Sphenoceramus, J. Bohm, 1915

Landman, Neil H., Plint, A. Guy & Walaszczyk, Ireneusz, 2017, Allostratigraphy And Biostratigraphy Of The Upper Cretaceous (Coniacian-Santonian) Western Canada Foreland Basin, Bulletin of the American Museum of Natural History 2017 (414), pp. 1-173 : 94-98

Sphenoceramus ex gr. pachti

( Arkhangelsky, 1912)

Figures 22 , 23 2006. Sphenoceramus ex gr. pachti ( Arkhangelsky, 1912) . Walaszczyk and Cobban: textfigs. 48.7, 48.10, 48.11.

MATERIAL: Three specimens from Cripple Creek : TMP 2016.041.0152, TMP 2016.041.0153, and TMP 2016.041.0154. Twenty-six specimens from West Thistle Creek ; TMP 2016.041.0246– TMP 2016.041.0271. Two specimens from Bighorn Dam , TMP 2016.041.0424 and TMP 2016.041.0430. Numerous uncataloged specimens from Cardinal River .

DESCRIPTION: The species is small to medium size for the genus, inequilateral, apparently equivalve (?semiequivalve). The valve is weakly to moderately inflated, unless geniculated, quite oblique, with d between 45° and 50°. The valve outline is trapezoidal. The disc is trigonal in outline. The posterior auricle is usually well developed and well separated from the disc. The distinct auricular sulcus is visible in some specimens. The very distinct radial sulcus runs posteriorly of the growth axis. It starts at ca. 10 mm from the beak and becomes stronger with age. The beak is pointed, curved anterodorsally, projects slightly above the hinge line. The anterior margin is relatively short, usually slightly above 50% of the growth axis length, quite steep, may be slightly concave just below the umbo. The anterior margin passes into the long, broadly convex anteroventral margin. The ventral margin is narrowly rounded. The posterior margin is straight, almost parallel to the growth axis, being about 60% of the growth-axis length. The growth axis is straight in the juvenile and early adult part, then curves posteriorly. The ligament (dorsal) margin straight, relatively short, below 50% of the respective growth-axis length.

The general outline and shape changes in geniculated specimens. The geniculation appears usually at between 30 and 40 mm axial distance from the beak. The geniculation angle varies but is approximately 60°. Rarely, double geniculation is observed. The geniculated forms are characterized by a steep (sometimes overhanging) anterior pachti has ever been reported from the Western wall, and sharp anterior margin (fig. 22J). Interior Basin, and our material also seems to

The ornament is composed exclusively of include adult specimens. Consequently, our specicommarginal rugae; very fine, discontinuous mens are regarded as representing a sample of a radial elements appear on some specimens (TMP nonradially ribbed natural population. 2016.041.0152, fig. 23D). Up to 20 mm of the The geniculated, and particularly the smallaxial length the rugae are poorly developed, and sized specimens of S. pachti described herein very the ornament is composed almost entirely of closely resemble the type of I. pontoni McLearn raised, sharp-edged growth lines. In some speci- (1926: pl. 20, fig. 1). The only differences that may mens the rugae appear, however, much earlier, be indicated, based on the limited material of and the growth lines are poorly visible. In most McLearn’s species, are: (1) weakening of the posof the specimens the rugae show a distinct step- terior radial sulcus and (2) general weakening of wise change from rather closely spaced, round- the ornament in the adult stages of I. pontoni (this topped ones, to widely spaced ones, with is not the case in S. pachti ). The typical specimens distinctly sharper edges. In the geniculated spec- of McLearn’s species, collected from the Smoky imens, the change is associated with the change River area, western-central Alberta, and from the of growth plane. The growth lines become less Kevin section, northern Montana, come from and less regular with age. The rugae do not pass beds slightly younger than S. pachti . Consequently, onto the posterior auricle. In the geniculated it is suggested herein that I. pontoni may represent specimens they pass onto the anterior wall. an evolutionary descendant of S. ex gr. pachti .

REMARKS: The studied material looks at first OCCURRENCE: The material studied is from very variable. To a large extent this is, however, the lower Santonian of Alberta, Canada. Very because of the genicution; the geniculation similar specimens occur higher in the lower and causes changes in the character of the anterior middle? Santonian. margin and wall, and in the general shape of the whole specimens. Other than that, the speci- Sphenoceramus ex gr. cardissoides mens are quite stable with respect to general ( Goldfuss, 1835) outline and ornament.

Figure 24

The general outline, ornament and ontogenetic changes make our specimens clearly representa- 1965. Inoceramus cardissoides subsp. indet . Seitz: tive of Sphenoceramus pachti . The difference 47, pl. 4, figs. 1–4. between the Arkhangelsky’s types and the studied 2006. Sphenoceramus ex gr. cardissoides (Goldmaterial is the lack of radial ornament. In this fuss, 1835). Walaszczyk and Cobban: textrespect, our specimens correspond to S. pachti figs. 48.3–48.6. subsp. indet . as described by Seitz (1965: pl. 9, figs. MATERIAL: Single RV, RTM 2016.041.0272,?1, 3–4). Seitz suggested the possibility that his from 126 m level of West Thistle Creek. undescribed variety could represent juveniles of DESCRIPTION: The studied specimen (fig. 24) is otherwise typical (i.e., radially ornamented) S. the small-sized internal mold of the RV; its hmax pachti . Although this is also possible in the case of = 48.5. The valve is strongly inflated as for S. our material, no radially ribbed specimen of S. cardissoides ; its b/h= 0.45. The disc is subquadrate FIG. 23. Sphenoceramus ex gr. pachti ( Arkhangelsky, 1912) . Wapiabi Formation, basal Santonian. A, C, TMP 2016.041.0254, LV (geniculated), West Thistle Creek, 129.5 m, A, lateral view, C, anterior view. B, E, TMP 2016.041.0247, LV (geniculated), West Thistle Creek, 123 m, B, anterior view, E, lateral view. D, G, TMP 2016.041.0152, LV, Cripple Creek, 47.5 m, D, lateral view, G, oblique, dorsoposterior view. F, TMP 2016.041.0265, LV, West Thistle Creek, 125.3 m, lateral view. All photographs are ×1. in outline, prosogyrate. The beak is pointed, curved anterodorsally, projecting above the hinge line. Its anterior margin is distinctly concave, relatively long, with AM/h = 0.65, which is distinctly higher than in S. ex gr. pachti (ca. 0.5 or less). The anterior wall is steep to overhanging. The ventral margin is rounded. The radial sulcus is well developed. The posterior auricle is not preserved.

The valve is ornamented with strong commarginal rugae, sharp-edged, with flat, and relatively large interspaces. The rugae are superimposed by raised, sharp-edged growth lines. Traces of weak radial ribs are visible in the axial part of the valve.

REMARKS: The specimen clearly belongs to the Sphenoceramus pachti-cardissoides group. Its relatively long anterior margin is a characteristic of S. cardissoides . The lack of radial ornament makes it close to forms referred by Seitz (1965: 47) to Inoceramus cardissoides subsp. indet . Similarly, as with nonradially ribbed variety of S. pachti, Seitz did not name this variety, suggesting that it might comprise only the juvenile stages of other, radially ribbed species. The presence of this morphotype in the U.S. Western Interior suggests that the variety may represent a separate taxon.

OCCURRENCE: The studied specimen comes from the lower Santonian of the West Thistle Creek section; the US Western Interior specimens come from the Santonian. The morphotype is also known from the lower and middle Santonian of Germany ( Seitz, 1965).

Figures

FIG. 22. Offshore facies typical of the Muskiki Member. A. Weakly bioturbated (BI 0-1) cm-scale beds of very fine-grained sandstone interstratified with mudstone forming an upward-coarsening succession culminating at flooding surface CS4. The rusty-weathering colour is typical of this facies and is due to abundant disseminated pyrite. Overlying rock comprises weakly bioturbated mudstone with mm-scale siltstone interbeds. Example: “Brown Creek” section (fig. 6, 11–19 m). B. Stratified mudstone (BI 1-2) with mm-scale coarse siltstone interbeds and abundant, dispersed siderite nodules; scale bar = 20 cm. Example: Oldfort Creek (fig. 10, 42–46 m). C. Heavily bioturbated (BI 4-5) sandy siltstone with dispersed siderite nodules. Example: Oldfort Creek (fig. 10, 49–52 m).

FIG. 24. Sphenoceramus ex gr. cardissoides (Goldfuss, 1835), TMP 2016.041.0272, RV, Wapiabi Formation, West Thistle Creek, 125.3 m, A, lateral view, B, posterior view, C, anterior view. The photograph is ×1.