AMMONITE MARKINGS ON THE SEAFLOOR OF THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

Deposits from the Upper Cretaceous (middle Cenomanian – upper Coniacian) of the Western Interior Seaway exhibit markings that were produced by ammonite shells resting or touching on the seafloor after death. Most of the deposits are siltstones of relatively shallow water origin. The markings consist of impressions of the ventral and lateral surfaces of the ammonites. We have tried to match up the markings with the ammonite species that may have produced them. Altogether, there are at least 22 sites representing 11 ammonite zones from the Dakota Sandstone of western Colorado, the Juana Lopez Member of the Mancos Shale of New Mexico, the Tropic Shale of Utah, the Lincoln Member and Hartland Member of the Greenhorn Limestone of Colorado, the Floweree Member and the Ferdig Member of the Marias River Shale of Montana, the Cody Shale of Wyoming, and the Niobrara Formation of New Mexico. We have not observed similar markings from any strata above the upper Coniacian.