THE USE OF MEDICAL CT SCANNING TO ANALYZE AMMONOID SHELL WALL THICKNESS AS A RESPONSE TO PREDATION

Throughout the late Cretaceous, the diversity of durophagus, or shell crushing, predatory taxa increased globally. Several traits observed in prey taxa, such as ammonites, have been suggested as a response to the enhanced predation threat, including increased shell ornamentation and septal thickness. A potentially more direct indicator of a response to predation would be shell wall thickening, as exhibited by other mollusks. The goal of this study is to use non-invasive imaging methods to test the hypothesis that ammonites exhibited thicker shell walls during the late Cretaceous, which may have been in response to increased predation.

Non-invasive methods like medical computerized tomography (CT) have been demonstrated as effective for cephalopod research by prior studies, with fast scan times (minutes) and adequate resolutions (~500 to 250μm) to visualize internal structures. Preliminary scans of ammonites from the late Cretaceous of South Dakota demonstrate that this method produces an image of internal shell structures useful for measurements, including outer walls and septa. The specimens examined are from the Pierre Shale Formation. Specimens varied in preservation from well-preserved to fragmentary to assess how this may impact scan quality.

From initial scans of 8 specimens, certain factors can cause interference. For example, pyrite content in several of the shells ranged from a few isolated grains to more pervasive precipitation. In all cases, scans of fossils with pyrite produced a starburst effect in the final image that can obscure many features. Furthermore mineral precipitation and recrystallization can also affect the viability of a shell for scanning. Incomplete shells still retained some internal structures, suggesting that even incomplete specimens may be useful for measurements. Advances in medical CT scanning technology allow for reliable measurements of ammonite shell walls, balancing cost, speed, and scale needed to address many morphological questions.

The use of medical CT scanning to observe ammonite shell structure can allow for the inclusion of more specimens than would be otherwise possible when using a destructive method or other higher-resolution approaches which are often expensive, time consuming, and require specialized training to handle post-processing of scans.