Placenticeras Meek, 1876

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 : 4-7

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

Ammonites syrtalis Morton, 1834: 40 , pl. 16, fig. 4.

Stantonoceras pseudocostatum Johnson. Scott and Cobban, 1964 : pl. 8.

Placenticeras syrtale (Morton, 1834) . Wolleben, 1967: 1161 (with additional synonymy).

Placenticeras syrtale syrtale (Morton, 1834) . Wolleben, 1967: 1161, pl. 150, fig. 5; pl. 151, figs. 1 , 2, 5–7; text­fig. 7e, f.

Placenticeras syrtale adkinsi Wolleben, 1967: 1164 , pl. 151, figs. 8, 9; pl. 152, figs. 1 , 2, 5– 8; text­fig. 89.

Placenticeras syrtale rooneyi Wolleben, 1967: 150 , figs. 6, 7; pl. 151, figs. 3, 4; pl. 152, figs. 3, 4; text­fig. 7d, g.

Placenticeras (Stantonoceras) guadalupae Roemer. Cobban, 1976: 125 , pl. 2, fig. 7.

Placenticeras syrtale (Morton, 1834) . Kennedy and Cobban, 1991: 176, figs. 7.1–7.4, 11.5.

Placenticeras syrtale (Morton, 1834) . Kennedy and Cobban, 1993: 835, figs. 5.4–5.6.

Placenticeras syrtale (Morton, 1834) . Kennedy et al., 1995: pl. 2, figs. 1 , 2.

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 Trans­Pecos 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 .

SUBORDER ANCYLOCERATINA WIEDMANN ,

1966

SUPERFAMILY TURRILITACEAE GILL, 1871

FAMILY DIPLOMOCERATIDAE SPATH, 1926

SUBFAMILY DIPLOMOCERATINAE SPATH, 1926

Figures

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