Encrustation of inarticulate brachiopods on scaphitid ammonites and inoceramid bivalves from the Upper Cretaceous U. S. Western Interior

Abstract

AbstractThe inarticulate brachiopodDisciniscais a rare faunal element in the Upper Cretaceous of the U.S. Western Interior. We report two occurrences of encrustation ofDisciniscaon a scaphitid ammonite (scaphite) and several inoceramids from the lower MaastrichtianBaculites baculus/Endocostea typicaBiozones of the Pierre Shale at two localities. Six specimens ofDisciniscaare present on a single specimen ofHoploscaphites crassusfrom east-central Montana. They occur along the furrow at the mature apertural margin. Because the brachiopods are restricted to the margin and do not occur on the rest of the shell, it is likely that they encrusted the ammonite during its lifetime. If so, this implies that the soft body of the scaphite did not cover the outside surface of the aperture, leaving this area vulnerable to epizoan attachment. A total of 13 specimens ofDisciniscaare also present on four specimens ofCataceramus?barabinifrom east-central Wyoming. The brachiopods occur in crevices on the outside of the shells and may have encrusted the inoceramids after their death as the shells began to break down and delaminate, resulting from the decomposition of the organic matrix holding them together. Based on the faunal assemblages at both localities, the presence ofDisciniscamay indicate environments with either low oxygen levels and/or few predators or competitors.