Ammonite record from the Pierre Shale of northeastern New Mexico
— William A. Cobban

In northeastern New Mexico, the Upper Cretaceous Pierre Shale is a dark-gray marine unit about 1,600 ft (488 m) thick (Johnson and others, 1966, p. 88). Invertebrate fossils, chiefly mollusks, are locally abundant in gray- to brown-weathering limestone concretions and dusky-red- to rusty-brown-weathering clay ironstone concretions.

The first large collections of invertebrate fossils were made by 0. H. St. John (1876, p. 280) near Cimarron (Fig. 1) from several levels of concretions between 90 and 570 ft (27-174 m) below the top of the shale; these fossils were identified by C. A. White who published the list in 1879 (White, 1879b, p. 267, 268). The list included only three species of ammonites, which were identified as Baculites ovatus Say, Scaphites nodosus Owen, and Placenticeras placenta (DeKay).

W. T. Lee investigated the geology of the Raton basin at various times from 1902 to 1917 and collected from the Pierre Shale at 21 localities in northeastern New Mexico. In a generalized stratigraphic section for the Raton coal field, Lee (1912, p. 611) listed the following ammonites, identified by T. W. Stanton, from the upper part of the Pierre Shale: Ancyloceras sp., Baculites ovatus Say, B. cornpressus Say, Heteroceras cheyensis Meek and Hayden, Heteroceras sp., Placenticeras intercalare Meek and Hayden, P. whitfieldi Hyatt?, P. sp., Ptychoceras sp., Scaphites nodosus Owen, and S. sp. Later, in his large summary work, Lee (1917, p. 45-47) gave more specific information regarding localities and stratigraphic positions of these ammonites in the Pierre Shale.

Since the publication of Lee's list of fossils, the only additional ammonites recorded from the Pierre Shale of north-eastern New Mexico are Scaphites hippocrepis (DeKay), Baculites rugosus Cobban, and B. undatus Stephenson. Scaphites hippocrepis was collected by Lee in 1913 at U.S. Geological Survey Mesozoic locality 8352, 2.5 mi (4 km) southeast of Clifton House (Fig. 1), but the species was not included in the lists of fossils in his summary paper (Lee, 1917). The species was identified as Scaphites hippocrepis (DeKay) var. tenuis by Reeside (1927, p. 23 and table opposite p. 2), and later it was assigned to S. hippocrepis III by Cobban (1969, p. 24). Baculites rugosus was recorded near Raton (Cobban, 1962b, p. 134), where the species was collected by J. B. Mertie in 1908 at locality 5566 about 1.5 mi (2.4 km) northeast of Raton. Baculites undatus Stephenson was noted at Casa Grande (locality D7484) and 2 mi (3.2 km) west of Cimarron (locality D4826) by Cobban (1973, p. 460).

Several collections made in recent years by G. R. Scott, C. L. Pillmore, the late J. R. Gill, and myself have added much to our knowledge of the faunal content of the Pierre Shale of northeastern New Mexico. The present paper was prepared to update some of the collections of W. T. Lee and to present data concerning the more recently collected ammonites.