Solenoceras bearpawense, KENNEDY & LANDMAN & COBBAN & SCOTT, 2000

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 : 73-77

Solenoceras bearpawense , new species Figures 58–60 , 61G–Q, 62

Solenoceras n. sp. Gill et al., 1972: 95.

Solenoceras sp. Larson et al., 1997: 47, two un-

numbered figs.

TYPES: The holotype is USNM 482510,

from a limestone concretion in the Didymoceras nebrascence zone of the Bearpaw Shale at USGS Mesozoic locality D 2629 in the NE1/4 sec. 31, T. 10 N., R. 36 E., Rosebud County, Montana. Paratypes are USNM 482511–482514 , from the same locality ; USNM 482515 , from the Pierre Shale at USGS Mesozoic locality D1422 (fig. 2, loc. 13) ; USNM 482516 , from the Pierre Shale at USGS Mesozoic locality D1421 (fig. 2, loc. 12) ; USNM 482517 , from the Pierre Shale at USGS Mesozoic locality D1235 (fig. 2, loc. 55) ; and USNM 482518–482520 , from the Pierre Shale at USGS Mesozoic locality D3935 (fig. 2, loc. 61) .

ETYMOLOGY: For the Bearpaw Shale of Montana.

MATERIAL: About 275 fragments, mostly uncrushed internal molds, from 25 localities. The largest collection contains about 100 fragments from limestone concretions in the Bearpaw Shale at USGS Mesozoic locality D 2629 in Rosebud County, Montana (fig. 2, loc. 3).

DIAGNOSIS: A slender species of Solenoceras with periodic constrictions bounded by flared ribs on the smaller limb, weakened ornament or loss of ornament on the elbow, and a finely ribbed larger limb (fig. 58).

DESCRIPTION: The holotype (fig. 61M, N) consists of two tightly appressed limbs 43.0 mm long, connected by a narrowly rounded elbow, 10.2 mm in diameter. The body chamber occupies the elbow and larger limb. The larger limb, which lacks its adapertural end, has a maximum costal height of 6.2 mm and width of 5.7 mm. The smaller limb has a circular cross section at the smaller end, where the diameter is 3.8 mm. Three or four very weak constrictions bounded by high ribs are present on the smaller limb. Ribs are prorsiradiate and rather weak on the smaller limb, where the rib index is 4; they bear minute nodate tubercles that border the narrow venter. Opposite tubercles are connected across the venter by very weak transverse ribs. Ornament weakens and almost disappears on the larger end of the elbow. Ribs and tubercles rejuvenate on the larger limb; ribs are rursiradiate, with a rib index of 6. Ribs are narrow on the flanks; they are straight, rounded, and narrower than the interspaces. Each rib bears a very small, nodate tubercle at the margin of the venter. Opposite tubercles are connected by transverse flattened ribs.

Most specimens in the collections resemble the holotype in size and ornament. Ribbing is weak on the smaller limb but strong on the larger limb, where the rib index is 5, occasionally 4 or 6. Well-preserved specimens have three minute longitudinal siphonal ridges on the elbow and adjoining part of the larger limb. Constrictions are widely spaced on the smaller limb, commonly at every ninth rib; they are occasionally present at every fourth to sixth rib on the larger limb.

The smallest limbs in the collections are straight to slightly curved, circular in whorl section with diameters as small as 0.8 mm (USNM 482517, not illustrated) and smooth except for periodic constrictions. The largest specimen (USNM 482516, not illustrated) in the collections has a diameter of 7.8 mm at the aperture. The final rib or two becomes nontuberculate and irregular in height. The aperture may be preceded by a short area that has striae following the course of the ribs. A small, broad, convex projection is present on the dorsum.

Dimorphism cannot be definitely demonstrated. Diameters of the 25 measurable elbows present in the collection from USGS Mesozoic locality D1422 (fig. 2, loc. 13) are summarized in figure 59. Two body cham- bers in the BHMNH collection from the Didymoceras nebrascense zone of the Pierre Shale southeast of Rapid City, South Dakota, have body chambers 78 and 88.6 mm long (N. Larson, written commun., 1994).

The suture is fairly simple (fig. 60). The external lobe is rectangular, the lateral lobe is deeply bifid; the saddle that separates these lobes is bifid and about the size of the lateral lobe.

DISCUSSION: Solenoceras bearpawense , n. sp., is the oldest known species of the genus in the Western Interior. The species is closely related to Solenoceras bembense Haas, 1943 (p. 11, figs. 4, 14) from Angola. The holotype of the African species, represented by the smaller of the two parallel limbs, has five ribs per limb height and a constriction bordered by a strong apical rib. The ribbing of the holotype of S. bembense is a little denser and stronger than that of the smaller limb of S. bearpawense . Of the American species, S. bearpawense is nearest to Solenoceras texanum (Shumard, 1861: 189) , from which it differs in having more inflated limbs. The smaller limb of both species is weakly ribbed and bears constrictions. S. texanum is present, however, in the Western Interior at a higher level (zones of Baculites cuneatus and Baculites reesidei ). Solenoceras mexicanum Anderson, 1958 (p. 211, pl. 72, fig. 8) also has a weakly ornamented smaller limb, but constrictions seem to be absent, and the California species is smaller. Other described species of Solenoceras differ considerably from S. bearpawense .

OCCURRENCE: Solenoceras bearpawense seems to range throughout the upper Campanian zone of Didymoceras nebrascense (fig. 62). Specimens of S. bearpawense have been collected from the lower part of the Bearpaw Shale in east-central Montana (Gill et al.; 1972: 95, Solenoceras , n. sp., of unit 49), and from the middle part of the Pierre Shale in southeastern Montana, western South Dakota, and eastern Colorado. Other specimens have been found in the Mesaverde Formation (below the Teapot Sandstone Member) in east-central and south-central Wyoming and in the Mancos Shale (below the Cozzette Sandstone Member of the Iles Formation) in west-central Colorado. The species also occurs in the Lewis Shale in the eastern side of the San Juan Basin in northwestern New Mexico (N. Larson, written commun., 1994).

Figures

Fig. 58. Restoration of most of a Solenoceras bearpawense, n. sp. Stipple board drawing by John R. Stacy. Figure is ×3.

Fig. 59. Histogram showing range in diameters of the elbows of 25 specimens of Solenoceras bearpawense, n. sp., USGS Mesozoic locality D1422 (fig. 2, loc. 13).

Fig. 60. Part of the suture of Solenoceras bearpawense, n. sp. E is the external lobe and L is the lateral lobe. The heavy, straight line marks the middle of the venter. USNM 482513, USGS Mesozoic locality D2629 (fig. 2, loc. 3).

Fig. 62. Didymoceras nebrascense (Meek and Hayden, 1856a) and Solenoceras bearpawense, n. sp. BHMNH 4040, collected by Neal L. Larson, Pierre Shale in the SE1/4 sec. 12, T. 7 S., R. 7 E., Fall River County, S. Dak. Figure is reduced ×0.90.