Exiteloceras Hyatt, 1894

KENNEDY, W. J., LANDMAN, N. H., COBBAN, W. A. & SCOTT, G. R., 2000, Late Campanian (Cretaceous) Heteromorph Ammonites From The Western Interior Of The United States, Bulletin of the American Museum of Natural History 2000 (251), pp. 1-88 : 51-65

Genus Exiteloceras Hyatt, 1894

TYPE SPECIES: Ancyloceras jenneyi Whitfield, 1877, p. 42 , by the subsequent designation of Diener, 1925, p. 88.

DIAGNOSIS: Planispiral, loosely coiled, earliest whorls hamitid with two or more open shafts connected by elbows. Adult whorls somewhat hamitid to criocone. All whorls have subelliptical cross sections. Ornament of narrow ribs bearing small, sharp spines (fig. 47). Size dimorphism present.

DISCUSSION: Specimens of the type species are quite variable in their form, but most have hamitid early growth stages, becoming open criocone when adult. The genus is represented only by the type species in the area under consideration.

Exiteloceras has been referred to both Nostoceratidae and Diplomoceratidae by previous authors. We place it in the former with no great confidence, given the uncertainty surrounding its phylogenetic relationships.

OCCURRENCE: Exiteloceras occurs in the Western Interior in the upper Campanian zone of Exiteloceras jenneyi . Specimens have been found in the Bearpaw Shale in Montana; the Pierre Shale in Montana, Wyoming, South Dakota, Colorado, and New Mexico; the Lewis Shale in Colorado and New Mexico; and in the Mancos Shale and Rifle shale unit of the Iles Formation in Colorado. The genus has also been found in the Navesink Formation in New Jersey, the Mount Laurel Sand in Delaware, the Ripley Formation in Mississippi, and in Colombia (Kennedy, 1992).

Exiteloceras jenneyi jenneyi (Whitfield, 1877)

Figures 40 , 42 , 43 , 44A–C , 45–48

Ancyloceras Jenneyi Whitfield, 1877: 42 .

Ancyloceras Jenneyi Whitfield. Whitfield, 1880: 452 , pl. 15, fig. 5; pl. 16, figs. 7–9.

Ancyloceras jenneyi Whitfield. Stanton, 1888: 185 .

Ancyloceras jenneyi Whitfield. Boyle, 1893: 40 .

Exiteloceras (Ancyloceras) Jennyi (sic) Whitfield. Hyatt, 1894: 577.

Exiteloceras jenneyi (Whitfield) . Grabau and Shimer, 1910: 205, fig. 1474.

Turrilites (Exiteloceras) Jenneyi Whitfield. Diener, 1925: 88 .

Exiteloceras jenneyi (Whitfield) . Roman, 1938: 445.

Exiteloceras jenneyi (Whitfield) . Wright, 1957: L224, fig. 251, 7.

Exiteloceras jenneyi (Whitfield) . Scott and Cobban, 1965: unnumbered fig.

Exiteloceras jenneyi (Whitfield) . Gill and Cobban, 1966: A32.

Exiteloceras jenneyi (Whitfield) . Scott, 1969: pl. 2, unnumbered fig.

Exiteloceras jenneyi (Whitfield) . Cobban, 1970: D73, figs. 1g –k, n–r, 3.

Exiteloceras jenneyi (Whitfield) . Gill and Cobban, 1973: 10, fig. 6.

Exiteloceras jenneyi (Whitfield) . Hirsch, 1975: fig. 8a–c.

Exiteloceras jenneyi (Whitfield) . Scott and Cobban, 1975: unnumbered fig.

Exiteloceras jenneyi (Whitfield) . Kennedy and Cobban, 1976: fig. 4.

Exiteloceras jenneyi (Whitfield) . Scott and Cobban, 1986a: unnumbered fig.

Exiteloceras jenneyi (Whitfield) . Scott and Cobban, 1986b: unnumbered fig.

Exiteloceras jenneyi (Whitfield) . Bryant and Martin, 1988: fig. 23.

Exiteloceras jenneyi (Whitfield) . Kennedy, 1992: 174, figs. 11A, B, 14A, C, 15.

Exiteloceras jenneyi (Whitfield) . Wright, 1996: 250, fig. 195, 1a–d.

Exiteloceras jenneyi (Whitfield, 1877) . Kennedy and Cobban, 1997: 72 (pars).

Exiteloceras jenneyi Whitfield, 1877 . Larson et al., 1997: 45, unnumbered figs.

TYPES: The holotype and paratype have USNM number 12295. They came from ‘‘limestone referred to the Fort Pierre Group of the Upper Missouri Cretaceous , on the East Fork of Beaver Creek , three miles west of Camp Jenney , Black Hills’ ’ (Whitfield, 1877: 43). Beaver Creek is a southeastwardflowing stream that joins the Cheyenne River south of Newcastle on the west flank of the Black Hills in eastern Wyoming. The specimens came from a limestone concretion(s) in rocks now assigned to the upper part of the Pierre Shale (lower and middle parts of the Monument Hill Bentonitic Member or from the upper part of the underlying shale; Robinson et al., 1964: 89–93) .

MATERIAL: Abundant specimens come from over 100 localities mainly in Montana, South Dakota, Wyoming, and Colorado. Most specimens are from limestone concretions, and preservation is generally good. The largest collections consist of 61 specimens ( USGS Mesozoic locality D304) and 48 specimens ( USGS Mesozoic locality D2854), both from the Terry Sandstone Member of the Pierre Shale near Fort Collins, Colorado (fig. 2, locs. 35, 36).

DIAGNOSIS: A moderate-sized planispiral species characterized by juvenile loose, elliptical coiling and by adult loose, circular coiling. Ornament of mostly single ribs, each of which bears a small spinose tubercle at the margin of the venter. Suture deeply and intricately subdivided.

DESCRIPTION: The holotype (fig. 42B, C) consists of parts of a phragmocone and much of the body chamber of a fairly small specimen (microconch?). Coiling is planispiral and broadly elliptical; the preserved part of the larger limb has a diameter of about 95 mm (measurements from a plaster cast). Whorl cross sections are oval with a broadly rounded dorsum. The flanks are very broadly rounded and converge to a fairly narrow venter that is narrowly rounded in intercostal section and flattened in costal section. The maximum breadth of the whorl is a little below the middle of the flank. The ratios of whorl breadth to whorl height at two places on the outer whorl are 0.75 and 0.77 (fig. 39). Ornament consists of narrow ribs and a row of small tubercles on each side of the venter. Ribs are much narrower than the interspaces; they arise on the outer part of the dorsum and become highest on the lower half of the flank. Ribs are rursiradiate, most are slightly flexuous, and all support a small tubercle at the margin of the venter. An occasional weak secondary rib arises on the outer part of the flank and supports a ventral tubercle. All opposite tubercles are connected by a flattened rib that crosses the venter transversely. In places the flattened rib supports barely discernible looped ribs. Tubercles of the holotype are nodate to slightly bullate; many are flattened bases of septate spines. The rib index ranges from 5 to 6. The sutures of the holotype and paratype were illustrated by Whitfield (1880: pl. 15, fig. 5, pl. 16, fig. 9). Lateral and umbilical lobes are narrow-stemmed, expanded, and considerably denticulate. The saddles that separate ventral, lateral, and umbilical lobes are about equal in size and deeply bifid.

The bulk of the specimens of Exiteloceras jenneyi at hand are similar to the types, and have ratios of whorl breadth to whorl height between 0.7 and 0.8; the stouter form of the species that occurs low in the zone of E. jenneyi is treated herein as the new chronologic subspecies E. jenneyi camacki .

The earliest growth stage of Exiteloceras jenneyi jenneyi observed is a straight limb 19.2 mm long (USNM 482472, not illustrat- ed). The whorl heights at the ends are 0.7 mm and 1.6 mm. Ornament is lacking except near the larger end, where barely discernible ventral tubercles arise. A later growth stage is shown in USNM 482473 (fig. 44A). This has a straight limb that bends into a broadly elliptical, open planispiral coil followed by a circular coil. Juvenile coiling varies considerably. Some specimens have at least three subparallel limbs connected by narrowly curved to broadly curved elbows (fig. 46D), whereas others have circular coiling at a small diameter (fig. 45A). Adult specimens have circular to somewhat elliptical coiling (fig. 43B). The largest specimen (USNM 482474, not illustrated) in the USGS collections is a fragment of a body chamber 74 mm in whorl height from USGS Mesozoic locality D439 (fig. 2, loc. 15).

Ornament of Exiteloceras jenneyi jenneyi consists of narrow ribs that are generally rursiradiate. A few ribs may be rectiradiate, and a very few may be prorsiradiate. In this respect, the drawing of E. jenneyi presented by Scott and Cobban (1965) and reproduced in many works is not typical of the species; the drawing shows dominantly rectiradiate and prorsiradiate ribbing. Ribs of E. jenneyi jenneyi are usually single and arise on the outer part of the dorsum. An occasional secondary rib may arise high on the flank and supports a small ventral tubercle. The holotype has several of these short secondary ribs on the outer whorl (fig. 42C). An unusual individual (USNM 482475, not illustrated, from USGS Mesozoic locality 79) (fig. 2, loc. 52), is a segment of an adult body chamber that has six consecutive primary ribs each separated from the rest by a short tuberculate secondary rib. Beginning at a whorl height as small as 13 mm, ribs may become somewhat flexous (fig. 45A); they are especially flexuous and closely spaced near the aperture (fig. 46D). On many specimens, ventral ribs are represented by paired looped ribs that connect opposite tubercles (fig. 47B). Every rib on the flank supports a tubercle that borders the venter. Tubercles may be clavate, bullate, or nodate. Some are flat-topped, but all supported small, hollow spines (figs. 44A, C, 47A). In general, there are 5–7 tubercles in a distance equal to the whorl height; a few specimens have 3 or 4 and a few 8–10 in a distance equal to the whorl height.

Adult specimens show a reduction in size of tubercles near the aperture along with crowded ribs that may be irregular in strength. Secondary ribs may also occur near the aperture (fig. 46A). A complete aperture is not preserved in any of the USGS specimens. Part of the aperture of one adult (USNM 482476, not illustrated), from USGS Mesozoic locality 7201 (fig. 2, loc. 63), with a whorl height of 46 mm, shows crowded and irregular ribs in the last 24 mm of the individual. Tubercles are reduced in size toward the aperture and disappear on the last two ribs. A microconch (BHMNH 4034), from the Pierre Shale of Carter County, Montana, has the dorsal half of the adult aperture preserved. It follows the trend of the terminal, crowded, flexuous ribs and has a very broad dorsal projection and a slightly convex midflank projection. The crowded ribs and reduced tubercles on several body chambers indicate the mature stage in Exiteloceras jenneyi jenneyi . Specimens with these features that have whorl heights at their larger end of as little as 32 mm (for example, unfigured specimens USNM 482477, from loc. 52, and USNM 482490, from loc. D35) are probably macroconchs. Adult specimens that have whorl heights of as much as 74 mm are probably macroconchs (for example, unfigured specimen USNM 482474, from loc. 15).

The complex suture has been illustrated by Whitfield (1880: pl. 15, fig. 5; pl. 16, fig. 9) and Cobban (1970: fig. 3). A typical suture is shown in Figure 40.

DISCUSSION: Pathologic specimens are scarce. The largest collection, 61 specimens from USGS Mesozoic locality D304 (fig. 2, loc. 35), does not include a single pathologic specimen. The second largest collection, 48 specimens from USGS Mesozoic locality D2854 (fig. 2, loc. 36), has only one pathologic individual. This specimen (USNM 482479), part of an elliptical limb 75 mm long, has dents along one side of the inner part of the flank as well as a complete loss of ventral tubercles. Two pathologic specimens, however, are present in a much smaller collection of 16 specimens from USGS Mesozoic locality 23053 (fig. 2, loc. 5). One specimen, USNM 482480 (not illustrated), is part of an adult body chamber that has a whorl height of 44 mm. It has a longitudinal groove along the lower part of the flank accompanied by a loss of ribbing in the depression. The other specimen, USNM 482481 (not illustrated), part of an adult body chamber that has a whorl height of 36.5 mm, has a depression on the venter followed by a loss of ornament. Neal Larson (written commun., 1994) observed seven pathologic individuals out of a collection of over 100 specimens collected by Howard Ehrle from the Pierre Shale of Carter County, Montana (fig. 2, general area of locs. 5–9). Two specimens have a single row of tubercles, three appear to have been broken and rehealed, one has a crease at midflank, and one has a bite that was healed.

OCCURRENCE: Exiteloceras jenneyi jenneyi is widely distributed in the Pierre Shale around the Black Hills in western South Dakota, northeastern Wyoming, and southeastern Montana (fig. 41). The subspecies also occurs in the Bearpaw Shale in east-central Montana. It has been found at many localities in the western two-thirds of Colorado in the Pierre Shale, Mancos Shale, and Iles Formation (Rifle shale unit and Cozzette Sandstone Member). Farther south, it occurs in the Pierre and Lewis shales in northern New Mexico. Kennedy (1992: 174, figs. 14A, C) described two flattened specimens from the Guadalupe Formation of Colombia that he assigned to E. jenneyi . One is a juvenile of three subparallel shafts linked by narrowly curved elbows; ribs are rursiradiate and have an index of 6 or 7. The other, a larger elliptical coil, has rursiradiate ribs that have an index of 3 or 4.

Exiteloceras jenneyi camacki , new subspecies Figures 44D, E , 49 , 50

Exiteloceras jenneyi (Whitfield) . Cobban, 1970: D73, figs. 1g –k, n–r, 3.

Exiteloceras jenneyi (Whitfield, 1877) (subspecies). Kennedy et al., 1995: pl. 3, figs. 14–17.

Exiteloceras jenneyi (Whitfield, 1877) . Kennedy and Cobban, 1997: 72 (pars), fig. 9.1–9.10.

TYPES: Holotype USNM 482491 is from the Pierre Shale northeast of Pueblo at USGS Mesozoic locality D 1232 in the SE1/4 SE1/ 4 sec. 23, T. 19 S., R. 64 W., Pueblo County, Colorado (fig. 2, loc. 53). Paratypes are USNM 482486 and 482492–482495 from the Pierre Shale at USGS Mesozoic localities D2619, D13525, D3466, and 1344 (fig. 2, locs. 18, 22, 49, 60, respectively).

ETYMOLOGY: For Walter G. Camack, Pueblo, Colorado, who first noted this form in his extensive collections of Pierre Shale ammonites.

MATERIAL: About 50 specimens from 25 localities in Montana, Wyoming, and Colorado.

DIAGNOSIS: A stout form of Exiteloceras jenneyi that has a ratio of whorl breadth to whorl height of 0.8 or more.

DESCRIPTION: The holotype (fig. 49) is a little more than half of a circular whorl 183 mm in diameter. About two-thirds is phragmocone. At the base of the body chamber, the whorl height is 35.0 mm and the whorl breadth is 30.5 mm (ratio of whorl breadth to whorl height is 0.87). The intercostal section has a broadly rounded dorsum, very broadly rounded flanks, and a more narrowly rounded venter. The costal section is similar, except that the venter is flat. Ornament consists of single, narrow, rursiradiate ribs that are flattened on the venter; the rib index is 6. The suture is like that of Exiteloceras jenneyi jenneyi .

Most of the specimens at hand are parts of adult whorls. The few juvenile whorls are fragments of elliptical limbs that have whorl heights of 8.5 mm or more and ratios of whorl breadth to whorl height of 0.8 or more. Ornament on juvenile and adult whorls is like that of Exiteloceras jenneyi jenneyi .

The largest specimen in the collections is part of a macroconch body chamber 71.0 mm high and 59.3 mm wide (ratio of whorl breadth to whorl height is 0.84) (fig. 50). Ribs are rursiradiate, flexuous, and become crowded toward the aperture. Tubercles are small; they are nodate on the earlier part of the specimen and bullate on the later part. The rib index is 7 on the earlier part of the specimen, 10 or 11 on the later part. The complete aperture follows the flexuous form of the ribbing.

DISCUSSION: The inflated whorl section of Exiteloceras jenneyi camacki separates it from E. jenneyi jenneyi , which has a more compressed whorl section. Some individuals of the former have ratios of whorl breadth to whorl height of as much as 0.95 (for example, unfigured paratypes USNM 482492 and 482493, from locs. 18 and 49, respectively).

OCCURRENCE: Exiteloceras jenneyi camacki occurs low in the zone of E. jenneyi and is clearly related to E. jenneyi jenneyi . Specimens of E. jenneyi camacki have been found at many localities in the Pierre Shale in southeastern Montana, eastern Wyoming, western South Dakota, and eastern Colorado. Specimens from the basal part of the Mount Laurel Sand of Delaware (Cobban, 1970: p. D73, figs. 1g –k, n–r; 3; Kennedy and Cobban, 1997: fig. 9.1–9.10) can be assigned to E. jenneyi camacki .

FAMILY DIPLOMOCERATIDAE SPATH, 1926

Figures

Fig. 40. Suture of Exiteloceras jenneyi jenneyi (Whitfield, 1877). E is the external lobe, L is the lateral lobe, U is the umbilical lobe, and I is the internal lobe. The heavy, straight line marks the middle of the venter. USNM 482478, USGS Mesozoic locality D1947 (fig. 2, loc. 19).

Fig. 42. Exiteloceras jenneyi jenneyi (Whitfield, 1877). A. USNM 482482, USGS Mesozoic locality D304 (fig. 2, loc. 35). B, C. USNM 12295, holotype, from ‘‘limestone of the Fort Pierre Group, on the west side of Beaver Creek, southwest Black Hills’’ (Whitfield, 1877: 40). Figures are ×1.

Fig. 43. A–C. Exiteloceras jenneyi jenneyi (Whitfield, 1877). USNM 450341, from USGS Mesozoic locality D2854 (fig. 2, loc. 36). Figures are ×1.

Fig. 44. A C. Exiteloceras jenneyi jenneyi (Whitfield, 1877). A. USNM 482473, USGS Mesozoic locality D304 (fig. 2, loc. 35). B. USNM 482483, USGS Mesozoic locality D1947 (fig. 2, loc. 19). C. Plaster cast of a specimen from the private collection of the late Karl F. Hirsch, Denver, Colo., Pierre Shale at Long Hollow north of Pueblo, Colo. D, E. Exiteloceras jenneyi camacki, n. subsp. Paratype, USNM 482494, USGS Mesozoic locality 1344 (fig. 2, loc. 60). Figures are ×1.

Fig. 45. Exiteloceras jenneyi jenneyi (Whitfield, 1877). A. USNM 482484, USGS Mesozoic locality D304 (fig. 2, loc. 35). B. USNM 482485, same locality. Figures are ×1.

Fig. 46. Exiteloceras jenneyi jenneyi (Whitfield, 1877). A, B. USNM 482486, USGS Mesozoic locality 1338 (fig. 2, loc. 59). C E. USNM 450342, USGS Mesozoic locality D79 (fig. 2, loc. 52). Figures are ×1.

Fig. 47. Exiteloceras jenneyi jenneyi (Whitfield, 1877). A. USNM 482487, USGS Mesozoic locality D304 (fig. 2, loc. 35). B, C. Private collection of the late Karl F. Hirsch, Denver, Colo., Pierre Shale of the Pueblo area, Colo. Figures are ×1.

Fig. 48. Exiteloceras jenneyi jenneyi (Whitfield, 1877). BHMNH 2140, collected by Howard Ehrle, Pierre Shale in Carter County, Mont. Figure is reduced ×0.92.

Fig. 49. Exiteloceras jenneyi camacki, n. subsp. Side view and two cross sections of the holotype, USNM 482491, locality D1232 (fig. 2, loc. 53). The solid line is the costal section, and the dashed line is the intercostal section. Figure is ×1.

Fig. 50. A, B. Exiteloceras jenneyi camacki, n. subsp. Paratype, USNM 482495, USGS Mesozoic locality D13525 (fig. 2, loc. 22). Figures are ×1.