The Late Cretaceous ammonite Baculites undatus Stephenson in Colorado and New Mexico

Baculites undatus Stephenson, originally described from the Nacatoch Sand of northeastern Texas, has been found in the Pierre Shale at 25 localities in Colorado and two in New Mexico. An early form of the species occurs in the zone of Didymoceras cheyennense (Meek and Hayden), and the typical form occurs in the overlying zones of Baculites compressus Say, B. cuneatus Cobban, and B. reesidei Elias. The species is characterized by its stout cross section, broad venter, conspicuous lateral and ventral ribs, and suture of about average complexity for the genus.