New Mexico Geological Society Annual Spring Meeting — Abstracts

The chronostratigraphically highest Placenticeras previously reported from the Western Interior is Placenticeras costatum Hyatt, 1903, from the late Campanian Baculites reesidei Zone (Larson et al., 1997; Cobban, 2016). We report here Placenticeras as high as the B. baculus Zone in the Pierre Shale of northeastern New Mexico. The B. baculus Zone is three ammonite zones higher than the B. reesidei Zone. P. costatum was recovered by us from the B. reesidei, B. jenseni, and B. baculus zones, and P. meeki Böhm, 1898 from the B. baculus Zone in the Cimarron area. The B. jenseni Zone is one ammonite zone higher than the B. reesidei Zone. In a postscript, W. J. Kennedy (Cobban, 2016) reported that, in the Western Interior, the youngest species of Placenticeras, P. costatum, extends into the lower half of the B. reesidei Zone, and P. meeki disappears a little lower in that zone.

Two well preserved, compressed half whorls of Placenticeras costatum with nacreous layer from NMMNH (New Mexico Museum of Natural History) localities 12260 and 12261 are from the Baculites baculus Zone. There are five moderately strong, but small, umbilical bullae in the half whorl. The small, weaker, outer lateral nodes are about one-third the distance from the ventrolateral shoulder to the umbilical shoulder. Low, weak flexuous ribs connect the umbilical bullae to the outer lateral nodes and finally to ventrolateral clavi as delicate riblets. The weakly concave venter bears two rows of small, alternating, ventrolateral clavi. Five ventrolateral clavi occur between every two outer lateral nodes (Kennedy et al., 1996, p. 6), and the shells have distinct, sinuous growth striae, both of which are characteristic of P. costatum (Cobban, 2016, p. 597). Suture is not visible.

One specimen from locality 12259 is a well preserved whorl that is part of a larger Placenticeras meeki with iridescent nacreous layer. It has a highly compressed shell with a very narrow, tabulate, concave venter. Fragments of the larger, outer whorl are less compressed with a tabulate venter that becomes less concave on the largest fragment, which is still septate. The shell is completely smooth and unornamented. Suture is not preserved.

The occurrence of Placenticeras costatum in the upper Campanian Baculites jenseni Zone and the lowermost Maastrichtian B. baculus Zone, and P. meeki, in or a little above the B. baculus Zone in the Raton Basin, significantly extends the known presence of Placenticeras in the Western Interior.

References:

1. Cobban, W. A., 2016, A survey of the Cretaceous ammonite Placenticeras Meek, 1876, in the United States Western Interior, with notes on the earliest species from Texas: Acta Geologica Polonica, v. 66, p. 587-608.
2. Kennedy, W. J., Cobban, W. A., and Landman, N. H., 1996, Two species of Placenticeras (Ammonitina) from the Upper Cretaceous (Campanian) of the Western Interior of the United States: American Museum Novitates, no. 3173, 13 p.
3. Larson, N. L., Jorgensen, S. D., Farrar, R. A. and Larson, P. L., 1997, Ammonites and the other cephalopods of the Pierre Seaway: Tucson, AZ, Geoscience Press, 148 p.