Scaphites of the “Nodosus Group” from the Upper Cretaceous (Campanian) of the Western Interior of North America

Abstract

Scaphitid ammonites (scaphites) are common in the Upper Cretaceous Pierre Shale and Bearpaw Shale of the Western Interior of North America. We redescribe Hoploscaphites nodosus (Owen, 1852) and H. brevis (Meek, 1876) from the Baculites compressus–B. cuneatus zones of the upper Campanian. The types of both of these species were collected in the mid-19th century in what was then called Nebraska Territory, and included parts of present-day South Dakota, North Dakota, and Montana. Based on our present knowledge of the distribution of these species, the type material was probably collected from the B. compressus–B. cuneatus zones in the Pierre Shale at Sage Creek, a tributary of the Cheyenne River, Pennington County, South Dakota.Traditionally, the more robust, more coarsely ornamented scaphites (comprising the “nodosus group”) from the Pierre Shale and Bearpaw Shale were assigned to Jeletzkytes Riccardi, 1983, and the more slender, more finely ornamented scaphites were assigned to Hoploscaphites Nowak, 1911. However, our large collections of these scaphites from the Baculites compressus–B. cuneatus zones reveal a complete intergradation between the two morphological extremes, and for many specimens, the choice of genus is arbitrary. In addition, our studies of other biostratigraphic zones in the Pierre Shale and Bearpaw Shale reveal that cooccurring species of these two “genera” share more in common with each other than they do with congeneric species from other horizons. Furthermore, contrary to earlier assumptions, Jeletkytes is not endemic to the Western Interior Basin of North America and occurs, for example, in the U.S. Atlantic Coastal Plain and Europe. We thus provisionally treat Jeletzkytes as a junior subjective synonym of Hoploscaphites. This expanded definition of Hoploscaphites is consistent with present-day concepts of other scaphitid genera such as Discoscaphites Meek, 1876, and Trachyscaphites Cobban and Scott, 1964.In Hoploscaphites nodosus and H. brevis, the juvenile shell is planispirally coiled with a small umbilicus. The whorl section is initially depressed and becomes more compressed through ontogeny. The angle of the body chamber in juveniles is approximately two-thirds of a whorl. At the approach of maturity, the shell uncoils, forming a relatively long shaft and recurved hook. The ratio of whorl width to whorl height reaches a minimum value at midshaft. The apertural margin at maturity is constricted and terminates in a flared lip. Commonly, the last two or three septa, corresponding to the formation of the hook, are more closely spaced (approximated). These features indicate that the rate of growth decreased and eventually stopped at maturity (“morphogenetic countdown” associated with determinate growth). Both species of scaphites occur as dimorphs, which are referred to as macroconchs (presumably females) and microconchs (presumably males). In samples of specimens of the same species within a single concretion, macroconchs are approximately 20% larger than microconchs. In addition to size, dimorphs are distinguished by differences in shape, including the presence or absence of an umbilical bulge, the size of the umbilical diameter, the outline of the umbilical shoulder relative to that of the venter in side view, and the relative change in whorl height in passing from the mature phragmocone to the shaft of the body chamber.The holotype of Hoploscaphites nodosus, by monotypy, is UC 6381, the original of Scaphites nodosus Owen (1852: 581, pl. 8, fig. 4). Adults exhibit a range of variation in size, degree of compression, and coarseness of ornament. The exposed phragmocone occupies most of the coiled portion of the shell, and is approximately two-thirds of a whorl in angular length. Adults are large (LMAX averages 91.8 mm in macroconchs and 78.0 mm in microconchs) and ellipsoidal in side view, with a strongly recurved hook (apertural angle averages 73° in macroconchs). The ratio of whorl width to whorl heigh