THE UNIQUENESS OF THE ENDEMIC BACULITES OF THE WESTERN INTERIOR

The Family Baculitidae inhabited the Western Interior Seaway during the Late Cretaceous with approximately 50 known species from several different genera. Baculites appeared in the latest Cenomanian with Sciponoceras and survived, nearly continuously, until the latest Maastrichtian with Baculites larsoni. The extensive fieldwork and research of W.A. Cobban helped lead to the division of the thick, homogenous Pierre Shale and equivalent age strata into clear and concise biostratigraphic ammonite zones. Cobban and others helped designate 18 species of Baculitidae as biostratigraphic index fossils within the Campanian and Maastrichtian because of their unique characters and wide distribution across much of the Western Interior (Cobban et al. 2006). Those biozone species were selected from a lineage of 23 distinct species that began in the Campanian with Baculites aquilensis and ended with Baculites eliasi in the earliest Maastrichtian. Baculites baculus migrated into the Western Interior, reportedly from the Gulf Coast, soon after the disappearance of Baculites eliasi, giving rise to a new, shorter-lived lineage that became endemic to the Western Interior and ended with Baculites clinolobatus. This zonation has been successfully utilized by scores of researchers over the last 60 plus years. We examine the shape, ornamentation, and septal patterns of many of the Campanian and Maastrichtian baculites and discuss the characters that make them unique.

Cobban, W.A., I. Walaszczyk, J.D. Obradovich, and K.C. McKinney, 2006. A USGS Zonal table for the Upper Cretaceous middle Cenomanian-Maastrichtian of the Western Interior of the United States based on ammonites, Inoceramids, and radiometric ages, U.S. Geological Survey, Open-File Report 2006-1250, 46 p.