New Mexico Geological Society Annual Spring Meeting — Abstracts

Both Late Cretaceous selachians and marine invertebrates have biostratigraphic utility in the North American Western Interior. Here, we report a selachian assemblage (NMMNH locality 5617, SDSM locality V2001-04) from the Clay Mesa Member of the Mancos Shale near the town of Galisteo in Santa Fe County, New Mexico (T13N, R10E, near the Cañada Estacada). The selachians and associated marine invertebrates are in a 0.6-m-thick bed of hematitic bioclastic conglomerate in the middle part of the Clay Mesa Member that yields the ammonites Acanthoceras amphibolum, Tarrantoceras sellardsi and the bivalve Inoceramus arvanus. The ammonties place this fauna in the A. amphibolum ammonite zone. The selachain fauna consists of Ptychodus occidentalis, Squalicorax curvatus, Cretoxyrhina mantelli and Cretodus semiplicatus, further confirming these taxa in the middle Cenomanian. 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. S. curvatus specimens have a distinguishing concave labial crown face. C. mantelli specimens lack cusplets and a nutrient groove on the lingual root; distinguishing them from Cretolamna and Paranomotodon. Distinct cusplets that are continuous with the root identify specimens of C. semiplicatus. This is the oldest Late Cretaceous selachian assemblage reported from New Mexico.