MODE OF LIFE AND HABITAT OF SCAPHITES IN THE LATE CRETACEOUS OF THE U.S. WESTERN INTERIOR

Scaphitid ammonites (scaphites) are among the most common ammonites in the Upper Cretaceous of the U.S. Western Interior. As part of a monographic study, we have examined species of Hoploscaphites from the Campanian and Maastrichtian Pierre Shale and Bearpaw Shale. Examination of the morphology of these ammonites (shell, septa, muscle scars, jaws, hooks, radula), faunal associations, facies distribution, isotopic composition, mechanical strength, incidence and kinds of injures, reveals clues about their mode of life and habitat. As adults, these scaphites probably lived just above the sea bottom. They preferred well oxygenated water, as indicated by the fact that they are generally associated with a diverse molluscan community. Habitat depths are estimated at less than 100 m, based on studies of the mechanical properties of the septa and siphuncle (Tsujita and Westermann, 1998). The high angle of orientation of the aperture at maturity is incompatible with a nektobenthic mode of life, involving scavenging or searching for food on the bottom. The apertural margin is also constricted and ends in a thin lip, which would have prevented unimpeded movement of the soft body outside of the aperture. Instead, scaphites may have consumed small prey in the water column, which is consistent with the presence of an aptychus-type lower jaw. Adults were probably poor swimmers, based on a comparison of their musculature with that of nautilus (assuming that such a comparison is valid). The uncoiling of the body chamber at maturity also increased the coefficient of drag and decreased hydrodynamic efficiency. As with modern cephalopods, these scaphites were probably preyed upon by fish, reptiles, crustaceans, and other cephalopods. Lethal injuries are common on the adapical end of the body chamber. The large ventrolateral tubercles on the shaft undoubtedly served as an anti-predatory defense against such attacks from behind.