Metaplacenticeras subtilistriatum (Jimbo, 1894)

LANDMAN, NEIL H., TSUJITA, CAMERON J., COBBAN, WILLIAM A., LARSON, NEAL L., TANABE, KAZUSHIGE & FLEMMING, ROBERTA L., 2006, Jaws of Late Cretaceous Placenticeratid Ammonites: How Preservation Affects the Interpretation of Morphology, American Museum Novitates 3500, pp. 1-48 : 29-33

Metaplacenticeras subtilistriatum

Yasukawa Formation, Hokkaido

UMUT MM28896 is a lower jaw that occurs in a calcareous concretion with numerous plant fragments and shells ( figs. 26E–H, 27 ). It is 25.4 mm long and the left wing is 13.4 mm wide (W/L of the left wing 5 0.53). The jaw is folded along the midline with most of the right wing smashed in.

The anterior margin is slightly indented on each side of the apex. The apical angle is approximately 1308. There is a step­like feature near the anterior margin but most of the space between this feature and the anterior margin is filled in with coarsely crystalline black material eliminating any relief.

A narrow slit 0.15 mm wide is present at the apex and extends approximately 1.5 mm, after which it is covered in black material for a distance of 1 mm. It is subsequently replaced by a narrow groove 3 mm long, which is bordered by narrow ridges on each side, posterior of which, the jaw is torn apart.

The left wing consists of multiple layers. The basal layer is the steinkern, which is covered with coarsely crystalline black material. This layer is very thick in the median region and shows a honeycomb­like texture. It is overlain by a very thin sheath of brown­ ish, finely crystalline material. This is overlain, in turn, by patches of a white crystalline layer approximately 8 mm thick, which seems to be composed of elongate sections forming an angle of 458 with the midline ( figs. 26F, 27 ). This layer may actually consist of two sublayers.

Ridges and grooves are visible on the black layer on most of the left wing, and parallel the posterior margin, becoming coarser and more widely spaced posteriorly. The posterior one­half of the wing also shows irregular bumps suggesting plastic deformation. The steinkern shows a fingerprint­like pattern, reflecting the texture of the overlying black material.

UMUT MM28897 is an isolated lower jaw preserved in a piece of calcareous concretion with plant and shell fragments ( figs. 26A–D, 28 ). It is 17 mm long and the left wing is approximately 9 mm wide (W/L of the left wing 5 0.53). Most of the jaw is intact, but the posterior portion is crumpled and ragged. The jaw is composed of three layers: the steinkern, an overlying layer of coarsely crystalline black material, and a top layer of finely crystalline yellowish material (calcite, see below) in the form of two plates (aptychi).

The apex is pointed, and the anterior margin is slightly indented on each side. The apical angle is approximately 1208. The usual step­like feature near the anterior margin is not present and is presumably covered over. The apex shows a slit that extends approximately 4 mm along the midline. This slit is filled in with matrix, but black material is visible underneath—at least, at the posterior end of the slit. A groove is present on the rest of the jaw and is partly lined with black material, indicating that the two wings are continuous across the groove. The two calcitic plates (aptychi) terminate at the midline in a well­defined beveled edge. Thus, the underlying black layer is continuous across the middle, whereas the overlying calcitic plates are separate.

The calcitic plates (aptychi) are ornament­

24), ventral view, anterior end on top, 33. A. Uncoated. B. Coated.

ed with ridges and grooves that parallel the posterior margin. The ornament becomes coarser, more irregular, and more widely spaced toward the posterior end. Longitudinal striations are also present. The underlying black layer shows the same ornament as the aptychi, but the longitudinal striations are more prominent.

COMPOSITION AND MICROSTRUCTURE OF THE JAWS

The lower jaws of placenticeratids consist of several layers. The same layers are probably present in the upper jaws, but there are not enough data to confirm this.

The surface of the steinkerns of the lower jaws of AMNH 47275 and 47277 shows a network of small, irregular craters, each approximately 80 mm long ( fig. 29C, D ). Tanabe and Fukuda (1983) noted a similar pattern on the dorsal surface of the outer lamella of the lower jaw of the Late Cretaceous ammonite Gaudryceras . They interpreted this pattern as an impression of beccublast cells, which attach the jaws to the musculature (see Dilly and Nixon, 1976), as in the jaws of coleoids and modern Nautilus . If the pattern in Placenticeras also reflects the imprints of beccublast cells, it would imply that the dorsal surface of the outer lamella of the lower jaw was attached to the jaw muscles.

The principle component of the placenticeratid lower jaws is coarsely crystalline black material with a honeycomb­like texture. This material probably represents diagenetically altered chitin, which is very resistant to decay (Lehmann, l981; Kear et al., 1995). X­ray diffraction analysis of samples of this material from AMNH 47275, UWO.KMC.100126, BHMNH 5456, and BHMNH 5454b indicates that it consists of magnesium enriched calcite and amorphous material (organic compounds). In addition, the sample from AMNH 47275 contains pyrite, and the sample from UWO.KMC. 100126 contains siderite and illite, which probably reflect the composition of errant pieces of matrix.

The thickness of the coarsely crystalline black material varies on different parts of the lower jaw. It is thickest in the anterior region—for example, it is 0.3 mm thick at this point in AMNH 47277. However, no specimen is sufficiently well enough preserved to measure the thickness on the entire jaw. Compaction and exfoliation of the chitin after death have undoubtedly altered the original thickness of this layer. As a point of reference, the thickness of the lower jaw of Nautilus belauensis Saunders, 1981 , is approximately 0.5 mm near the posterior end.

Clear to yellowish crystals are commonly embedded in the interstices in the black material. These crystals also occur as elongate struts on each side of the central boss at the junction of the inner and outer lamellae in BHMNH 5456 ( fig. 20 ). Analysis of this material indicates that it consists of calcite enriched in magnesium. The calcitic crystals embedded in the black material are probably diagenetic in origin. However, it is conceivable that the struts represent an original calcitic thickening in the apical region of the jaw.

In many specimens, there is a finely crystalline yellowish to golden material overlying the black layer. X­ray diffraction analysis of a sample of this material from AMNH 47275 indicates the presence of calcite enriched in magnesium, organic compounds, and pyrite. This layer demarcates the junction between the inner and outer lamellae in AMNH 47277 ( fig. 7 ), suggesting that although it may be diagenetically altered, this layer actually represents an original deposit.

A thin outer calcitic layer is present in the two specimens of Metaplacenticeras subtilistriatum . In UMUT MM28896, this layer is only 8 mm thick and shows a fibrous microstructure ( fig. 29A,B ). It is covered with fine ridges and grooves that parallel the posterior margin. X­ray diffraction analysis of samples of this material from both specimens indicates that it consists of magnesium enriched calcite. In addition, the sample from UMUT MM28897 contains quartz, which probably reflects the composition of errant pieces of matrix. A crystalline layer also covers part of the right wing in BHMNH 5456 ( fig. 20 ). However, unlike the layer in M. subtilistriatum , the layer in BHMNH 5456 is very thick (1.2 mm), pockmarked due to weathering, and devoid of any morphological features. It probably represents a diagenetic deposit. Xray diffraction analysis of this layer reveals the presence of calcite.

A calcitic layer comparable to that in Metaplacenticeras is absent in the Placenticeras jaws from North America. Interestingly, the North American jaws sometimes occur in association with ammonite shells retaining their original aragonitic composition. Based on the differential solubility of calcite versus aragonite, one would have supposed that the aragonite would have dissolved before the calcite, a fact that is commonly cited in studies of ammonite jaws ( Morton and Nixon, 1987). Thus, the absence of a calcitic layer in these jaws would suggest that such a layer never existed at all.

We propose an alternative explanation, however. We suggest that a thin calcitic layer also covered the lower jaws of Placenticeras from North America, but that this layer was simply not preserved. This argument is based on the close affinity of Metaplacenticeras and Placenticeras , and homology with the lower jaws of closely related ammonites (see below). Judging from the jaws of M. subtilistriatum , the calcitic layer in Placenticeras would also have been very thin and fibrous, unlike the thick aragonitic layer of the outer shell. As a consequence, the calcitic layer of the lower jaw would have been very friable and easily broken. In addition, jaws, as internal structures embedded in the buccal bulb, experienced a different taphonomic history from that of the outer shell. The microenvironment created within the buccal bulb after death may have promoted local dissolution of the calcite compared with the conditions affecting the outer shell. Indeed, in their study of the lower jaws of scaphitid ammonites from the Upper Cretaceous Fox Hills Formation, Landman and Waage (1993: 60) noted that ‘‘The calcitic layer is usually absent on even slightly weathered surfaces, and in freshly exposed specimens it may be partly or entirely absent as a result of removal prior to formation of the enclosing concretion.’’

Figures

Fig. 27. Drawing of the left wing of the lower jaw of UMUT MM28896 (see fig. 26E–H), showing pieces of the outer calcitic layer (dotted area). Texture as defined in figure 4.

Fig. 28. Drawing of part of the anterior end of UMUT MM28897 (see fig. 26A–D) showing the ridges paralleling the posterior margin. Texture as defined in figures 4 and 27.

Fig. 29. A,B. Calcitic layer on the ventral side of the outer lamella of the lower jaw of UMUT MM28896 (see figs. 26E–H, 27). A. The surface is marked with growth lines. B. Cross section of the calcitic layer with fibrous microstructure. C, D. Dorsal surface of the outer lamella of the lower jaw of AMNH 47277 (see figs. 6, 7). C. The surface exhibits a network of small irregular craters, perhaps reflecting the impression of beccublast cells. D. Close­up of C.

Fig. 20. Lower jaw of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, BHMNH 5456, Baculites cuneatus Zone, Pierre Shale, Meade County, South Dakota, ventral view, anterior end on top. The upper arrow indicates the edge of the outer lamella. The lower arrow indicates the separation between the two lamellae, fortuitously exposed at a break in the jaw, 32.7.

Fig. 7. Close­up of the anterior end of AMNH 47277 (see fig. 6). A. Coated. B. Uncoated. The arrow indicates the edge of the outer lamella, 33.