The last Western InteriorBaculitesfrom the Fox Hills Formation of South Dakota

Abstract

Species ofBaculitesare important marker fossils in the Upper Cretaceous rocks of the Western Interior of the United States and provide indices for 20 of the 29 Campanian and Maastrichtian zones recognized by Cobban (inGill and Cobban, 1966; Cobban, 1977). They often occur in rock-forming proportions (Gill and Cobban, 1966, Pl. 11, fig. 3) and are common up through the lower MaastrichtianBaculites clinolobatuszone. In the type area of the Fox Hills Formation in west-central South Dakota,B. clinolobatusis present in the lower part of the Mobridge Member of the Pierre Shale, butBaculitesare rare or absent in the rest of the member as well as in the overlying Elk Butte Member that forms the uppermost part of the Pierre Shale (Waage, 1968, p. 50, 51, fig. 6). Only the diminutiveBaculites columnaMorton, 1834, has been noted from the succeeding Fox Hills Formation (Waage, 1968). The highest marine Cretaceous rocks of the Western Interior are characterized instead bySphenodiscusand a range of scaphitid species (Hoploscaphites, Discoscaphites). It is therefore of some interest to describe, for the first time, the baculitids from the very high Cretaceous of the Western Interior. The material described below was collected from the Fox Hills Formation by N. L. Larson, P. L. Larson, and R. A. Farrar of the Black Hills Institute of Geological Research, Hill City, South Dakota. We are grateful to them for allowing us to describe this interesting collection. Specimens cited below are deposited in the collections of the Black Hills Institute (BHI) and in the U.S. National Museum of Natural History (USNM) in Washington, D.C.