Hoploscaphites sp.

LANDMAN, NEIL H. & COBBAN, WILLIAM A., 2003, Ammonites from the Upper Part of the Pierre Shale and Fox Hills Formation of Colorado, American Museum Novitates 3388, pp. 1-46 : 33-35

Hoploscaphites sp. cf. H. birkelundae

Landman and Waage, 1993 Figures 21, 22

Compare: Hoploscaphites birkelundi Landman and Waage, 1993: 119 , figs. 60, 85–90 (incorrect original spelling).

MATERIAL: CSM 5209, a macroconch, from the upper transition member of the Pierre Shale, Green Mountain , near Golden, Jefferson County, Colorado (fig. 1, loc. 4) .

DESCRIPTION: CSM 5209 is a large, compressed macroconch 106.8 mm in maximum length (fig. 21). It is somewhat crushed producing a slight offset from one side to the other. The specimen is an internal mold with most of the outer shell missing. The adapical part of the phragmocone and part of the venter and flanks of the body chamber are missing on the left side. The specimen is nearly circular in side view. The umbilicus is tiny but due to postmortem breakage, it is not measurable. The exposed phragmocone is 81.4 mm in maximum diameter, compressed, and very involute. It is 0.67 whorl long and terminates well below the line of maximum length. The angle between the last septum and the line of maximum length is approximately 60°.

The body chamber consists of a short shaft and slightly reflected hook. The body chamber is tightly curved in side view and is 0.44 whorl in angular length. There is very little gap, if any, between the phragmocone and reflected hook. The umbilical shoulder shows a slight bulge, especially on the left side. The apertural angle is 53.5° and the apertural margin is fairly straight with a constriction on the right side.

Despite the poor preservation, it is clear that the whorl section of the phragmocone was compressed ovoid with very broadly rounded flanks. The whorl height at the base of the body chamber is 50.4 mm. The whorl section of the body chamber is also compressed ovoid. The ratio of whorl width to whorl height at midshaft is 0.54. The umbilical wall is steep and convex and the umbilical shoulder is sharply rounded. The inner one­half of the flanks is nearly flat or inclined very slightly outward with maximum width at midwhorl height. The outer flanks converge in a broad convexity toward the ventrolateral shoulder, which is sharply rounded. The venter is very broadly rounded. At the point of recurvature, the whorl section is slightly more depressed because of a reduction in whorl height; the ratio of whorl width to whorl height is 0.68. The umbilical wall is steep and slightly convex and the umbilical shoulder is fairly sharply rounded. The flanks are very broadly rounded and converge to the ventrolateral shoulder, which is fairly sharply rounded; the venter is very broadly rounded.

Ornamentation on the exposed phragmocone consists of moderately widely spaced, broad prorsiradiate ribs. They appear swollen (or even bullate) on the adapical part of the phragmocone. They cross the venter with a slight adoral projection; there are 4 ribs/cm on the venter. Ventrolateral tubercles are preserved on the middle and adoral end of the exposed phragmocone and are evenly spaced at distances of approximately 6 mm. A row of three flank tubercles is also preserved on the middle of the phragmocone near the ventrolateral margin. Each tubercle occurs on a single rib separated by a nontuberculate rib.

The ribs on the body chamber are low and broad with wide interspaces. On the adapical part of the shaft, the ribs bend backward across the umbilical shoulder and form a weak concavity on the inner flanks. They then become broadly convex and weakly prorsiradiate on the outer flanks. There is a patch of adapically pointed chevrons on the middle of the flanks on the left side, indicating a healed injury. On the adoral part of the shaft and hook (as shown on the left side), the ribs become increasingly prorsiradiate. They are slightly concave on the umbilical shoulder and innermost flanks but then bend markedly forward and cross the middle and outer flanks in a weak convexity. The spacing between rib crests at midflank at the point of recurvature is approximately 4.5 mm. Ribs are not preserved on the venter of the body chamber. Even where shell is present on the adapical part of the shaft, there are no ventral ribs. Ventrolateral tubercles occur on the entire body chamber nearly to the aperture; they may indeed extend to the aperture, but the outer shell is not preserved there. The tubercles are more or less evenly spaced at intervals of 6–8 mm and are paired across the venter.

The last few sutures are preserved but they are strongly approximated and difficult to draw. A portion of the last suture, including parts of the first and second lateral saddles and the first lateral lobe, is illustrated in figure 22. The suture is similar to that of Hoploscaphites birkelundae except that the first lateral lobe is unusually narrow and the second lateral saddle is unusually wide.

DISCUSSION: This specimen is very similar to Hoploscaphites birkelundae in its ornamentation and proportions (see Landman and Waage, 1993: figs. 85F, 86E–G, 87D, E). However, it is much larger than any previously described specimen of this species. It is 106.8 mm in maximum length whereas the largest specimen of H. birkelundae described by Landman and Waage (1993: 125) is 76.5 mm in maximum length. As we develop a better understanding of the intraspecific variation within H. birkelundae , we may discover that this large specimen represents the upper size limit of the species. Indeed, in other species of Hoploscaphites , macroconchs show a broad variation in size (see Landman and Waage, 1993: figs. 58, 75). We presently refer this specimen to Hoploscaphites sp. cf. H. birkelundae .

OCCURRENCE: Upper transition member of the Pierre Shale, Jefferson County, Colorado .

Figures

Fig. 22. Last suture of Hoploscaphites sp. cf. H. birkelundae Landman and Waage, 1993, CSM 5209, Fox Hills Formation, Green Mountain, near Golden, Jefferson County, Colorado. The suture is drawn at a whorl height of 49.2 mm.