Solenoceras, 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 : 78-80

Solenoceras , n. sp. Gill et al., 1970: 22, 23.

TYPES: The holotype is USNM 482530, from the Rock River Formation at USGS Mesozoic locality D 1387 in the NW ¼ sec. 16, T. 20 N., R. 76 W., Albany County, Wyoming (fig. 2, loc. 27). Paratypes are USNM 482531–482538, 482823–482826, from the same locality, and 482539, from USGS Mesozoic locality D359 (fig. 2, loc. 26).

ETYMOLOGY: Latin, elegans , fine.

MATERIAL: About 200 well-preserved fragments from the type locality plus about 30 fragments from other localities.

DIAGNOSIS: A long, slender species of Solenoceras with a few constrictions near the adapical end and one near the aperture. Ribs are usually closely spaced with a rib index of 5.

DESCRIPTION: The holotype (fig. 61DD)

consists of two tightly appressed, slightly curved limbs 69 mm long, connected by a narrow elbow 9.6 mm in diameter. Owing to cover of nacreous shell material on nearly all of the smaller limb, the position of the base of the body chamber is not recognizable. The small end of the smaller limb is broken off at a limb height of about 2 mm. The height of the larger limb is about 6.5 mm at the aperture. Both limbs have stout cross sections. Ribs are straight, narrow, and single; each bears a small nodate tubercle at the margin of the flattened venter. Short, sharp spines are preserved near the aperture, where they arise from tubercles. Ribs are prorsiradiate on the smaller limb and on the older part of the elbow; they are rectiradiate on the middle of the elbow, and slightly rursiradiate on the younger part of the elbow and on the larger limb. The rib index is 5. The aperture is bordered by a thin rib.

Forty-five specimens from USGS Mesozoic locality D1387 have their elbows preserved. The diameters of the elbows are summarized in figure 65. The bulk of the specimens have elbow diameters from 8 to 10 mm.

One of the smallest individuals (USNM 482823, not illustrated) in the collection is a limb 30.5 mm long that has terminal whorl heights of 0.4 and 2.3 mm. Beginning at the small end, the limb forms a gentle curve for about 8 mm and then becomes straight for the rest of the limb. Given the small height of the limb at its smaller end, only a few millimeters of limb and ammonitella can be missing. The ornament is lacking except for a few widely spaced constrictions. Another juvenile limb (USNM 482824, not illustrat- ed), 5.5 mm long, that arises from the ammonitella, is 0.5 mm high at the larger end and forms a gentle curve. This limb is also smooth except for a few weak constrictions. These specimens as well as other very small limbs in the collection (USGS Mesozoic locality D1387) allow restoration of the early growth stages of Solenoceras elegans shown in figure 66.

The ornament of the specimens from USGS Mesozoic locality D1387 (fig. 61R– CC, EE–GG) is much like that of the holotype. Ribs are fairly strong, narrow, and closely spaced, and cross the venter transversely, where some may be looped between opposite tubercles. The ornament remains strong on the elbow.

Several specimens have the aperture preserved. A constriction is bordered by a high adoral rib, followed by a weak rib, and then by the aperture (fig. 66). The aperture has the form of the ribs and a broad convex dorsal projection.

The suture is typical of the genus. The lobes and saddles are bifid and of nearly equal size except for the narrow, trifid internal lobe (fig. 67).

DISCUSSION: Solenoceras elegans , n. sp., has a long body chamber and ribbing much like that of Solenoceras bearpawense , n. sp., but S. elegans lacks constrictions on the small limb. The long body chamber and density of ribs of Solenoceras texanum (Shumard, 1861) , as illustrated by Stephenson (1941: pl. 77, figs. 4, 5), resemble those of S. elegans , but the Texas species has a rectangular cross section as well as constrictions on the small limb.

OCCURRENCE: Solenoceras elegans is fairly common in part of the Rock River Formation in southeastern Wyoming, where the species is associated with Didymoceras stevensoni . At many localities in Colorado, S. elegans ranges upward into the zone of Exiteloceras jenneyi . BHMNH has 10 specimens collected by Howard Ehrle from the D. stevensoni and E. jenneyi zones of the Pierre Shale of Carter County, Montana (N. Larson, written commun., 1994).