Jeletzkytes 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 : 39-44

Jeletzkytes sp. cf. J. dorfi

Landman and Waage, 1993 Figure 24 Compare: Jeletzkytes dorfi Landman and Waage, 1993: 184 , figs. 141–148.

MATERIAL: USNM 519511 from near the base of the upper transition member of the Pierre Shale, SW ¼, SE ¼ sec. 2, T 1N, R 70W, Boulder County, Colorado (fig. 1, loc. 7).

DESCRIPTION: USNM 519511 is a large, robust, elongate macroconch (fig. 24). It is an internal mold; a portion of the phragmocone and the midflanks of the body chamber are crushed, so that the elongate form of the specimen may be due in part to postmortem distortion. The left side of the specimen and most of the venter are not preserved. It is difficult to pinpoint the position of the base of the body chamber but it is probably near the line of maximum length. The specimen is 117.7 mm in length. It is clearly a macroconch because the umbilical shoulder of the body chamber is straight in side view and shows a slight bulge. The hook is only slightly separated from the phragmocone; the apertural angle is estimated at 47°, to the extent that it can be measured at all, given that the tip of the aperture is missing. The umbilicus is 6.5 mm in diameter.

The whorl section of the adapical part of the phragmocone is compressed subquadrate. The umbilical wall is inclined outward, the umbilical shoulder is moderately well round­ ed, and the flanks are broadly rounded. The ventrolateral shoulder is sharply rounded and the venter is very broadly rounded to flat. The whorl section becomes more compressed with an increase in whorl height toward the line of maximum length, but it is unclear how much of this change is due to crushing. The umbilical wall is steep and subvertical, and the umbilical shoulder is sharply round­ ed. The flanks are very broadly rounded and gently slope toward the ventrolateral shoul­ der. The ventrolateral shoulder is sharply rounded and the venter is nearly flat. The whorl section at midshaft is compressed subquadrate; the ratio of whorl width to whorl height is 0.84. The umbilical wall is strongly convex and the umbilical shoulder is fairly sharply rounded. The flanks are very broadly rounded with maximum width at midwhorl height. The ventrolateral shoulder is fairly sharply rounded, and the venter is very broadly rounded. The whorl section becomes equidimensional at the point of recurvature because of a reduction in whorl height; the ratio of whorl width to whorl height is 1.00.

Ornament on the phragmocone is visible on the adapical part on the right side. Ribs are prorsiradiate, coarse, and widely spaced, with intercalation and branching on the outer flanks. Each long rib bears three or four flank tubercles and each short rib bears one or two tubercles, so that there are four rows of flank tubercles, with the outermost row being the most persistent. The innermost row probably represents a continuation of the row of umbilicolateral tubercles on the body chamber, but because of postmortem crushing, it is difficult to trace these tubercles onto the body chamber. The ribs bend forward near the ventrolateral margin and cross the venter with a very weak adoral projection; there are 5 ribs/ cm on the venter. Ventrolateral tubercles are evenly spaced at distances of approximately 5 mm and are paired on either side of the venter. They are larger and more widely spaced than the outermost flank tubercles.

The body chamber is also covered with strong, widely spaced ribs with broad interspaces between them. Intercalation and branching occur on the outermost flanks. On the adapical part of the shaft, the ribs are rectiradiate and cross the flanks with a broad convexity. At midshaft, the ribs are rectiradiate on the umbilical wall and recti­ or slightly prorsiradiate on the flanks. They form a slight concavity on the innermost flanks and a broad convexity on the middle and outer flanks. They are widely spaced on the outer flanks with interspaces of approximately 5 mm between them at midshaft. Ribs become progressively more prorsiradiate toward the aperture. The ribs bend forward on the hook near the ventrolateral margin, accompanied by branching and intercalation. No ribs are preserved on the venter. Umbilicolateral tubercles occur on the entire body chamber and form a broad arc along the inner flanks although, in general, only the bases of the tubercles are preserved. They attain maximum size and spacing (approximately every 8 mm) at midshaft. Ventrolateral tubercles are preserved on the adapicalmost and adoralmost parts of the body chamber (they are absent on the middle part but were no doubt worn away). These tubercles are evenly spaced on the hook at distances of approximately 6–7 mm and are slightly offset from one side of the venter to the other. A row of tubercles occurs on the outermost flanks of the hook. These tubercles are evenly spaced at distances of approximately 5 mm and are the sites of intercalation and branching. Another tiny tubercle is visible near the aperture on the middle of the flanks.

DISCUSSION: This is a very robust specimen characterized by coarse ribbing, umbilico­ and ventrolateral tubercles, multiple rows of flank tubercles on the adapical part of the phragmocone, and a single row of flank tubercles on the hook. In these respects, it matches the description of Jeletzkytes dorfi Landman and Waage, 1993 . In terms of shell shape and coarseness of ribbing, it most closely resembles YPM 23176 from Wyoming ( Landman and Waage, 1993: fig. 142A–D). However, our specimen differs from J. dorfi in its large size. It is 117.7 mm in maximum length, whereas the largest specimen of J. dorfi recorded by Landman and Waage (1993: 192) is 104.6 mm in maximum length. It also differs in its low apertural angle (47°). In contrast, the lowest apertural angle reported by Landman and Waage (1993: 192) is 53°.

OCCURRENCE: This specimen was collected near the base of the upper transition member of the Pierre Shale, Boulder County, Colorado. The label refers to impressions of Baculites clinolobatus 20 ft (6.1 m) below this specimen, suggesting that this specimen is in or just above the B. clinolobatus Zone , in the Hoploscaphites birkelundae Zone.

ACKNOWLEDGMENTS

We thank Paul Bartos and Bryan Cooney of the Colorado School of Mines and Cope MacClintock and Tim White of the Yale Peabody Museum for the loan of specimens. The USGS provided specimens for study and facilities for work. Susan Klofak, Kathy Sarg (both AMNH), and Neal Larson (Black Hills Institute, Hill City, South Dakota) helped collect fossils in the field. Stephen Thurston (AMNH) and Kathy Sarg prepared the illustrations, Susan Klofak prepared the specimens, and Stephanie Crooms (AMNH) word­processed the manuscript.

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Figures

Fig. 24. Jeletzkytes sp. cf. J. dorfi Landman and Waage, 1993. A–C. USNM 519511, macroconch, base of upper transition member of the Pierre Shale, SW¼, SE¼ sec. 2, T1N, R70W, Boulder County, Colorado. A, Right lateral; B, apertural; C, ventral. All figures are ×1.