Baculites sp smooth Pierre Shale South Dakota

The body chambers containing lower jaws are fragmentary and range from 43.2 mm to 120.9 mm in length (Figs. 13.4-14, Table 13.1). All of the body chambers are steinkerns but several retain pieces of the aragonitic outer shell (= composite internal molds). The body chambers consist of four stout and eight slender individuals, presumably representing mature macroconchs and microconchs, respectively.

In specimens that preserve the ultimate septum or in which the shell expands enough to detect the adoral direction, the jaw is located in the adoral part of the body chamber. In most specimens, the long axis of the jaw lies at an angle of 45°-90° to the long axis of the body chamber, with the anterior end of the jaw on the dorsal side of the shell, and the posterior end of the jaw on the ventral side of the shell (Figs. 13.5-7, 8D-F, 9, 10, 11, 12A-C, 13). The jaws are nearly perpendicular to the whorl section except in BHMNH 5494a, in which the jaw is displaced to the left hand side of the shell (Fig. 13.7). In BHMNH 5147 and 5497 (Figs. 13.8A-C, 12D-F), in contrast, the long axis of the jaw is approximately parallel to the long axis of the body chamber, with the jaw lodged in the dorsal and ventral half of the shell, respectively.

The jaw consists of two wings with the symphysis along the midline. Both wings are preserved in AMNH 47109, BHMNH 5494a, 5147, 5143, 5491, and 5496

Fig. 13.4 Size-frequency histogram t 3-gj i 1-LL

Baculites sp. (smooth)

of the wing width of the lower jaw of Baculites sp. (smooth) from the Gammon Ferruginous Member of the Pierre Shale, South Dakota. M = macroconch; m = microconch.

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Wing widlh (mm)

Fig. 13.5 Steinkern of part of the body chamber of Baculites sp. (smooth), macroconch, AMNH 47109, Pierre Shale, South Dakota, with lower jaw inside. A. Right lateral view. X1. B. Ventral view. X1. C. Lower jaw with parts of the aptychus preserved, dorsal side of the shell on top, coated. X1.5.

(Figs. 13.5, 7, 8A-C, 9, 11, 12A-C). The wings are folded in a U-shape in BHMNH 5147, 5143, and 5491 (Figs. 13.8A-C, 9, 11). In contrast, they are nearly flattened out in AMNH 47109 and BHMNH 5494a (Figs. 13.5, 7) and folded slightly outward in BHMNH 5496 (Fig. 13.12A-C). In the other six specimens, only one wing or one wing and part of the other wing is preserved. In BHMNH 5148 and 5495 (Figs. 13.8D-F, 13D-F), the partially preserved wing is oriented nearly perpendicular to the plane of the other wing.

Fig. 13.6 Steinkern of part of the body chamber of Baculites sp. (smooth), microconch, AMNH 51329, Pierre Shale, South Dakota, with the lower jaw inside. A. Right lateral view. XI. B. Ventral view. X1. C. Lower jaw with part of the right side of the aptychus preserved, coated. X1.5.

The jaw measurements are reported in Table 13.1. Jaw length ranges from 20.8 mm to 37.2 mm. Wing width ranges from 7.8 mm to 14.4 mm. Jaw width, which equals twice the wing width, ranges from 15.6 mm to 28.8 mm. The ratio of jaw width to jaw length ranges from 0.68 to 0.87 and averages 0.77, indicating an elongate shape.

A comparison of the shape of the jaws with that of the whorl cross section reveals that the jaws fit snugly into the body chamber (Table 13.1). The ratio of jaw length to whorl height ranges from 0.72 to 1.03, and averages 0.85. The ratio of jaw width to whorl width ranges from 0.81 to 1.04, and averages 0.94. The ratio of jaw width to jaw length is the same or slightly higher than the ratio of whorl width to whorl height. When one or both wings are splayed out across the whorl section, the sides of the jaw touch the sides of the shell, e.g., AMNH 47109, BHMNH 5148, 5144, and 5496 (Figs. 13.5, 8D-F, 10, 12A-C).

The variation in jaw size with respect to sexual dimorphism is illustrated in Fig. 13.4. The baculites were divided into macroconchs and microconchs based on the size and robustness of the shell. Wing width (= 1/2 aptychus width) was used as a measure of jaw size. The histogram of wing width shows no overlap between dimorphs.

The jaw is covered with a pair of calcareous plates or valves (= the aptychus). Each valve has a broadly rounded lateral margin, a straight symphysal margin, and

Fig. 13.7 Steinkern of part of the body chamber of Baculites sp. (smooth), macroconch, BHMNH 5494a, Pierre Shale, South Dakota, with lower jaw inside. A. Lateral view. XI. B. Dorsal view. XI. C. Lower jaw with part of the right side of the aptychus preserved, dorsal side of the shell on top. X1.5.

a narrowly rounded posterior margin. The anterior margin is missing or obscured in all specimens. The symphysis is bordered by a flange with a sharp crest, as shown in BHMNH 5144 and 5491 (Figs. 13.10, 11).

The aptychi are composed of calcite, as indicated by X-ray diffraction analysis of a sample from BHMNH 5491. The calcite in this specimen is 0.50 mm thick at a ridge and 0.17 mm thick near the symphysis. The aptychi are covered with coarse, irregular rugae characteristic of rugaptychi. The rugae parallel the lateral and posterior margins and approach the symphysis at nearly a right angle. In BHMNH 5144 (Fig. 13.10), with the best preserved aptychus, there are 13 rugae, although additional ones are undoubtedly obscured in the anterior region. The rugae are more broadly spaced in the middle of the aptychus than near the posterior margin.

In some specimens, parts of the aptychus are broken off exposing large portions of the underlying chitinous layer. The chitinous layer bears a midline groove

Fig. 13.8 Steinkerns of parts of the body chambers of Baculites sp. (smooth), Pierre Shale, South Dakota, with lower jaws inside. A-C. BHMNH 5147, microconch. A. Right lateral view. X1. B. Dorsal view. X1. C. Close-up ofthe lower jaw folded in a U-shape. X1.5. D-F. BHMNH 5148, microconch. D. Left lateral view. X1. E. Ventral view. X1. F. Lower jaw with part ofthe left side ofthe aptychus preserved, dorsal side ofthe shell on top. X1.5.

Fig. 13.8 Steinkerns of parts of the body chambers of Baculites sp. (smooth), Pierre Shale, South Dakota, with lower jaws inside. A-C. BHMNH 5147, microconch. A. Right lateral view. X1. B. Dorsal view. X1. C. Close-up ofthe lower jaw folded in a U-shape. X1.5. D-F. BHMNH 5148, microconch. D. Left lateral view. X1. E. Ventral view. X1. F. Lower jaw with part ofthe left side ofthe aptychus preserved, dorsal side ofthe shell on top. X1.5.

Fig. 13.9 Steinkern of part of the body chamber of Baculites sp. (smooth), BHMNH 5143, macroconch, Pierre Shale, South Dakota, with the lower jaw inside. A. Right lateral view. X1. B. Ventral view. X1. C. Lower jaw, dorsal side of the shell on top. X1.5.

bordered by flanges. In BHMNH 5494a (Fig. 13.7), this layer is smooth with subdued undulations. The two wings in this specimen diverge at the anterior end and the symphysis disappears, so that there is a triangular gap between the wings.

Two layers are visible below the calcitic aptychus in BHMNH 5491: a black crystalline layer (80 |im thick near the symphysis) and an underlying tan layer (60 |im thick near the symphysis). X-ray diffraction analysis of these two layers indicates that they consist of an amorphous material and magnesium enriched calcite, respectively.

A radula is present in BHMNH 5496 (Figs. 13.12A-C, 14). It occurs in the middle of the jaw at the anterior end. The aptychus is missing in this area and the surface is eroded away. The radula is approximately 4.1 mm long and 2.0 mm wide. The teeth

Fig. 13.10 Steinkern of part of the body chamber of Baculites sp. (smooth), BHMNH 5144, microconch, Pierre Shale, South Dakota, with the lower jaw inside. A. Left lateral view. X1. B. Ventral view. X1. C. Close-up of the lower jaw with the right side of the aptychus preserved, dorsal side of the shell on top. X1.5.

Fig. 13.10 Steinkern of part of the body chamber of Baculites sp. (smooth), BHMNH 5144, microconch, Pierre Shale, South Dakota, with the lower jaw inside. A. Left lateral view. X1. B. Ventral view. X1. C. Close-up of the lower jaw with the right side of the aptychus preserved, dorsal side of the shell on top. X1.5.

are dark brown with reddish overgrowths. Three elongate, slightly curved elements are visible in the upper right side of the radula and represent the marginal teeth (Fig. 13.14B). Each tooth is approximately 990 |im long and lies on a curved sheet. The teeth point forward and inward. They are ornamented with ridges spaced at equal intervals of approximately 70 |im (Fig. 13.14C).

Three blunter, rectangular elements are visible just to the right of the marginal teeth, and represent the marginal plates. They are aligned almost perpendicular to the marginal teeth and are spaced between them.

Additional teeth are present on the lower right side of the radular complex but are indistinct (Fig. 13.14A). The middle of the radula is recrystallized. A small triangular tooth is visible at the lower end flanked by longer teeth on either side, which point inward. The left side of the complex also contains elongate teeth like those in the upper right side, but they are broken and embedded in the matrix.

Fig. 13.11 Steinkern of part of the body chamber of Baculites sp. (smooth), BHMNH 5491, macroconch, Pierre Shale, South Dakota, with the lower jaw inside. A. Left lateral view. X1. B. Ventral view. X1. C. Close-up ofthe lower jaw, dorsal side ofthe shell on top. X1.5.

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