Volviceramus sp. A

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 : 89-93

Volviceramus sp. A

The posterior margin is not observed on our specimen; it apparently is very short.

The umbonal part is moderately to strongly inflated. The adult stage then becomes weakly or only moderately inflated, and becomes again strongly inflated in the ventral part (?gerontic stage). The valves are almost smooth, or bear only weakly developed, widely spaced, very low rugae; growth lines are not observed.

REMARKS: These two specimens resemble closely Volviceramus exogyroides and may easily be imagined as a morphotype of V. exogyroides , which very early in ontogeny changes its juvenile (very oblique) growth to the adult (almost straight). More material is needed to decide whether this morphotype is an extreme variant of V. exogyroides or it is an independent species.

OCCURRENCE: This morphotype is known from only two specimens from our collection. Both are from the lower middle Coniacian (both are from CA7) of Sheep River and Chungo Creek.

Figures 19, 20

Sphenoceramus J. Böhm, 1915 MATERIAL: Two specimens in total, both TYPE SPECIES: Inoceramus cardissoides Goldinternal molds of LVs. Single specimen from fuss, 1835.

Sheep River, TMP 2016.041.0334, from 106.5 m; GENERIC CHARACTERS: The genus is obliquely single specimen from Chungo Creek, TMP wedge shaped in outline, weakly to moderately 2016.041.0195, from 56.9 m. inflated, medium to large sized, equivalve and MEASUREMENTS: See table 1. inequilateral. The disc is triangular in outline, DESCRIPTION: The valves are subrounded to with three growth axes (Schalenkante of Seitz: suboval, weakly inequilateral, prosocline, moder- 1965, text-fig. 1), similar to the genus Cordiceraately inflated. The beak is pointed, curved strongly mus , commonly with a more or less well-develdorsoanteriorly. The umbo is subtrigonal. The oped radial sulcus in the posterior part of the umbonal part grows obliquely (umbonal δ is disc. The umbo is pointed, curved anteriorly, between 35° and 40°); then the valve becomes usually extending above the hinge line. The ornastraight, growing at d between 70° and 75°. The pos- ment is well developed over the disc, usually terior auricle apparently does not develop, or is very much weaker on the posterior auricle. Both consmall (and not preserved on our specimen). The centric and radial ornament elements occur.

anterior margin is relatively short (AM/h = 0.4); it OCCURRENCE: Sphenoceramus appeared in the is concave below the umbo, then straight. The ven- late Coniacian (see Heinz, 1926, 1928; Heine, tral margin is broadly and regularly rounded, long. 1929; Seitz, 1962; Tröger and Christensen, 1991; FIG. 18 . Volviceramus stotti , sp. nov. A, B, D, TMP 2016.041.0439, LV, A, anterior view; B, dorsal view, D, lateral view. Wapiabi Formation; Bighorn Dam, 91.5 m; the specimen is ×0.85. C, E, TMP 2016.041.0245, LV, C, lateral view, E, posterodorsal view. Wapiabi Formation, West Thistle Creek, 107.5 m; the photograph is ×0.9.

Walaszczyk and Wood in Niebuhr et al., 1999), and ranged up to the mid early Campanian ( Seitz, 1965). It is a Boreal species, rarely reported in the Mediterranean area (see, e.g., Tröger and Summesberger, 1994). The genus seems to be limited to the Northern Hemisphere; reports from the Southern Hemisphere ( Heinz, 1929; Kauffman in Kennedy et al., 1973) are not confirmed (see discussion in Walaszczyk and Cobban, 2006; Kennedy et al., 2008; Walaszczyk et al., 2014a). The genus is widely reported from the North Pacific Province, particularly from its western parts. How abundant the genus was in this province is, however, unclear. Of a wide variety of morphotypes referred to Sphenoceramus (e.g., Matsumoto et al., 1982; Toshimitsu, 1988; Noda, 1988), most should be referred to other genera (e.g., Glazunov, 1976; Zonova, 1992, 1993).

In the Western Interior Basin Sphenoceramus occurs regularly only in its northern part ; probably ranging south to Montana. Further south the genus appears sporadically.

Figures

FIG. 20. A. The upper part of the Marshybank Member at the Bighorn Dam section (fig. 7). Surface CS23, which is mantled with chert and siderite pebbles, marks a maximum regressive surface and is overlain by four subtle upward-coarsening successions culminating in surface SS0. B. Upper part of the Marshybank Member exposed on the Bighorn River (fig. 7) showing bioturbated sandy siltstone grading up into weakly stratified silty sandstone, the top of which is marked by surface CS23.

FIG. 18. A. Overview of the lower part of the Coniacian succession at Burnt Timber Creek (fig. 9) illustrating depositional cyclicity and bounding surfaces E7 and CS1 to CS5. Note the prominent sandier-upward succession culminating in surface CS4. B. Overview of part of the Sheep River canyon (fig. 11). The CS4 surface caps an upward-coarsening package that, in this view, is repeated by a thrust fault; Walaszczyk (circled) provides an indication of scale. C. Detailed view of the subtle, but sharp CS4 surface that is mantled with a few mm of chert and phosphate pebbles and granules and separates bioturbated siltstone and very fine-grained sandstone below from silty clay above.