Baculites sp smooth and weak flank ribs Mooreville Chalk Alabama

There are approximately 150 isolated aptychi. They are attributed to Baculites sp. (smooth) and Baculites sp. (weak flank ribs) by comparison with the jaws inside the body chambers of these species and the fact that these two species occur in age-equivalent strata elsewhere. In terms of measurable specimens, the collection contains 8 pairs of valves found in close association, 19 left valves, and 11 right valves. The valves range from 15.2 mm to 38.6 mm in length and 6.4 mm to 17.2 mm in width, although nearly all of the valves are slightly broken (Table 13.1). The ratio of jaw width (=twice the width of the valve) to jaw length (=length of the valve) ranges from 0.77 to 0.92.

The size distribution of 38 valves in our collection is illustrated in Fig. 13.16, using valve width rather than length as an indicator of size, because the specimens are more commonly broken in length than width. Only one valve of each pair was counted. As a cautionary note, the widths of valves in a pair do not always match, presumably due to breakage of one of the valves, but the difference is usually trivial. In case of a discrepancy, the larger value was used. The resultant histogram shows a bimodal distribution of valve width with modes at 9-10 mm and 13-14 mm.

Baculites Smooth
Fig. 13.14 Radula in Baculites sp. (smooth), BHMNH 5496, microconch, Pierre Shale, South Dakota. A. Overall view of the radula with the marginal teeth exposed on the right; dorsal side of the shell on top. B. The marginal teeth are long and ornamented with ridges. C. Close-up of the marginal teeth.
Baculites Smooth

Fig. 13.15 Steinkern of part of the body chamber of Baculites sp. (weak flank ribs), BHMNH 5331, macroconch, Cody Shale, Wyoming, with the lowerjaw inside. A. Right lateral (?) view. XI. B. Ventral (?) view. X1. C. Close-up of the lower jaw with part of the aptychus and underlying chitinous layer preserved, ventral (?) side of the shell on top. X1.5.

Fig. 13.15 Steinkern of part of the body chamber of Baculites sp. (weak flank ribs), BHMNH 5331, macroconch, Cody Shale, Wyoming, with the lowerjaw inside. A. Right lateral (?) view. XI. B. Ventral (?) view. X1. C. Close-up of the lower jaw with part of the aptychus and underlying chitinous layer preserved, ventral (?) side of the shell on top. X1.5.

Because some of the isolated valves may belong to the same jaw and are thus counted twice, we reevaluated the data by constructing histograms using only left valves (including the left valve of each pair), and histograms using only right valves (including the right valve of each pair). The histogram of left valves retains the mode at 13-14 mm but the other mode disappears. The histogram of right valves yields the original two modes at 9-10 mm and 13-14 mm.

This bimodal distribution may reflect sexual dimorphism. However, it is difficult to demonstrate that all of the aptychi were derived from adults. One indication of ontogenetic stage is the degree of convexity of the aptychus, with flatter aptychi characteristic of later ontogenetic stages (see below). According to this criterion, most of these aptychi are mature, with the exception of the two smallest specimens.

The shape of each valve is semilunate (Figs. 13.17-20). The lateral margin is broadly rounded and the posterior margin is more sharply rounded. The anterior

Fig. 13.16 Size-frequency histograms of aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Greene and Dallas counties, Alabama. The bimodal distribution with peaks at 9-10 mm and 13-14 mm may reflect sexual dimorphism, provided that the specimens are all mature.

margin usually ends in a notch that parallels the rugae and is probably the result of breakage. The symphysal edge is bordered by a flange, reflecting the morphology of the underlying chitinous layer.

The aptychi are weakly convex on the ventral side and weakly concave on the dorsal side. Smaller specimens are more strongly curved than larger specimens, which probably reflects a change in shape during ontogenetic development (compare

Fig. 13.17 Aptychi o/Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Greene and Dallas counties, Alabama. A-D. Aptychus, BHMNH 5406. A. Right valve, ventral view. B. Le/t valve, ventral view. C. Le/t valve, dorsal view. D. Right valve, dorsal view. E-H. Aptychus, BHMNH 5407. E. Right valve, ventral view. F. Le/t valve, ventral view. G. Le/t valve, dorsal view. H. Right valve, dorsal view. IL. Aptychus, BHMNH 5480. I. Right valve, ventral view. J. Le/t valve, ventral view. K. Le/t valve, dorsal view. L. Right valve, dorsal view. M-Q. Aptychus, BHMNH 5450. M, N. Right valve, ventral view. O. Le/t valve, ventral view. P. Le/t valve, dorsal view. Q. Right valve, dorsal view. M is XI; all other /igures X1.5.

Fig. 13.17 Aptychi o/Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Greene and Dallas counties, Alabama. A-D. Aptychus, BHMNH 5406. A. Right valve, ventral view. B. Le/t valve, ventral view. C. Le/t valve, dorsal view. D. Right valve, dorsal view. E-H. Aptychus, BHMNH 5407. E. Right valve, ventral view. F. Le/t valve, ventral view. G. Le/t valve, dorsal view. H. Right valve, dorsal view. IL. Aptychus, BHMNH 5480. I. Right valve, ventral view. J. Le/t valve, ventral view. K. Le/t valve, dorsal view. L. Right valve, dorsal view. M-Q. Aptychus, BHMNH 5450. M, N. Right valve, ventral view. O. Le/t valve, ventral view. P. Le/t valve, dorsal view. Q. Right valve, dorsal view. M is XI; all other /igures X1.5.

Fig. 13.18 Aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Greene and Dallas counties, Alabama. A-D. Aptychus, BHMNH 5412. A. Right valve, ventral view. B. Left valve, ventral view. C. Left valve, dorsal view. D. Right valve, dorsal view. E-H. Aptychus, BHMNH 5149. E. Right valve, ventral view. F. Left valve, ventral view. G. Left valve, dorsal view. H. Right valve, dorsal view. I-L. Aptychus, BHMNH 5479. I. Right valve, ventral view. J. Left valve, ventral view. K. Left valve, dorsal view. L. Right valve, dorsal view. M-Q. Aptychus, BHMNH 5408. M, N. Right valve, ventral view. O. Left valve, ventral view. P. Left valve, dorsal view. Q. Right valve, dorsal view. M is X1; all other figures X1.5.

Fig. 13.18 Aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Greene and Dallas counties, Alabama. A-D. Aptychus, BHMNH 5412. A. Right valve, ventral view. B. Left valve, ventral view. C. Left valve, dorsal view. D. Right valve, dorsal view. E-H. Aptychus, BHMNH 5149. E. Right valve, ventral view. F. Left valve, ventral view. G. Left valve, dorsal view. H. Right valve, dorsal view. I-L. Aptychus, BHMNH 5479. I. Right valve, ventral view. J. Left valve, ventral view. K. Left valve, dorsal view. L. Right valve, dorsal view. M-Q. Aptychus, BHMNH 5408. M, N. Right valve, ventral view. O. Left valve, ventral view. P. Left valve, dorsal view. Q. Right valve, dorsal view. M is X1; all other figures X1.5.

Fig. 13.19 Aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Greene and Dallas counties, Alabama. A-C. Right valve, BHMNH 5156. A, B. Ventral view. C. Dorsal view. D, E. Left valve, BHMNH 5409. D. Ventral view. E. Dorsal view. F, G. Right valve, BHMNH 5152. F. Ventral view. G. Dorsal view. H, I. Right valve, BHMNH 5157. H. Ventral view. I. Dorsal view. J, K. Left valve, AMNH 51342. J. Ventral view. K. Dorsal view. A is X1; all other figures X1.5.

Fig. 13.19 Aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Greene and Dallas counties, Alabama. A-C. Right valve, BHMNH 5156. A, B. Ventral view. C. Dorsal view. D, E. Left valve, BHMNH 5409. D. Ventral view. E. Dorsal view. F, G. Right valve, BHMNH 5152. F. Ventral view. G. Dorsal view. H, I. Right valve, BHMNH 5157. H. Ventral view. I. Dorsal view. J, K. Left valve, AMNH 51342. J. Ventral view. K. Dorsal view. A is X1; all other figures X1.5.

Fig. 13.20 Aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Greene and Dallas counties, Alabama. A, B. Right valve, AMNH 51316. A. Ventral view. B. Dorsal view. C, D. Right valve, BHMNH 5411. C. Ventral view. D. Dorsal view. E, F. Right valve, BHMNH 5451. E. Ventral view. F Dorsal view. G, H Right valve, BHMNH 5151. G. Ventral view. H. Dorsal view/. I, J. Right valve, AMNH 51293. I. Ventral view. J. Dorsal view. K-M. Left valve, BHMNH 5485. K, L. Ventral view. M. Dorsal view. K is X1; all other figures X1.5.

Fig. 13.20 Aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Greene and Dallas counties, Alabama. A, B. Right valve, AMNH 51316. A. Ventral view. B. Dorsal view. C, D. Right valve, BHMNH 5411. C. Ventral view. D. Dorsal view. E, F. Right valve, BHMNH 5451. E. Ventral view. F Dorsal view. G, H Right valve, BHMNH 5151. G. Ventral view. H. Dorsal view/. I, J. Right valve, AMNH 51293. I. Ventral view. J. Dorsal view. K-M. Left valve, BHMNH 5485. K, L. Ventral view. M. Dorsal view. K is X1; all other figures X1.5.

Figs. 13.17M-Q and 20K-M). Some valves, such as BHMNH 5152, also show longitudinal flexures, associated with fractures on the dorsal side (Fig. 13.19F, G).

The aptychi vary in thickness. For example, the thickness at the posterior margin ranges from 2.8 mm in BHMNH 5149 to 0.8 mm in BHMNH 5485. In any one specimen, the valve is thickest in the center and thinnest at the symphysis, which

Fig. 13.21 Cross section through the long axis of an aptychus of Baculites sp. (smooth or weak flank ribs), AMNH 54277, Mooreville Chalk, Alabama. A. The aptychus is composed of thin cal-citic increments secreted at the posterior end. Dorsal surface is on the top, ventral surface on the bottom. B. Each increment is approximately 20 um thick.

Fig. 13.21 Cross section through the long axis of an aptychus of Baculites sp. (smooth or weak flank ribs), AMNH 54277, Mooreville Chalk, Alabama. A. The aptychus is composed of thin cal-citic increments secreted at the posterior end. Dorsal surface is on the top, ventral surface on the bottom. B. Each increment is approximately 20 um thick.

Fig. 13.22 Dorsal surface of aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Alabama. The symphysal edge is on the left side of each photo and the anterior direction is toward the top. A. BHMNH 5479. View of the symphysal edge showing delicate diagonal stria-tions. B. BHMNH 5479. The increments composing the aptychus bend forward as they approach the symphysis. C. BHMNH 5152. Thin longitudinal striations are common near the symphysis. D. BHMNH 5151. The area near the symphysis is covered with chevrons in this specimen, which may be the result of an injury or growth pathology.

Fig. 13.22 Dorsal surface of aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Alabama. The symphysal edge is on the left side of each photo and the anterior direction is toward the top. A. BHMNH 5479. View of the symphysal edge showing delicate diagonal stria-tions. B. BHMNH 5479. The increments composing the aptychus bend forward as they approach the symphysis. C. BHMNH 5152. Thin longitudinal striations are common near the symphysis. D. BHMNH 5151. The area near the symphysis is covered with chevrons in this specimen, which may be the result of an injury or growth pathology.

ends in a flat, beveled edge (e.g., 2.8 mm versus 0.6 mm, respectively, in BHMNH 5154). Variations in thickness are also associated with injuries and pathologies. The thick ridge and accompanying fold at the posterior margin in BHMNH 5406 is such an example (Fig. 13.17A-D).

X-ray diffraction analysis indicates that the aptychi are composed of calcite. A cross section through the long axis of a valve reveals a series of thin increments, each approximately 20 |im thick (Fig. 13.21). The increments are stacked shingle-style with more recent increments lying underneath older increments, forming an angle of 30° with the dorsal edge. The increments seem to be bundled into clusters, but this requires further research.

The dorsal surface of the aptychus is relatively smooth. There are broad undulations covered with fine lines that parallel the lateral and posterior margins. The lines bend forward at the symphysis, which they approach at an angle of 45° (Fig. 13.22B). It is unclear if the fine lines become more closely spaced toward the posterior end. Many specimens also show delicate longitudinal or diagonal striations, especially near the symphysis (Fig. 13.22A, C). In BHMNH 5151 (Fig. 13.22D), this area is covered with chevrons, which may be the result of a repaired injury.

Fig. 13.23 Ventral surface of aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Alabama. The symphysal edge is toward the left of each photo and the anterior direction is toward the top. A-C. AMNH 51342. A. The rugae turn almostperpendicularly(left) toward the symphysis, forming a geniculation. B. The rugae, especially on the lateral margin, are twisted over, resembling flexible sheets of metal. C. Rugae with botryoidal excrescences. D. BHMNH 5485. The increments composing the aptychus are visible where the rugae have eroded away.

Fig. 13.23 Ventral surface of aptychi of Baculites sp. (smooth and weak flank ribs), Mooreville Chalk, Alabama. The symphysal edge is toward the left of each photo and the anterior direction is toward the top. A-C. AMNH 51342. A. The rugae turn almostperpendicularly(left) toward the symphysis, forming a geniculation. B. The rugae, especially on the lateral margin, are twisted over, resembling flexible sheets of metal. C. Rugae with botryoidal excrescences. D. BHMNH 5485. The increments composing the aptychus are visible where the rugae have eroded away.

The ventral surface of the aptychus is ornamented with coarse ridges or rugae that parallel the lateral and posterior margins. The rugae usually attain their widest spacing at midlength (Figs. 13.18E, F, I, J, M-O, 19B, D, J), and sometimes bunch up at the posterior end (compare Figs. 13.18E, F, 20C). In general, the rugae are closely spaced along the lateral margins (Fig. 13.17A, B, E, F). They turn almost perpendicularly toward the symphysis, forming a geniculation (Figs. 13.18I, J, 20A, 23A), but do not continue onto the symphysal fold, which is covered instead with fine ridges that are convex toward the posterior end.

In addition to the geniculation, the rugae sometimes develop a series of kinks (zigzags) that are aligned longitudinally (Fig. 13.19A, B, H). These kinks may or may not correspond to fractures on the dorsal side. Conversely, fractures on the dorsal side may or may not correspond to kinks on the ventral side.

The rugae vary in their degree of coarseness and waviness, which may reflect ontogenetic development. For example, the rugae in BHMNH 5450 (Fig. 13.17M-O), the smallest specimen in our collection, and presumably from a juvenile, are fine and relatively straight, whereas those in BHMNH 5149 (Fig. 13.18E, F), one of the larger specimens in our collection, and presumably from an adult, are coarse and wavy. However, specimens of nearly the same size also show wide variation in coarseness and sinuosity (compare Fig. 13.20A, I), which may be related to different states of preservation.

The rugae are asymmetric in profile, with one side more steeply sloping than the other. Commonly, the crests of the rugae bend backward (anteriorly) at an acute angle, e.g., AMNH 51316 (Fig. 13.20A). In AMNH 51342, the rugae on the lateral margin are twisted over, resembling flexible sheets of metal (Fig. 13.23B). In some specimens, the edges of the rugae are very sharp, whereas in others, they are worn down. In general, the rugae are sharper on the posterior end, suggesting erosion of earlier formed rugae during ontogeny, but this pattern requires further documentation.

Close examination of the ventral side of the aptychi reveals several additional features: a thin, chalky outer layer covering the surface; botryoidal excrescences (Fig. 13.23C), which may be accentuated by weathering; fine ridges that cross the sides of the rugae diagonally and disappear in the troughs (e.g., AMNH 51316); and thin increments in areas where the rugae are worn down (Fig. 13.23D).

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