Trilophosaurus, so the putative procolophonid genus Chinleogomphius is a junior objective synonym of Trilophosaurus. Features of T. jacobsi that diagnose it from T. buettneri include asymmetrical, tricuspate teeth in which the central cusp is taller than the marginal cusp, polygonal in cross-section and displaced slightly lingually; the lingual cusp is low, transversely broadened and anteroposteriorly compressed; teeth with prominent cingula along the mesial and distal margins; and paired sagittal (parasagittal) crests composed of portions of the postfrontals and the parietals, as well as a relatively broad skull roof between the supra-temporal fenestrae. We also re-illustrate and re-describe the holotypes of both Trilophosaurus buettneri Case and T. jacobsi Murry. T. jacobsi is primarily Adamanian in age, and the lowest occurrence of T. jacobsi is stratigraphically above that of T. buettneri. Therefore, T. buettneri is an Otischalkian– Adamanian index taxon, and T. jacobsi is an Adamanian–earliest Revueltian index taxon. Trilophosaurus teeth are readily identifiable to species and therefore iden-tifiable as isolated fossils. Andrew B. Heckert, Spencer G. Lucas, Larry F. Rinehart, Justin A. Spielmann, Adrian P. Hunt & Robert Kahle (2006) "Revision of the Archosauromorph Reptile Trilophosaurus, With a Description of the First Skull of Trilophosaurus Jacobsi, From the Upper Triassic Chinle Group, West Texas, Usa" Palaeontology Volume 49 pp. 621-640 Version of Record Available from (www.amazonaws.com) During Triassic time, continental tetrapod faunas changed in composition from temnospondyland primitive-synapsid-dominated Early Triassic faunas to archosaur-dominated Late Triassic faunas. An intriguing subplot that is often lost in discussion of this evolutionary transition is the changes that occurred in the nature of some specialized, herbivorous tetrapods. During the Early Triassic, procolophonids comprised a substantial proportion of the herbivorous fauna (as did dicynodonts and gomphodont cynodonts), whereas Late Triassic procolophonids are relatively rare, apparently having been supplanted by herbivorous archosauromorphs, including rhynchosaurs and trilophosaurs (e.g. Benton 1984, 1991; Reisz and Sues 2000). Rhynchosaurs are all but unknown in the Upper Triassic of North America (Hunt and Lucas 1991; Long and Murry 1995; Lucas et al. 2002), yet in places the trilophosaur Trilophosaurus is abundant. While rhynchosaurs have undergone significant revision in the past two decades (e.g. Benton 1983, 1984, 1990; Langer and Schultz 2000; Langer et al. 2000a, b), the less well-known trilophosaurs have had but one new taxon identified (Murry 1987), which was subsequently re-identified as a procolophonid (Sues and Olsen 1993). Even Late Triassic procolophonids in North America have received far more recent attention than trilophosaurs (Sues and Olsen 1993; Small 1997; Sues and Baird 1998; Sues et al. 2000). Numerous workers have collected and described Upper Triassic fossils from various localities in West Texas, including Cope (1892), Case (1922, 1928a, b, 1932), Gregory (1945), Murry (1982, 1986) and Chatterjee (1986) (Text-figure 1). Among the fossils collected were the original specimens of the unusual archosauromorph reptile Trilophosaurus buettneri Case, 1928a, subsequently the subject of a monograph by Gregory (1945) and described further by Parks (1969), DeMar and Bolt (1981) and Murry (1982). To date, almost all other verifiable Trilophosaurus records come from low in the Chinle Group in Texas (Gregory 1945; Parks 1969; Murry 1982, 1986; Heckert et al. 2001) or the Chinle Group in Arizona and New Mexico (Murry 1987; Murry and Long 1989; Long and Murry 1995). The only exceptions are isolated teeth collected by Kirby (1989, 1990, 1991, 1993) in the stratigraphically higher Owl Rock Formation of northcentral Arizona (Text-fig. 1). Here, we provide a detailed history of the study of Trilophosaurus, outline the stratigraphy of a new locality yielding fossils of Trilophosaurus and describe a nearly complete juvenile skull of Trilophosaurus from this locality. We also re-describe and re-illustrate the holotypes of T. buettneri and T. jacobsi, compare these with the referred fossils from the new locality, and comment on the biostratigraphical and biochronological significance of Trilophosaurus. MATERIAL AND METHODS For this paper, the principal fossils studied were an extensive collection of associated skulls, mandibles and postcrania at the New Mexico Museum of Natural History and Science (NMMNH) from NMMNH locality 3775 (Heckert et al. 2001). Additionally, we borrowed and examined the holotype specimens of T. buettneri Case and T. jacobsi Murry and made comparisons with other fossils described by Gregory (1945) and in the NMMNH collections from Gregory’s localities. Scanning electron microscopy (SEM) work was accomplished on a JEOL-JSM5800 housed at the Institute of Meteoritics at the University of New Mexico Department of Earth and Planetary Sciences. Digital macrophotography was conducted using both a Nikon Coolpix 995 and a Minolta F100. Images were manipulated on a Macintosh G4 computer using Adobe PhotoShop Elements 2.0, and sketches were constructed by tracing digital images in Adobe Illustrator 9.0. Institutional abbreviations. MNA, Museum of Northern Arizona, Flagstaff; NMMNH, New Mexico Museum of Natural History and Science, Albuquerque; TMM, Texas Memorial Museum, Austin; UMMP, University of Michigan Museum of Paleontology, Ann Arbor.