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The Placerias/Downs' Quarry complex in eastern Arizona, USA, is the most diverse Upper Triassic vertebrate locality known. We report a new short-faced archosauriform, Syntomiprosopus sucherorum gen. et sp. nov., represented by four incomplete mandibles, that expands that diversity with a morphology unique among Late Triassic archosauriforms. The most distinctive feature of Syntomiprosopus gen. nov. is its anteroposteriorly short, robust mandible with 3-4 anterior, a larger caniniform, and 1-3 "postcanine" alveoli. The size and shape of the alveoli and the preserved tips of replacement teeth preclude assignment to any taxon known only from teeth. Additional autapomorphies of S. sucherorum gen. et sp. nov. include a large fossa associated with the mandibular fenestra, an interdigitating suture of the surangular with the dentary, fine texture ornamenting the medial surface of the splenial, and a surangular ridge that completes a 90 degrees arc. The external surfaces of the mandibles bear shallow, densely packed, irregular, fine pits and narrow, arcuate grooves. This combination of character states allows an archosauriform assignment; however, an associated and similarly sized braincase indicates that Syntomiprosopus n. gen. may represent previously unsampled disparity in early-diverging crocodylomorphs. The Placerias Quarry is Adamanian (Norian, maximum depositional age similar to 219 Ma), and this specimen appears to be an early example of shortening of the skull, which occurs later in diverse archosaur lineages, including the Late Cretaceous crocodyliform Simosuchus. This is another case where Triassic archosauriforms occupied morphospace converged upon by other archosaurs later in the Mesozoic and further demonstrates that even well-sampled localities can yield new taxa. Department of Geological Sciences at Appalachian State University; College of Arts and Sciences at Appalachian State University; Honors College at Appalachian State University Published version Funding to support open access publication of this contribution was provided by the Department of Geological Sciences, the College of Arts and Sciences, and the Honors College at Appalachian State University.

The early history of dinosaurs in North America is obscured by an incomplete fossil record, taxonomic uncertainties and speculative correlations of tetrapod-bearing rocks, as well as poor calibration of the Late Triassic time scale. High-precision U-Pb geochronology provides a reliable means of correlating terrestrial rock formations independent of equivocal lithostratigraphy or vertebrate biostratigraphy, and hence the possibility of properly evaluating models for the early radiation and diversification of Dinosauria. Here we present new, high-precision, U-Pb ID-TIMS zircon geochronology from the presumed lowermost strata of the Upper Triassic Chinle Formation of the Colorado Plateau in Southwest United States, including a mean 206Pb/238U date of 219.39 ± 0.16 Ma from the renowned Placerias Quarry Bone Bed in eastern Arizona. The new results prompt revisions to the chronostratigraphy of the lower Chinle and provide a new temporal context for its rich tetrapod fauna. The oldest documented dinosaurs of North America coexisted with their non-dinosaurian near-relatives for a minimum of 12 m.y., from ca. 223 Ma to ca. 211 Ma, in the Norian. This early dinosauromorph record follows a ca. 6 m.y. period from which no tetrapod fossils have been documented and which was itself preceded by a ca. 10 m.y. depositional hiatus spanning nearly the entire Ladinian and Carnian stages of the terrestrial North America. The supposed late appearance of dinosauromorphs in North America compared to those in South America thus appears to be an artifact of incomplete preservation, as well as unsubstantiated age interpretations. This, together with the conspicuous biogeographic distinctions among the Triassic dinosauromorph assemblages, invalidates a simple diachronous model for the transcontinental radiation of early dinosaurs.

Patterns of bone deposition are reported and deduced from mid-shaft sections of 21 limb bones of the dicynodont Placerias hesternus from the Placerias Quarry (Upper Triassic), Arizona, USA. All sampled elements of P. hesternus have a large medullary cavity completely filled with bony trabeculae surrounded by dense cortical bone. Dense Haversian bone extends from the perimedullary region to at least the mid-cortex in all sampled bones. Primary bone in the outer cortex of limb elements of P. hesternus is generally zonal fibrolamellar with a peripheral layer of parallel-fibred bone. These data suggest periodic rapid osteogenesis followed by slower growth. Among dicynodonts, this strategy is most similar to growth previously reported in other Triassic (Lystrosaurus, Wadiasaurus) and some Permian taxa (Oudenodon, Tropidostoma). An external fundamental system (EFS), suggesting complete or near complete cessation of appositional growth, is present in the largest tibia. This is the first report of EFS in dicynodonts and may represent the attainment of maximum size in P. hesternus. Slow-growing peripheral bone was observed in elements of varying size in our sample and may support a differential growth pattern between P. hesternus individuals from this locality. A complete growth series of P. hesternus, analysis of Placerias specimens from other localities, and further sampling of other Upper Triassic dicynodonts are needed to better understand a more complete picture of the growth and remodelling patterns that we have initially investigated.

Previous detailed descriptions of relatively complete drepanosaurid material make it possible to recognize isolated drepanosaurid elements from other localities. The identification of isolated elements from the USA and Great Britain extended the geographical distribution of the group and encouraged a review of Triassic collections for characteristic elements of this family. In this paper, isolated vertebrae previously described as problematic reptiles from the famous Placerias Quarry, near St. Johns,Arizona, USA are re-identified as drepanosaurids.These specimens represent the oldest occurrence of this family, which is earliestAdamanian. w e i z e r b a r t x x x a u t h o r

The Placerias Quarry of northeastern Arizona is one of the most diverse Triassic vertebrate fossil localities in the world. Several long-standing hypotheses are tested about depositional environment, paleoecology, and thanatocoenotic influences. Sedimentological features indicate a low-energy depositional environment. Bones are associated mostly with mudstones and a layer that contains numerous carbonate nodules. Thin-sections and field characteristics reflect that this nodule zone is pedogenic in origin. Taphonomic data show that the bones were not transported into the site by fluvial processes. The frequency of modification features is minimal and there is little evidence of postmortem alteration of the bones from prolonged exposure, trampling, or predation. Element survivorship curves were generated for the five most abundant taxa in the quarry. Curves for left and right elements tracked each other closely, but no pattern of preservation of cranial and postcranial elements was evident either w...

Faunal lists of taxa from the continental Permian and Triassic vertebrate localities from Algeria (Zarzaitine Series) and Morocco (Argana Group) are reported and their stratigraphical implications discussed. The nectridean Diplocaulus minimus and a captorhinid moradisaurine support a Permian age (Ufimian-Kazanian) for the top of Member T2 of the Argana Group. The base of the Lower Sandstones of the Zarzaitine Series is considered Late Spathian-Lower Anisian on the basis of capitosaurid, trematosaurid, benthosuchid and brachyopoid amphibians. Both these localities have yielded Upper Triassic vertebrates. The aetosaurs ( Paratypothorax, Desmatosuchus ) support a Late Carnian age for the top of the Lower Sandstones, whereas metoposaurs, parasuchians, an aetosaur and a Placerias -like form are indicative of Middle to early Late Carnian age for Member T5 of the Argana Group.

Three incomplete dorsal lateral scutes from the Irohalene member of the Timesgadiouine Formation, Argana Group in the Essaouira basin of Morocco can be assigned to Longosuchus meadei (Sawin). This supports correlation of the vertebrate fossil assemblage of the Irohalene Member, which also includes Paleorhinus, Angistorhinus and Placerias, to the Otischalkian and Sanfordian land-vertebrate faunachrons of the North American Late Triassic. The Irohalene Member, therefore, is of Late Carnian (late Tuvalian) age.