Turonian ammonites from the Upper Cretaceous D-Cross Member of the Mancos Shale, Cebollita Mesa, Cibola County, New Mexico
— Paul L. Sealey and Spencer G. Lucas

At Cebollita Mesa southeast of Grants in Cibola County (T6N, R10W), the D-Cross Member of the Mancos Shale is ~50 m thick and is underlain by the Tres Hermanos Formation and overlain by the Gallup Sandstone. We collected the following ammonoids from the DCross Member: Prionocyclus wyomingensis Meek, Coilopoceras inflatum Cobban and Hook, Scaphites ferronensis Cobban and Prionocyclus novimexicanus (Marcou). P. wyomingensis is fairly common from near the base of the D-Cross to up to ~10 m above its base. C. inflatum is common and is restricted to the lowermost 5-6 m of the D-Cross Member. This is the first confirmed report of P. wyomingensis associated with C. inflatum in the D-Cross Member at Cebollita Mesa. The adult robust specimens of C. inflatum are characterized by a distinctive inflated appearance with a prominently ribbed shell possessing radial swellings; all but one specimen collected at Cebollita Mesa are the robust form of C. inflatum. S. ferronensis is locally abundant in the D-Cross Member of the Mancos Shale at Cebollita Mesa, and specimens are clustered in small concretions low in the section, ~8 m above the base of the D-Cross. P. novimexicanus is fairly common in the D-Cross Member at Cebollita Mesa and occurs throughout most of the D-Cross Member’s thickness, from ~10 m above its base to up to ~9 m below the D-Cross-Gallup contact. Scaphites whitfieldi Cobban has not been recovered by us from the D-Cross at Cebollita Mesa, but the middle-upper part of the member there should be in that zone. Likewise, Scaphites warreni Meek and Hayden has not been recovered by us from this section, but the lower part of the D-Cross Member there should be in that zone. Ammonoid distribution in the D-Cross Member at Cebollita Mesa is concentrated in three intervals and provides a better range zone biostratigraphy than an assemblage zone biostratigraphy.