Genus Placenticeras Meek, 1876

Type Species: Ammonites placenta Dekay, 1828: 278, pl. 5, fig. 2 (not 5).

Placenticeras syrtale (Morton, 1834) Figures 2, 3I–P

Type: Holotype, by monotypy, is the original of Morton, 1834 (p. 40, pl. 16, fig. 4), from the “older Cretaceous” of Greene County, Alabama. It was refigured by Hyatt (1903: pl. 27, fig. 15; pl. 28, figs. 1, 2) and is no. 282 in the collection of the Academy of Natural Sciences of Philadelphia.

Description: There are 27 specimens, including USNM 490961–490969. Most specimens are juveniles, the majority of which are body chambers, with whorl heights ranging from 18 to 50 mm. Specimens display a broad range of variation. In more compressed specimens, the ratio of whorl breadth to height is as low as 0.56; the coiling is very involute, with a tiny craterlike umbilicus. The umbilical wall and shoulder slope outward, with the greatest breadth just outside the umbilical shoulder. The inner flanks are feebly convex, the outer flanks are flattened and convergent, and the venter is very narrow and tabulate. Ornament consists of delicate falcoid ribs that form prominent crescentic ridges on the outer flank, and delicate ventral clavi that alternate across the venter, numbering around 20 per half whorl. As the whorl section becomes less compressed, the ornament coarsens. Low primary ribs strengthen across the umbilical wall and shoulder and form umbilicolateral bullae. These give rise to one or two low prorsiradiate ribs that are straight on the inner flank and concave on the outer flank, where they are joined by intercalated ribs, all of which form crescentic bullae on the outer flank (fig. 2A–J). The ventral clavi are more numerous than the bullae. In the most robust specimens, the ratio of whorl breadth to height is as high as 0.8; the greatest breadth is at the umbilicolateral bullae. The flanks are convergent and the venter is broad and feebly convex. Coarse umbilicolateral bullae give rise to pairs of ribs, with prominent outer lateral nodes and ventral clavi that are two to three times as numerous as the nodes (figs. 2L–O, 3N–P). The largest specimen has an estimated diameter of 110 mm and appears to be a microconch body chamber. It shows eccentric coiling and a decline in ornament over the last 90% of the body chamber.

Discussion: Kennedy and Cobban (1991:175) discussed the temporal changes in the Gulf Coast Placenticeras syrtale lineage: “At any one stratigraphic level there is great intraspecific variation, and many names have been introduced for different morphological variants. . . . Good illustrations of variation (and use of many names) are to be found in Hyatt (1903), Reeside (1927) and Young (1963). Wolleben (1967) is the latest to have tackled this group, studying a collection of over 600 specimens from the San Carlos and Ojinaga areas of Presidio County in Trans-Pecos Texas and Chihuahua, Mexico, respectively. . . . He used the unidirectional trend of whorl height–umbilical node distance coefficients to divide what he termed the ‘polymorphic Placenticeras chronocline’ into chronologic polymorphic subspecies.” According to Wolleben Placenticeras syrtale syrtale (Morton, 1834) (upper Santonian to lower Campanian) is a subspecies that includes specimens corresponding to P. syrtale of previous authors, P. syrtale adkinsi Wolleben, 1967 (lower Campanian) is a subspecies that includes specimens corresponding to Placenticeras pseudosyrtale of previous authors, and P. syrtale rooneyi Wolleben, 1967 (lower Campanian) is a subspecies that includes specimens of Placenticeras newberryi of previous authors. Although Wolleben favored this approach to “avoid future nomenclature confusion” (Wolleben, 1967: 1161), it overlooks the principles of priority set out in the International Code of Zoological Nomenclature.

In the Blossom Sand assemblage, specimens range from compressed and feebly ornamented to less compressed and moderately strongly ornamented. They correspond to the earliest subspecies of Wolleben (1967). The incidence of weakly ornamented individuals is lower than that in the assemblage described by Kennedy and Cobban (1991) from the correlatives of the Santonian Texanites shiloensis Zone of Young (1963) in Mississippi, suggesting a higher level than this assemblage, but probably still within the Santonian as here defined.

Occurrence: Blossom Sand at USGS Mesozoic locality 18961. The species ranges from upper Santonian to lower Campanian, and is known from northern Mexico, Texas, Alabama, Mississippi, New Mexico, Colorado, Wyoming, Montana, and Utah.

Genus Glyptoxoceras Spath, 1925

Type Species: Hamites rugatus Forbes, 1846:117, by the original designation of Spath, 1926:81.

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 post-mortem 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 narrower 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 post-mortem 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, blunter 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.

Occurrence: As for type.

Genus Baculites Lamarck, 1799

Type Species: Baculites vertebralis Lamarck, 1801:103, by subsequent designation by Meek, 1876:391.

Baculites sp. cf. haresi Reeside, 1927 Figure 3A–H

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.