TAXONOMIC AND BIOSTRATIGRAPHIC IMPLICATIONS OF RECENT AMMONOID DISCOVERIES FROM THE UPPER CRETACEOUS OF EAST-CENTRAL UTAH

Long-term, continuing research on the Upper Cretaceous rocks and fossils of east-central Utah has resulted in new biostratigraphic and taxonomic refinements. Following the recently proposed changes to stratigraphic nomenclature, fossils were collected from three concretionary beds in what is now recognized as the lower Smoky Hill Member of the Blue Gate Shale of the Mancos Group southwest of the town of Emery, Utah. The mostly siliciclastic mudrocks of the Mancos reflect deposition within the Western Interior Seaway of North America, and the thickness of this unit exceeds 1500 m in some places. The fossil-bearing concretionary beds lie within the Scaphites preventricosus Zone, indicative of the Lower Coniacian Substage. Fossil collections are comprised of a well-preserved, high-diversity ammonoid assemblage that includes the zonal name-bearer, as well as Scaphites impendicostatus, Forresteria alluaudi, Placenticeras fritschi, Baculites yokoyamai, Baculites mariasensis, and Allocrioceras hazzardi. In addition, the first Coniacian Yezoites frontierensis has been identified. Also, the first Peroniceras in Utah was discovered, P. dravidicum; other species of this genus have previously been reported from as far north as Montana, but from the overlying Scaphites ventricosus Zone (Middle Coniacian Substage). The cephalopod fauna also includes rare nautiloids, which have been provisionally identified as Anglonautilus sp. Along with the cephalopods, abundant inoceramids, including Cremnoceramus crassus crassus and Cremnoceramus deformis deformis, allow correlation of these rocks to the uppermost part of the Lower Coniacian Substage of the Upper Cretaceous Series.