Glyptoxoceras texanum, KENNEDY & LANDMAN & COBBAN, 2001

KENNEDY, W. J., LANDMAN, N. H. & COBBAN, W. A., 2001, Santonian Ammonites from the Blossom Sand in Northeast Texas, American Museum Novitates 3332, pp. 1-10 : 7-8

Glyptoxoceras texanum , new species

Figure 4

DERIVATION OF NAME: From the state of Texas.

TYPE: Holotype is USNM 490970 (fig. 4C–E), paratype is USNM 490971 (fig. 4A, B), from the top of the Blossom Sand at USGS Mesozoic locality 18120, between Detroit and Bagwell, Red River County, Texas.

DIAGNOSIS: A large species of Glyptoxoceras ; body chamber bears rursiradiate ribs and periodic constrictions, each of which is flanked by a flared adapertural rib.

DESCRIPTION: The holotype USNM 490970 is a body chamber fragment 120 mm long, with replaced shell (fig. 4C–E). The maximum preserved whorl height is 31 mm. The specimen has suffered some post­mortem crushing, but the whorl section of the uncrushed portion is compressed ovoid with the venter more narrowly rounded than the dorsum; the ratio of whorl breadth to height is 0.75. For most of the length of the fragment ,

the ornament consists of coarse, narrow ribs; the rib index is 6. The ribs are much narrow­ er than are the interspaces; ribs are weak and transverse to feebly concave across the dorsum, stronger and convex across the dorsolateral margin, strong, straight, and rursiradiate on the flanks, and transverse on the venter, where they attain their maximum height. There are two broad, deep constrictions on the fragment, each of which is flanked by a flared adapertural rib. The adoral of these two constrictions is succeeded by a section of shell 40 mm in length on which the ornament changes markedly, indicating that the specimen is part of the terminal adult body chamber immediately prior to the final aperture. Ribbing is much finer, lower, and weaker, and the rib index increases to 11. There is a weak constriction and flared rib immediately adapical of the aperture.

The paratype USNM 490971 is much smaller, 52 mm long, crushed, with a maximum preserved whorl height of 11 mm and whorl breadth of 13.5 mm (fig. 4A, B). The depressed whorl section is probably an artifact of post­mortem deformation. Ornament consists of narrow, distant ribs that are weak and feebly concave across the dorsum; they strengthen across the dorsolateral margin, where they are sharp and high and much narrower than the interspaces. The ribs are markedly prorsiradiate on the flanks and strong and feebly convex across the venter. The sutures are not exposed.

DISCUSSION: The holotype of Glyptoxoceras ellisoni Young, 1963 (p. 46, pl. 1, figs. 10–14, 16–20; pl. 73, fig. 9; pl. 78, fig. 6) from the Dessau Chalk in central Texas is a body chamber fragment 125 mm long with a maximum preserved whorl height of 22 mm (fig. 5). It overlaps in size with the holotype of G. texanum but it has much coarser, blunt­ er ribbing (even when the differences of preservation are taken into account), with a rib index of around 4; there are no constrictions. Glyptoxoceras rubeyi (Reeside, 1927) (p. 14, pl. 3, figs. 8–10; pl. 5, figs. 3–11) has much more widely separated and coarser ribs. The holotype of Glyptoxoceras novimexicanum (Reeside, 1927) (p. 8, pl. 4, figs. 1–4 ), a body chamber fragment that is comparable in size to the holotype of G. texanum , lacks constrictions.

DESCRIPTION: There are two body chamber fragments, USNM 490972 (fig. 3A–D) and USNM 490973 (fig. 3E–H). The latter has a maximum preserved whorl height of 20 mm and a ratio of whorl breadth to height of 0.74; the whorl section is ovoid and the dorsum is more broadly rounded than the venter. Both fragments are virtually smooth, with traces of delicate riblets and striae that are strongly prorsiradiate on the outer flank and cross the venter in a broad convexity (fig. 3H).

DISCUSSION: These fragments belong to a very generalized Baculites , and are most comparable to Baculites haresi Reeside, 1927 (e.g., pl. 6, figs. 5, 6).

OCCURRENCE: Blossom Sand at USGS Mesozoic locality 18961.

OCCURRENCE: As for type.

FAMILY BACULITIDAE GILL, 1871

Figures

Fig. 4. Glyptoxoceras texanum, n. sp. A, B. Paratype USNM 490971; C–E. holotype, USNM 490970. Both specimens are from the Blossom Sand at USGS Mesozoic locality 18120 in Red River County, Texas. Arrows indicate the base of the body chamber. All figures are X1.

Fig. 1. The outcrop of the Blossom Sand in northeast Texas, showing the localities cited in the text.