ORGANIZED MICRO-TUBULES OF UNKNOWN ORIGIN IN WELL-PRESERVED AMMONITE SHELLS

The preservation of fossil molluscan shell material is commonly assessed by visual inspection of aragonitic microstructure, often evaluated using the Preservation Index (PI) qualitative scale (Cochran et al., 2010; Knoll et al., 2016). This assessment is performed when shell carbonate is targeted for stable isotope geochemical approaches. Ammonite shells typically preserve nacre tablets, the shape of which can be observed easily using SEM at magnifications of around 10,000×. As part of an ongoing study of the stable isotopic composition of ammonites from the Western Interior of the continental US, we have examined the shell microstructure of 44 ammonites of the genus Baculites, with ongoing work on other ammonite groups, including scaphitids. Baculites species were common and widespread in the latest Cretaceous, and form most of the biostratigraphic zonation for the Western Interior.

Our examination of these fossils reveals that the microstructural preservation of these ammonites (screened with an initial visual inspection) was generally very good (PI > 4). During this inspection, it was revealed that many specimens had pervasive tubules penetrating the nacre tablets. These tubules are typically oriented perpendicularly, or nearly so, to the nacre tables, and their roughly circular openings can be seen in plan view of the shell. These tubules are narrow, ranging from 0.5 to 2 µm in diameter, but generally 1 µm or less, and they are many tens of µm long. We have not observed any clear indication of branching, though that may be a consequence of the viewing orientation. At present, we cannot be certain whether these tubules are an intrinsic part of ammonite shell material or are the result of microbial borings, which we consider more likely. These tubules resemble those recorded from cyanobacterial on modern bivalves, but are different in size and organization from those previously recorded in fossil and modern cephalopod shell material.