Late Cretaceous (Campanian-Maastrichtian) Scaphitid Ammonites from the U.S. Western Interior

Scaphitid ammonites (scaphites) are common in the Upper Cretaceous Pierre Shale and Fox Hills Formation of the U.S. Western Interior. We are in the process of redescribing the species that belong to the genera Jeletzkytes Riccardi and Hoploscaphites Nowak. These species range from the Baculites asperiformis to the Jeletzkytes nebrascensis zones (approximately 12 my). There are altogether about 30 species, many of them still undescribed. In general, each species ranges through two or three biozones. For example, Jeletzkytes nodosus (Owen), Jeletzkytes brevis (Meek), and Hoploscaphites landesi Riccardi all co-occur and range from the Didymoceras cheyennense to the B. cuneatus zones.

All of these species are dimorphic, as indicated by differences in adult size and shape. Microconchs (presumably males) are approximately two-thirds the size of macroconchs (presumably females). Within a single dimorph, there is wide variation in the degree of robustness and coarseness of ornament. Finely ribbed, compressed forms intergrade with more coarsely ribbed, robust forms. The most important characters used to diagnose species are the presence and distribution of tubercles, including umbilicolateral, ventrolateral, and flank tubercles, the spacing of the ribbing, the degree of whorl compression (expressed by both whorl width/whorl height and whorl height/venter width), the flatness of the flanks, and the degree of uncoiling of the adult body chamber. The suture is of only modest importance. Starting in the Baculites eliasi Zone, scaphites are more tightly coiled, and the ventrolateral tubercles are more closely spaced. Starting in the B. grandis Zone, flank tubercles appear on the exposed part of the phragmocone, and starting in the H. nicolletii Zone, flank tubercles occasionally extend onto the body chamber. Most of these scaphite species are restricted to the Western Interior, but closely related forms occur on the Gulf and Atlantic Coastal Plain and in northern Europe.