Series: Bulletin of the American Museum of Natural History (AMNH Bulletins) Volume: 215
Well-preserved scaphitid ammonites from the type area of the Upper Cretaceous Fox Hills Formation in north-central South Dakota are the primary focus of this study; also included are scaphites from the uppermost Pierre Shale in this area and from the Fox Hills Formation in the Lance Creek-Red Bird area of eastern Wyoming. The Fox Hills beds represent the marginal marine phase of the progradational sequence marking the final withdrawal of the Cretaceous sea from the Western Interior. Within this marine sequence, which lies above the Baculites clinolobatus Range Zone, the following three range zones based on scaphites are recognized: Jeletzkytes nebrascensis Range Zone; Hoploscaphites nicolletii Range Zone; Hoploscaphites birkelundi Range Zone.
Based on recent correlation studies using macro- and microfossils, the H. nicolletii and J. nebrascensis zones equate with the lower Upper Maastrichtian of the European boreal province; the H. birkelundi Zone is either uppermost Lower Maastrichtian or possibly lowermost Upper Maastrichtan. More than 2500 adults and several hundred juveniles were examined in this study. These specimens fall into three genera: Hoploscaphites Nowak, 1911, Jeletzkytes Riccardi, 1983, and Discoscaphites Meek, 1876. Hoploscaphites includes H. nicolletii (Morton, 1842), H. comprimus (Owen, 1852), H. melloi, n. sp., and H. birkelundi, n. sp. This genus consists of compressed shells with or without ventrolateral tubercles, and less commonly, umbilical tubercles or bullae. Jeletzkytes includes J. spedeni, n. sp., J. nebrascensis (Owen, 1852), and J. dorfi, n. sp. and is characterized by medium to large shells with conspicuous ventrolateral and umbilicolateral tubercles. Discoscaphites includes D. conradi (Morton, 1834), D. gulosus (Morton, 1834), and a micromorphic species, D. rossi, n. sp. This genus consists of small to medium shells with multiple rows of tubercles.
A cladistic analysis using the genus Scaphites Parkinson, 1811 as an outgroup indicates that Hoploscaphites and Jeletzkytes are more closely related to each other than either is to Discoscaphites. The excellent preservation of these scaphites permits study of their ontogeny. The embryonic shell or ammonitella is approximately 700 [micrometers] in diameter and exhibits a tuberculate micro-ornamentation. The ammonitella is spheroidal in Hoploscaphites and Jeletzkytes and ellipsoidal in Discoscaphites. The juvenile shell is closely coiled and expands gradually in whorl width and whorl height. There is a slight change at approximately 5 mm shell diameter, coincident with the first appearance of ornament in the form of ribs. Ventrolateral tubercles appear as early as 10-15 mm shell diameter followed by the development of flank tubercles in species of Discoscaphites and Jeletzkytes. Rows of flank tubercles develop in succession from the ventrolateral margin toward the umbilicus. The angular length of the juvenile body chamber averages approximately 240°. The mature shell is not as tightly coiled as that of the juvenile. The body chamber consists of an elongate shaft and recurved hook terminating in a constricted aperture.
The change in the shape of the body chamber coincides with a change in the pattern of ornamentation. Large ventrolateral tubercles occur on the shaft in species of Jeletzkytes and in some species of Hoploscaphites. Muscle attachment areas on the body chamber are indicated by thin layers of myostracum preserved on steinkerns. All species are strongly dimorphic at maturity. The average size of macroconchs is significantly larger than that of microconchs, but dimorphs are primarily distinguished on the basis of shape. The body chamber of macroconchs increases abruptly in height, resulting in a nearly straight umbilical shoulder and a relatively small umbilicus whereas the body chamber of microconchs expands only gradually in size; the umbilical shoulder parallels the curve of the venter and the umbilicus is relatively broad.
Macroconchs tend to be more sharply differentiated morphologically between species than are microconchs. In species of Hoploscaphites and Discoscaphites, septal approximation occurs over more chambers and is more marked in macroconchs than in microconchs. Macroconchs are more abundant than microconchs in most species. Remains of upper and lower mandibles are associated with all three scaphite genera. The lower mandible is composed of paired, externally convex plates that lie opposite one another, their ventral commissure aligned along the plane of bilateral symmetry. The plates are chitinous with an outer layer of calcite. The upper mandible, which is composed only of chitin, is a single element with two vertical, winglike, lateral parts that converge anteriorly to form a beaklike structure. Like the plates of the lower mandible, wings of the upper mandible are ornamented with concentric ribs that follow finer growth lines. Commonly occurring with mandible remains are hollow chitinous structures consisting of an inverted cup-shaped base 2-5 mm in diameter from which two hornlike projections taper upward to sharp points. Their identification is uncertain but, based on their uniform shape, they may represent hooklike structures rather than radular elements.
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