Preservation of Pseudosutures

Whether or not pseudosutures are present in an etched specimen depends on the mineral composition of the pseudosutures and on how the fossil breaks. Pseudosutures that are preserved as calcite ought to dissolve, and in some cases this occurred. In a few instances, however, pseudosutures were not dissolved by acid etching. We attribute this to an overlying phosphatic coating sometimes left behind when the fossil broke out of the matrix (Fig. 9.15). When specimens are removed from the matrix, both the cameral lining and the shell wall may adhere to the external mold (a concave fragment; scenario 1, Fig. 9.15).

Alternatively, the cameral lining may adhere to the steinkern while the shell wall adheres to the surrounding matrix (scenario 2, Fig. 9.15). These different breakage patterns affect whether pseudosutures are visible after etching of the specimens. If the matrix breaks away from the internal mold with both the calcitic shell and the phosphatic chamber lining adhering to it (scenario 1), the pseudosutures will be visible as ridges on the inner surface of the external mold, and as grooves on the internal mold ("r" and "g," respectively, Fig. 9.15). If the pieces are then etched, the pseudosutures should still be visible on the external mold regardless of their

Fig. 9.15 Illustration of two possibilities that may occur during preparation of a specimen, with the specimens shown in median cross section (parallel to the siphuncle). This diagram explains why pseudosutures sometimes remain visible after etching. In both scenarios 1 and 2, the unbroken specimen has a mineralized layer that includes the shell wall (sh) and the pseudosutures (ps) on the inside surface of the wall. There is a chamber lining (cl) overlying the mineralized layer. In scenario 1, both layers break away from the internal mold, and the pseudosutures are visible as grooves on the internal mold (g) and as ridges on the external mold (r). After etching, the grooves on the internal mold are no longer visible, but the impressions of the ridges on the external mold (r) are preserved by the overlyingphosphatic layer. In scenario 2, the mineralized layer breaks away from the internal mold, but the phosphatic chamber lining(cl) adheres to the internal mold. As in scenario 1, the pseudosutural ridges (r) are visible on the external mold, and the impressions are visible on the internal mold as grooves (g). After etching, however, the mineralized ridges are dissolved away on the external mold, yet the impression of the ridges remains on the internal mold because the phosphatic lining (cl) resists etching.

Fig. 9.15 Illustration of two possibilities that may occur during preparation of a specimen, with the specimens shown in median cross section (parallel to the siphuncle). This diagram explains why pseudosutures sometimes remain visible after etching. In both scenarios 1 and 2, the unbroken specimen has a mineralized layer that includes the shell wall (sh) and the pseudosutures (ps) on the inside surface of the wall. There is a chamber lining (cl) overlying the mineralized layer. In scenario 1, both layers break away from the internal mold, and the pseudosutures are visible as grooves on the internal mold (g) and as ridges on the external mold (r). After etching, the grooves on the internal mold are no longer visible, but the impressions of the ridges on the external mold (r) are preserved by the overlyingphosphatic layer. In scenario 2, the mineralized layer breaks away from the internal mold, but the phosphatic chamber lining(cl) adheres to the internal mold. As in scenario 1, the pseudosutural ridges (r) are visible on the external mold, and the impressions are visible on the internal mold as grooves (g). After etching, however, the mineralized ridges are dissolved away on the external mold, yet the impression of the ridges remains on the internal mold because the phosphatic lining (cl) resists etching.

mineral composition, because the chamber lining protects and preserves the pseudosutures it overlies. On the internal mold, however, the grooves would disappear, as would all other calcitic material.

Alternatively (scenario 2, Fig. 9.15), if the specimen breaks between the calcitic layer and the phosphatic cameral lining, the calcitic layer and pseudosutural ridges (r) would adhere to the external mold, and the cameral lining would adhere to the internal mold. Before etching, the pseudosutures would be visible as ridges on the external mold and as grooves on the internal mold, as before. After etching, however, the pseudosutural ridges on the external mold, if they are calcitic, would disappear, leaving nothing on the internal surface of the shell wall fragment, while the pseudosutural grooves would still be preserved on the internal mold as impressions in the cameral lining (scenario 2, Fig. 9.15). This model thus explains our observations of the variation in position and presence of pseudosutures before and after etching.

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