Redescription of the Late Cretaceous (late Santonian) ammonite Desmoscaphites bassleri Reeside, 1927, from the Western Interior of North America

Abstract

The Late Cretaceous scaphitid ammonite Desmoscaphites bassleri Reeside, 1927, is redescribed based on newly collected material from Montana. This species is an important index fossil and demarcates the uppermost Santonian in the U.S. Western Interior. It is widely distributed and extends from Alberta to New Mexico, primarily along the western margin of the Interior Seaway. The adult shell is moderately stout and closely coiled, with small ventrolateral tubercles on the body chamber. Microconchs are smaller than macroconchs but otherwise similar in morphology. There is an abrupt change in ornamentation on the late juvenile whorls from coarse, widely spaced ribs to threadlike, more closely spaced ribs, with a pronounced sinuosity. The ribs later become more widely spaced on the shaft of the body chamber and more closely spaced again near the aperture. The early whorls are characterized by constrictions, which represent flexures in the shell wall. The constrictions first appear at a shell diameter of approximately 3 mm and occur regularly thereafter at intervals of 65–90° up to a shell diameter of approximately 15 mm. The suture is characterized by a trifid first lateral lobe. This lobe does not pass through an asymmetrically bifid stage during ontogenetic development.