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Scaphites of the 'Nodosus Group' from the Upper Cretaceous (Campanian)

Journal / Magazine

Series: Bulletin of the American Museum of Natural History (AMNH Bulletins) Volume: 342

By: Neil H Landman (Author), W James Kennedy (Author), William A Cobban (Author), Neal L Larso (Author)

American Museum of Natural History

Paperback | #208240
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About this book

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 height at midshaft averages 0.99 in macroconchs and 1.02 in microconchs. The whorl section is subquadrate/ovoid to reniform, with broadly rounded to flat flanks, and a broadly rounded venter. Ribs are straight to slightly flexuous and cross the venter with a weak adoral projection. There are 5–7.25 ribs/cm on the venter at midshaft in macroconchs and 6–8 ribs/cm on the venter at midshaft in microconchs. Umbilicolateral tubercles usually appear midway on the exposed phragmocone, but may already be present near the point of exposure. At midshaft, they occur at one-third to one-half whorl height and are relatively widely and evenly spaced, and usually extend to the aperture. Ventrolateral tubercles are generally present at the point of exposure. They are unevenly spaced on the exposed phragmocone, becoming more evenly spaced on the shaft. They attain their maximum size at midshaft, sometimes forming large, subspinose clavi that project out to the side. They become smaller and more closely spaced on the hook, and usually extend to the aperture. The suture is characterized by a broad, asymmetrically bifid first lateral saddle and narrow, symmetrically to asymmetrically bifid first lateral lobe.

The holotype of Hoploscaphites brevis, by original designation and monotypy, is USNM 367, the original of Scaphites nodosus var. brevis Meek (1876: 428, pl. 25, fig. 1). Adults exhibit a wide range of variation in size, degree of compression, and coarseness of ornament. Adults are small to large (LMAX ranges from 29.5 to 101.5 mm in macroconchs and from 26.7 to 81.2 mm in microconchs), and rounded to ellipsoidal in lateral view. The body chamber consists of a relatively short shaft and a weakly recurved hook (apertural angle averages 59° in macroconchs). The ratio of whorl width to whorl height averages 0.73 at midshaft in both macroconchs and microconchs. The whorl section is subquadrate/ovoid, with fairly flat flanks, and a broadly rounded venter. Ribs are fine and flexuous and cross the venter with a weak adoral projection. There are 6–14 ribs/cm on the venter at midshaft in macroconchs and 8–18 ribs/cm on the venter at midshaft in microconchs. In most small specimens, umbilicolateral tubercles are absent on the phragmocone and, instead, the primary ribs are strong and adorally concave in this area. In large specimens, umbilicolateral tubercles usually appear midway or near the adoral end of the exposed phragmocone. They are small and relatively evenly spaced. In almost all specimens, umbilicolateral tubercles are present on the body chamber, and occur at one-fourth to one-third whorl height. They are more or less uniformly spaced with, occasionally, some approximation near the point of recurvature. In specimens that are rounded in lateral view, the umbilicolateral tubercles are arranged in a broad arc, which is one of the hallmarks of this species. Ventrolateral tubercles are stronger than umbilicolateral tubercles, and are usually present on the phragmocone starting anywhere from the point of exposure to the adoral end of the phragmocone. In general, they are unevenly spaced on the phragmocone, commonly occurring in pairs or clusters, becoming more evenly spaced on the shaft. As in H. nodosus, they attain their maximum size at midshaft. The ventrolateral tubercles usually die out near the point of recurvature, but if not, they become smaller, more bullate, and more closely spaced toward the aperture. The suture is the same as that in H. nodosus.

Hoploscaphites landesi Riccardi, 1983, is a junior subjective synonym of H. brevis. The holotype is a small, compressed, finely ornamented microconch of H. brevis. It grades into larger, more robust specimens, with coarser ornament. We thereby expand the definition of H. brevis to include a wide range of variation in adult size, and argue that such variation reflects variation in the age (and size) at which individuals reach maturity. Establishment of separate species for different size specimens, given that all other aspects of their morphology (shell shape, whorl cross section, pattern of ribs, distribution of umbilicolateral and ventrolateral tubercles, and suture) are the same, seems unwarranted.

Hoploscaphites nodosus and H. brevis are widespread in the Baculites compressus–B. cuneatus zones in the Western Interior of North America, which represent a time interval of approximately 580 ky (Cobban et al., 2006). They are also present in parts of the underlying Didymoceras cheyennense Zone and the overlying B. reesidei Zone, but their exact distribution in these zones is not yet known. Hoploscaphites nodosus and H. brevis occur in the Bearpaw Shale in Alberta and Saskatchewan, the Bearpaw Shale and Pierre Shale in Montana, and the Pierre Shale in North Dakota, South Dakota, Wyoming, Colorado, and Kansas. They occur in nearshore deposits such as the unnamed shale member of the Pierre Shale in Grand County, Colorado, in offshore deposits such as the DeGrey Member of the Pierre Shale in Buffalo County, South Dakota, and in cold methane seeps in the Pierre Shale in Custer County, South Dakota. They are absent in the U.S. Gulf and Atlantic Coastal plains and northern Europe, although similar forms are present in both these areas in strata above and below the biostratigraphic interval containing the ranges of H. nodosus and H. brevis. Thus, although H. nodosus and H. brevis are endemic to the Western Interior of North America, they are part of a more broadly distributed clade that is also present in the U.S. Gulf and Atlantic Coastal plains and northern Europe.

As adults, these scaphites probably lived just above the sea bottom. They preferred 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. The large ventrolateral tubercles on the shaft served as an antipredatory defense against attacks from behind. Healed injuries, as well as lethal injuries, are common on adult shells, especially on the adapical end of the body chamber. As with modern cephalopods, these scaphites were probably preyed upon by fish, reptiles, crustaceans, and other cephalopods.

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