New Mexico Geological Society Annual Spring Meeting — Abstracts

The heteromorph ammonite Didymoceras nebrascense is locally abundant 285-290 m above the base of the Lewis Shale near Waterflow, New Mexico. Numerous fragments, including a large final whorl section and a complete early whorl section, were collected from concretions in these sediments. The associated fauna includes Placenticeras syrtale, P. placenta, cf. P. meekj, Baculites spp., Eutrephoceras dekayi, and Inoceramus (Endocostea) barabini.

Didymoceras is a heteromorph ammonite with two rows of nodes on each side of the venter. D. nebrascense is distinguished from D. stevenson and D. cheyennense by its finer, more closely spaced ribs. The lack of an impressed area where adjacent whorls made contact further distinguishes our specimens from D. stevensoni.

Specimens of Didymoceras nebrascense have also been collected from the Lewis Shale on the eastern side of the San Juan Basin, New Mexico and in the Pierre Shale in northeastern New Mexico. The Didymoceras nebrascense Zone is of late Campanian age. The co-occurrence of Inoceramus (Endocostea) barabini indicates the presence of the inoceramid zone of Endocostea barabini, which is a temporal correlative of the Didymoceras nebrascense Zone.

The occurrence of Didymoceras nebrascense thus indicates a late Campanian age for the middle part of the Lewis Shale north of the San Juan River near Waterflow, New Mexico. The presence of Didymoceras nebrascense also suggests correlation with part of the Lewis Shale on the eastern side of the San Juan Basin, New Mexico and part of the Pierre Shale in northeastern New Mexico.