Baculites Baculus Meek and Hayden, 1861 (Earliest Maastrichtian) from the Uppermost Pierre Shale in the Raton basin of Northeastern New Mexico and its Significance
— Paul L. Sealey and Spencer G. Lucas

We document the ammonite Baculites baculus Meek and Hayden, 1861 from the uppermost part of the Pierre Shale (22.5 m below the base of the Trinidad Sandstone) west of Raton in northeastern New Mexico. B. baculus was also collected from the upper Pierre, 80 km southwest of Raton near Cimarron, about 40 m below the base of the Trinidad Sandstone. The highest ammonite zone previously reported from northeastern New Mexico was the upper Campanian Baculites reesidei Zone, and the earliest Maastrichtian B. baculus Zone is three ammonite zones higher. B. baculus in the uppermost Pierre Shale thus places the Campanian-Maastrichtian stage boundary at the base of that zone in the uppermost Pierre and demonstrates that the Pierre Seaway regressed from northeastern New Mexico later than previously thought.