ANATOMICALLY LINKED OCCURRENCE OF CONCRETIONARY PRESERVATION OF MOSASAURS FROM THE PIERRE SHALE OF SOUTH DAKOTA

Vertebrate fossils are often found preserved in concretions, hard, cemented mineral masses. Little is understood about this mode of fossilization. Fossils in concretions are difficult to prepare and are often neglected, resulting in inaccessible data, such as element identification and count and species identification. Mosasaur preservation in the Cretaceous Pierre Shale is often in carbonate, phosphate, and iron concretions. However, these marine reptiles rarely preserve within a single concretion. Whether articulated or disarticulated, only parts of the organisms occur in concretion. This study tests if certain anatomical regions are preferentially preserved in concretions. If those regions are the skull and torso, which are characterized by higher fat content, then concretion formation may be driven by microbial decay of higher concentrations of adipose tissue.

Data were collected from preparatory notes and specimens at SDSMT’s Museum of Geology. The skeleton was divided into four anatomical regions: skull, torso, limbs, and tail. The presence or absence of concretions was documented for each region along with type of concretion and stratigraphic interval. These data will be used to test if concretion formation is random with respect to anatomical region or if some regions have proportionately more concretions than expected by chance. Preliminary data suggest that regions with more fat content, the skull and torso, are more often preserved in concretions. No concretions were documented in limb bones or the end of the tail, which contain less fat tissue. However, there are many mosasaur skulls found that are not preserved in concretions suggesting that multiple factors, such as temperature or water chemistry, also influence concretionary preservation of vertebrate fossils. Understanding growth preference offers insight into the mechanism of concretion formation and will allow researchers to account for biases in element count and diversity studies due to data loss.