New Mexico Geological Society Annual Spring Meeting — Abstracts

Late Cretaceous selachians provide a marine biostratigraphy throughout the North American Western Interior. Here, we report a selachian assemblage from the Bridge Creek Member of the Mancos Shale near the town of Carthage in Socorro County, New Mexico (sec. 8, T05S, R02E). The selachian fossils are in a 0.3-0.6 m-thick bed of sandy limestone/calcarenite at the top of the Bridge Creek Member that yields numerous shells of the bivalves Ostrea beloiti and Mytiloides mytiloides. This fossil assemblage occurs within the Sciponoceras gracile ammonite zone. Specimens of Ptychodus dominate the assemblage, with rare and fragmentary remains of bladeshaped shark teeth, some of which can be identified as Squalicorax sp. Ptychodus specimens belong to P. occidentalis and P. anonymus and further confirm their presence in Upper Cenomanian strata. The P. occidentalis specimens are characterized by their overall square shape and their transverse ridges bifurcating numerous times distally, grading into finer and finer parallel to subparallel ridges. The P. anonymus specimens are characterized by their transverse ridges that extend down the cusps then divide and curl around as they enter the marginal area. In addition, numerous P. anonymus specimens have a prominent cusp that superficially resembles P. whipplei, though the presence of transverse ridges that extend down the sides of the cusp confirm these specimens pertain to P. anonymus. The lithology of the site indicates deposition in deep-water, which suggests that Ptychodus had a preference for deep water, as previously noted by other workers.