SCAPHITID AMMONITES FROM THE UPPER CRETACEOUS WESTERN INTERIOR OF NORTH AMERICA

Scaphitid ammonites (scaphites for short) are abundant and well preserved in the Upper Cretaceous Western Interior of North America. The oldest scaphites in the Cenomanian belong to the genera Yezoites and Worthoceras (which extends into the Turonian), which represent the sister taxa of all other scaphites. The genera in the Turonian-Santonian are Scaphites (which extends into the Campanian), Pteroscaphites, Clioscaphites, Desmoscaphites, and Haresiceras (which extends into the Campanian). The genera in the Campanian and Maastrichtian are Platyscaphites, Trachyscaphites, Discoscaphites, Hoploscaphites, and Rhaeboceras. Notably absent from North America are Acanthoscaphites and Indoscaphites. Dimorphism is especially well expressed in more advanced taxa such as Hoploscaphites in which the umbilical shoulder of the adult body chamber is straight in macroconchs (females?) versus curved in microconchs (males?). Examination of the morphology of the shell, septa, and muscle scars, as well as a consideration of facies distribution (both litho- and biofacies), evidence of healed and fatal injuries, and isotopic composition of the outer shell of Hoploscaphites and Discoscaphites suggest that these animals lived close to the bottom in well oxygenated environments, probably less than 100 m deep. Inspection of the buccal apparatus also suggests that they ate small prey in the water column, Several patterns have been observed in the evolutionary lineages of scaphites in the Upper Cretaceous Western Interior: 1) increase in adult size; 2) repeated instances of progenesis (with adult sizes less than 1 cm in length); 3) recoiling of the adult body chamber usually accompanied by an increase in adult size; and 4) changes in whorl shape (compressed versus depressed) and degree of tuberculation, apparently reflecting changes in the extent of the Seaway. In addition, individual species exhibit broad morphological variation and show little or no directional change in adult size or morphology over time. Scaphites disappear in association with the withdrawal of the Seaway in the late Maastrichtian, with the last common occurrences of scaphites in the Elk Butte Member of the Pierre Shale in eastern South Dakota (representing the H. nebrascensis Zone).