BIOEROSION IN LATE CRETACEOUS AMMONITES: CHARACTERIZING MORPHOLOGY OF UNKNOWN MICROBIAL BORINGS

This study explores microborings in ammonite shell nacre that were initially recorded during the preservation assessment phase of an ongoing stable isotope study of Late Cretaceous ammonites from the Western Interior of North America. That study used SEM imaging to examine nacreous layer microstructure among multiple genera, including Baculites, Scaphites, and various complex heteromorph species. The prevalence of microborings in these specimens prompted our morphological study documenting the unknown penetrative features.

We hypothesize that the tubules are not a naturally occurring feature within ammonite shell material but rather a result of microbial boring. A diverse array of microboring morphologies in modern and fossilized calcium carbonate have been attributed to endolithic algae, fungi, and cyanobacteria trace-makers. The tubules in our specimens are mostly non-branching and of continuous width, with a preferred orientation perpendicular to the shell microstructure. Previous ammonite studies conducting SEM analysis note similar microboring morphologies but do not attempt systematic quantification of these features. To characterize these microborings, we measured the tubule width, length, quantity, and angle of the tubule relative to the nacre tablet structure. The morphological findings we present are consistent with recent work detailing the ichnotaxon Scolecia filosa produced by the endolithic cyanobacterium Plectonema terebrans.

We differentiated two morphologically similar types of tubules by tubule width. The secondary type, with a larger width, occurred infrequently but was found within at least one representative of each taxonomic group. We compared multiple ammonite genera from the same biostratigraphic time period but found no statistically significant variation in tubule morphology. Additionally, we found that tubules are simultaneously present in the septum and both outside surfaces of the shell, but identified no systematic pattern in tubule density between outside surfaces. Preliminary results did indicate a higher density of microborings in scaphitid ammonites, but further work is needed to confirm this observation. Ultimately, we intend to confirm our identification of S. filosa and determine the paleoecological significance associated with the presence of microborings in our specimens.