Sphenodiscus Meek, 1871

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 : 15-23

Genus Sphenodiscus Meek, 1871

TYPE SPECIES: Ammonites lenticularis

Owen, 1852: 579 (non Young and Bird,

1828: 269, fig. 5), by original designation, = Ammonites lobata Tuomey, 1856: 168.

Sphenodiscus pleurisepta ( Conrad, 1857)

Figures 12–15

Ammonites pleurisepta Conrad, 1857: 159 , pl. 15, fig. 1.

Sphenodiscus lenticularis (Owen) . Kellum, 1962: 68, pl. 4, figs. 3, 4; pl. 5, fig. 1; pl. 6, figs. 1, 2.

Sphenodiscus pleurisepta ( Conrad, 1857) . Cobban and Kennedy, 1995: 12, fig. 8.5 (with full synonymy).

Sphenodiscus pleurisepta ( Conrad, 1857) . Kennedy et al., 1996: 11, figs. 4A, 5–12.

TYPE: The holotype is USNM 9888, said to be from ‘‘Jacun, 3 miles [4.8 km] below Laredo,’’ but probably from the Escondido Formation of the Rio Grande Region, probably in Maverick County, Texas ( Stephenson, 1941, 1955).

MATERIAL: Five USNM specimens, most of which are fragments: USNM 77358 from the Milliken Sandstone Member of the Fox Hills Formation, sec. 35, T 7N, R 66W, Weld County, Colorado (fig. 1, loc. 18); USNM 519507 from the upper part of the Pierre Shale, NW ¼ sec. 3, T 2N, R 68W, Weld County, Colorado (fig. 1, loc. 16); USNM 519506 from the Fox Hills Formation, 5 mi (8.0 km) west and 2.5 mi (4.0 km) north of Ault, Weld County, Colorado (fig. 1, loc. 14); and USNM 519510 and 519519 from near the top of the Pierre Shale or lower part of the Fox Hills Formation, SW ¼, SE ¼ sec. 35, T 6N, R 67W, Weld County, Colorado (fig. 1, loc. 20).

DESCRIPTION: USNM 519506 is a fragment of an internal mold nearly one­half whorl long (fig. 12). It consists of the adoral part of the phragmocone and the adapical part of the body chamber. It retains part of the shell on the right side. The whorl height at the adoral end of the specimen is 80.8 mm. The whorl section is compressed oxyconic with maximum width at midwhorl height; the ratio of whorl width to whorl height is 0.35. The specimen is involute with a tiny umbi­ licus. The umbilical wall is inclined outward and the umbilical shoulder is broadly round­ ed on the adapical end of the specimen. The umbilical wall becomes steeper and the umbilical shoulder more sharply rounded toward the adoral end of the specimen. The inner flanks are concave, the midflanks are very broadly rounded, and the outer flanks are flat and converge to the acute venter. There is a row of midflank bullae—five on the left side—that are more or less equally spaced at distances of 11–13 mm. These bullae seem to disappear toward the adoral end of the specimen, which may be preservational. There is also a row of low swellings on the outer flanks—eight on the left side. They are more or less evenly spaced at distances of approximately 20 mm. Some of these swellings, especially those on the right side, appear to be crescentic. The patches of outer shell on the right side bear fine concave lirae. The suture is characterized by narrow stemmed saddles (fig. 13).

USNM 519519 (not illustrated) is a completely septate fragment approximately 0.25 whorl long. It is a slightly distorted internal mold missing part of the inner flanks on the left side. The whorl height at the adoral end of the specimen is approximately 87 mm. The whorl section is oxyconic and there is a row of swellings on the outer flanks (the inner flanks are too poorly preserved to detect bullae).

USNM 519510 (fig. 14) is in the same lot as USNM 519519. It is a fragment of the adapical part of the body chamber 0.125 whorl long. The two specimens may belong to the same individual but they do not obviously fit together. The whorl height at the adoral end of USNM 519510 is approximately 130 mm. The left side and the venter of the body chamber are missing. The flanks are very broadly rounded and converge toward the venter. There are two sharp tubercles on the midflanks. What appears to be a lobate muscle scar occurs just adoral of the ultimate septum on the right side (fig. 14). It covers the umbilical wall and extends onto the inner one­quarter of the flanks. The scar is approximately 27 mm wide at its widest point. The scar is demarcated by a slight groove on its adapical side and a faint ridge on its adoral side. It is covered in part with a thin layer of nacre. An impression of the inner whorls occurs on the left side of the specimen. The impression reveals a row of sharp tubercles on the inner flanks and another row of low swellings on the outer flanks.

USNM 519507 is an internal mold about one­half whorl long consisting of the adoral part of the phragmocone and adapical part of the body chamber (fig. 15). The whorl height at the adoral end of the specimen is approximately 74 mm. The whorl section is oxyconic with the maximum width at midwhorl height; the ratio of whorl width to whorl height is 0.29. There is a row of sharp tubercles at approximately one­third whorl height and a row of low nodate swellings on the outer flanks.

USNM 77358 (not illustrated) is from approximately the same locality as USNM

SUBORDER ANCYLOCERATINA WIEDMANN, 1966

519506. It is a fragment of a body chamber about one­quarter whorl long missing most of the left side. The whorl section is oxyconic with maximum whorl width at midwhorl height. A row of swellings occurs on the out­ er flanks. The row of tubercles on the midflanks is mostly absent, perhaps due to preservation—there is one small tubercle on the adapical end of the specimen. We provisionally include this specimen in Sphenodiscus pleurisepta , although the absence of midflank tubercles is suspicious.

DISCUSSION: The presence of two rows of tubercles or nodes distinguishes Sphenodiscus pleurisepta from Sphenodiscus lobatus ( Tuomey, 1856) . Sphenodiscus pleurisepta differs from Coahuilites sheltoni Böse, 1928 , in its compressed, oxyconic whorl section.

OCCURRENCE: USNM 519510 and 519519 are in the same lot as USNM 519516, a microconch of Jeletzkytes dorfi . Sphenodiscus pleurisepta occurs in the Hoploscaphites birkelundae Zone of the Pierre Shale in Meade and Pennington counties, South Dakota ( Kennedy et al., 1996; Larson et al., 1997), the Fox Hills Formation in Niobrara County, Wyoming ( Kennedy et al., 1996), and the upper part of the Pierre Shale and Fox Hills Formation in Weld County, Colorado ( Kennedy et al., 1996) (fig. 11, appendix). This species has also been reported from the Escondido Formation in Trans­Pecos Texas and northern Mexico ( Stephenson, 1941, 1955), the Corsicana Formation in northern Texas ( Kennedy and Cobban, 1993), the Owl Creek Formation in Tennessee, Mississippi, and Missouri ( Kennedy and Cobban, 2000), the Prairie Bluff Chalk in Alabama and Mississippi ( Cobban and Kennedy, 1995), and the Severn Formation in Maryland ( Kennedy et al., 1997).

SUPERFAMILY TURRILITACEAE GILL, 1871

FAMILY BACULITIDAE GILL, 1871

Genus Trachybaculites Cobban and Kennedy, 1995

TYPE SPECIES: Baculites columna Morton, 1834: 44 , pl. 19, fig. 8, by subsequent designation by Cobban and Kennedy, 1995: 29.

Trachybaculites sp. cf. T. columna ( Morton, 1834)

Figure 16

Compare: Trachybaculites columna ( Morton, 1834) . Cobban and Kennedy, 1995: 29, figs. 10.1, 10.3, 13.4–13.6, 14.3, 14.9, 17.1–17.14, 17.17–17.31 (with complete synonymy).

Trachybaculites columna ( Morton, 1834) . Kennedy and Jagt, 1998: 161, pl. 2, figs. 1–6.

Trachybaculites columna ( Morton, 1834) . Klinger and Kennedy, 2001: 267, fig. 191.

MATERIAL: CSM 5612 from the Fox Hills Formation near Wellington, Larimer County, Colorado.

DESCRIPTION: The specimen is 63 mm long and is embedded in a sandstone block with the dorsum, right side, and part of the venter exposed. No sutures are visible and the specimen is presumably all body chamber. The specimen is slightly crushed and twisted but the original whorl section appears to have been compressed ovoid. Ribs are sharp, straight, and prorsiradiate on the flanks. There are four ribs in a distance equivalent to the whorl height (rib index = 4). Ribs are strong and slightly convex on the venter and weak on the dorsum.

DISCUSSION: The rib index of this specimen, which equals 4, is higher than that in other specimens of Trachybaculites columna , in which the rib index is 2 or 3 ( Cobban and Kennedy, 1995; see especially fig. 13.4–6 with a rib index of 3). The lectotype (ANSP 72867a) and paralectotypes (ANSP 72867b– f) all have a rib index of 3 ( Klinger and Kennedy, 2001: fig. 191). BHI 2035 from the Fox Hills Formation of north­central South Dakota also has a rib index of 3 ( Cobban and Kennedy, 1992:683, fig. 1.7–9). Nevertheless, the size of our specimen and its coarse ribbing suggest that it is closely related to Trachybaculites columna .

OCCURRENCE: This specimen is from the Fox Hills Formation near Wellington, Colorado. The only recorded specimens of Trachybaculites columna from the Western Interior are from the Hoploscaphites nicolletii and Jeletzkytes nebrascensis zones of the Fox Hills Formation in north­central South Dakota ( Cobban and Kennedy, 1992). The species is also reported from the Prairie Bluff Chalk of Alabama and Mississippi ( Cobban and Kennedy, 1995), the Corsicana Formation in Guadalupe County, Texas ( Stephenson, 1941), and the Garzas Formation in the San Joaquin Valley, California ( Matsumoto, 1959).

SUPERFAMILY SCAPHITACEAE GILL, 1871

FAMILY SCAPHITIDAE GILL, 1871

SUBFAMILY SCAPHITINAE GILL, 1871

GENUS HOPLOSCAPHITES NOWAK, 1911

TYPE SPECIES: Ammonites constrictus J. Sowerby, 1817: 189, pl. A, fig. 1, by original designation.

Figures

Fig. 12. Sphenodiscus pleurisepta (Conrad, 1857). A–D. USNM 519506, Fox Hills Formation, 5 mi (8.0 km) west and 2.5 mi (4.0 km) north of Ault, Weld County, Colorado. A, Left lateral; B, apertural; C, ventral; D, right lateral. All figures are ×0.94.

Fig. 13. Composite suture (last suture plus parts of fourth from last suture) of Sphenodiscus pleurisepta (Conrad, 1857), USNM 519506, Fox Hills Formation, 5 mi (8 km) west and 2.5 mi (4 km) north of Ault, Weld County, Colorado. Suture is drawn at a whorl height of 61.8 mm.

Fig. 14. Sphenodiscus pleurisepta (Conrad, 1857). A–C. USNM 519510, top of Pierre Shale or lower Fox Hills Formation, SW¼, SE¼ sec. 35, T6N, R67W, Weld County, Colorado. A, Sketch of reconstructed specimen showing the location of the muscle scar; B, close­up of the muscle scar, ×1.5; C, sketch of the muscle scar shown in B, ×1.5.

Fig. 16. Trachybaculites sp. cf. T. columna (Morton, 1834). A, B. CSM 5612, Fox Hills Formation, near Wellington, Larimer County, Colorado. A, Right lateral, ×1; B, close­up of adoral end, ×2.8.