Coahuilites Böse, 1928

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 : 13-15

Genus Coahuilites Böse, 1928

TYPE SPECIES: Coahuilites sheltoni Böse, 1928: 283 , pl. 13, figs. 4–11, by original designation .

Coahuilites sheltoni Böse, 1928

Figures 7–10

Coahuilites sheltoni Böse, 1928: 283 , pl. 13, figs. 4–11.

Coahuilites sp. Scott and Cobban, 1965, 1975, 1986a, 1986b.

Coahuilites sheltoni Böse, 1928 . Cobban and Kennedy, 1995: 12, figs. 2.8, 2.9, 7.1, 8.1–8.3 (with full synonymy).

Coahuilites sheltoni Böse, 1928 . Kennedy et al., 1996:6, figs. 2, 3, 4B.

TYPE: Holotype, by original designation, is the original specimen of Böse (1928: pl. 13, fig. 7) from the Escondido Formation about 3 mi (4.8 km) south of Alamo Viejo in the region of Villa de Juarez, Coahuila, Mexico.

MATERIAL: Seven specimens, mostly whorl fragments without outer shell: USNM 519508 from the upper transition member of the Pierre Shale, about 400 ft (121.9 m) below the Laramie coal, SW ¼ sec. 34, T 13S, R 58W, Elbert County, Colorado; USNM 519509, 519517, and 519518 from the Fox Hills Formation, about 40 ft (12.2 m) below the Milliken Sandstone Member, NW ¼, NE ¼ sec. 34, T 4N, R 67W, Weld County, Colorado (fig. 1, loc. 15); DMNH 23359 from the upper transition member of the Pierre Shale, NE ¼ sec. 27, T 6S, R 58W, Elbert County, Colorado (fig. 1, loc. 17); AMNH 47397 from the Fox Hills Formation, about 40 ft (12.2 m) below the Milliken Sandstone Member, center, line between secs. 23 and 26, T 4N, R 67W, Wildcat Mound, about 2 mi (3.2 km) south of Milliken, Weld County, Colorado (fig. 1, loc. 25); and USNM 519520 from the base of the upper transition member of the Pierre Shale, NW ¼, SW ¼ sec. 5, T 5S, R 69W, Jefferson County, Colorado (fig. 1, loc. 27).

DESCRIPTION: DMNH 23359 is a fragment of a body chamber, a little less than one­half whorl in angular length, with most of the right side worn away (fig. 7C, D). The specimen is an internal mold with patches of nacreous shell still attached in places. An imprint of a large bryozoan colony occurs on the adapical half of the specimen. The estimated whorl height at the adoral end of the specimen is 65.0 mm. Coiling is very involute, but the umbilicus is not preserved. The inner flanks are slightly concave and the out­ er flanks are very broadly rounded to nearly flat and converge toward the venter. The ventrolateral shoulder is broadly rounded. The venter is moderately well rounded with a blunt midventral ridge on the adapical end of the specimen, which weakens adorally. The patch of shell on the inner flanks bears delicate, straight, prorsiradiate ribs. There is a row of four sharp nodes on the midflanks. The nodes are evenly spaced at distances of approximately 18–20 mm. They give rise to weak ribs that join six low, elongate nodes on the ventrolateral margin. These nodes are evenly spaced at distances of approximately 20–25 mm. The venter is smooth.

USNM 519508 is a fragment of an internal mold of the adoral end of the phragmocone and adapical end of the body chamber (fig. 7A, B). It is approximately one­third whorl in angular length. It is very involute, but the umbilicus is not preserved. The estimated whorl height at the adapical end of the specimen is 46.0 mm. The whorl section is compressed with maximum width at one­third whorl height. The ratio of whorl width to whorl height at the adapical end of the specimen is 0.48. The inner one­third of the flanks is very concave and the outer twothirds is very broadly rounded and converges toward the venter. The venter is sharply rounded with a very weak midventral ridge on the phragmocone. There is a row of six concave bullae at one­third whorl height. They are evenly spaced at distances of approximately 10–11 mm. There is a row of seven low swellings on the ventrolateral margin. The venter is smooth.

The suture has a broad shallow external lobe (E) with a broad, little incised median saddle (fig. 8). The first lateral saddle (E/L) is split in two by a broad, moderately incised lobe. All adjacent saddles have entire terminations.

USNM 519509 is a partially crushed internal mold 119.0 mm in diameter (fig. 9). It comprises part of the inner whorls of the phragmocone and approximately one­half whorl of the body chamber. The whorl section at the adoral end of the specimen is compressed with maximum width at midwhorl height; the ratio of whorl width to whorl height is 0.47. As in the other specimens, the shell is very involute. The inner flanks are concave and the outer flanks are very broadly rounded and converge toward the venter. The ventrolateral shoulder is moderately well rounded and the venter is well rounded to weakly fastigate. There is a row of 10 nodes on the midflanks of the last whorl, five on the phragmocone, and five on the body chamber. The nodes are more or less evenly spaced at distances of approximately 15 mm. A row of five nodate swellings occurs on the ventrolateral margin of the body chamber. No ribbing is present.

USNM 519520 is one­half whorl long and 125 mm in diameter (fig. 10). It comprises the adapical end of the phragmocone and most of the body chamber. It is an internal mold with patches of shell preserved. Most of the venter and left side of the specimen are missing. The inner whorls are preserved, showing an ontogenetic change in whorl shape, especially a progressive rounding of the venter. The whorl width and height at the base of the body chamber are 21.6 mm and 48.5 mm, respectively; the ratio of whorl width to height is 0.44. The inner one­third of the flanks is very concave and the outer two­thirds is broadly rounded and converges toward the venter. The venter is well rounded with a weak midventral ridge. Maximum width occurs at midwhorl height. There is a row of seven sharp bullae at one­third whorl height. The bullae become progressively more widely spaced adorally; the distance between the two most adoral bullae is about 17 mm. A row of low nodate swellings is also present near the ventrolateral shoulder.

AMNH 47397 (not illustrated) is a small scrap of the ventral part of the phragmocone. The venter is well rounded to weakly fastigate. The suture is well exposed and matches that of USNM 519508 (fig. 8).

DISCUSSION: USNM 519508, which was collected by J.B. Reeside, Jr., in 1932, was described by Lavington (1933: 404) from the ‘‘transition zone’’ of the Pierre Shale below the Fox Hills Formation. All of the Colorado specimens are similar to those reported from the Fox Hills Formation of Niobrara County, Wyoming ( Kennedy et al., 1996: 6–11, figs. 3, 4B).

OCCURRENCE: Coahuilites sheltoni occurs in the Baculites clinolobatus and Hoploscaphites birkelundae zones of the Western Interior (fig. 11, appendix). It is recorded from 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 Elbert, Jefferson, and Weld counties, Colorado ( Kennedy et al., 1996). The species has also been reported from Trans­Pecos Texas and northern Mexico ( Cooper, 1970) and from the Prairie Bluff Chalk in Alabama and Mississippi ( Cobban and Kennedy, 1995).