Baculites compressusB cuneatus Pierre Shale South Dakota

BHMNH 5498 is a right wing of a lower jaw (Fig. 13.24F, Table 13.1). It is 7.9 mm wide and 19.5 mm long. The estimated ratio ofjaw width to jaw length is 0.81. The wing is convex with a prominent flange along the symphysal edge. The anterior margin of the wing is broadly rounded and the posterior margin is more sharply rounded; the lateral margin is obscured.

The jaw consists of a dark brown to black crystalline layer approximately 200 |im thick. X-ray diffraction analysis indicates that it consists of magnesium enriched calcite. This layer shows broad folds that parallel the posterior margin. In areas where the black layer is broken away, it exposes a smooth brown layer bearing finer ridges, spaced at approximately equal distances of 0.5 mm.

An analysis of the relationship between the shape of the jaw and the whorl cross section is complicated because the jaw is not preserved inside the body chamber,

Fig. 13.24 A-E. Lower jaws o/Pseudobaculites natosini (Robinson, 1945), Lewis Shale, Natrona County, Wyoming. A, B. BHMNH 5501. A. Right wing, anterior direction toward the right. B. Le/t wing, anterior direction toward the le/t. C. Right wing, BHMNH 5500, anterior direction toward the right. D, E. AMNH 51328. D. Le/t wing, anterior direction toward the le/t. E. Right wing, anterior direction toward the right. F. Right wing o/a lower jaw attributed to Baculites compressus Say, 1820, or Baculites cuneatus Cobban, l962, BHMNH 5498, Pierre Shale, Meade County, South Dakota. Anterior direction toward the right. G. Aptychus attributed to Baculites sp., FHSM 968:1, Smoky Hill Chalk Member, Niobrara Formation, Rooks Co., Kansas. G is X2; all other /igures are X1.

Fig. 13.24 A-E. Lower jaws o/Pseudobaculites natosini (Robinson, 1945), Lewis Shale, Natrona County, Wyoming. A, B. BHMNH 5501. A. Right wing, anterior direction toward the right. B. Le/t wing, anterior direction toward the le/t. C. Right wing, BHMNH 5500, anterior direction toward the right. D, E. AMNH 51328. D. Le/t wing, anterior direction toward the le/t. E. Right wing, anterior direction toward the right. F. Right wing o/a lower jaw attributed to Baculites compressus Say, 1820, or Baculites cuneatus Cobban, l962, BHMNH 5498, Pierre Shale, Meade County, South Dakota. Anterior direction toward the right. G. Aptychus attributed to Baculites sp., FHSM 968:1, Smoky Hill Chalk Member, Niobrara Formation, Rooks Co., Kansas. G is X2; all other /igures are X1.

and, therefore, no measurements are available for the corresponding whorl cross section. Instead, we measured specimens of Baculites compressus/B. cuneatus from the same concretion as the jaws, as well as specimens from other concretions in the same stratigraphic interval. Our calculations indicate that the shape of the jaw is much broader than the whorl cross section (Table 13.1).

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