Using Taphonomic Disparity to Understand Preservation Biases in the Western Interior Seaway: An Example from the Pierre Shale (Upper Cretaceous)

Preservation potential of marine mollusks is believed to be mostly a result of shell mineralogy and depositional environment. However, little is known about how mineralogy and depositional environment interplay and affect preservation quality within a regional setting over a relatively short depositional period. To better understand preservation potential under these conditions, we examined mollusks from the Pierre Shale in the University Colorado Museum of Natural History invertebrate paleontology collection. This collection contains mostly ammonites and bivalves from 197 localities from the Western Interior Seaway that span an approximately 12 million year period of time. Specimens exhibit a range of preservation, from excellent (mostly unaltered original shell present) to very poor (highly altered shell to only molds/casts present). Depositional settings include dysoxic marine, methane seeps, prodeltas, beaches and barrier islands. Preservation quality of 121 ammonites and 203 bivalves, from 64 localities were examined for percent of original shell present, the degree to which that shell has been altered, the depth of alteration, and the quality of detail preserved in mold or cast. We examined the relationship between preservation quality, taxonomy, mineralogy, depositional setting, estimated carbonate saturation levels, and relative distance from shore. Preservation quality was not correlated with taxonomy or the original mineralogy of shell material. Instead, we found that depositional setting and the carbonate saturation levels within these settings were a more important determinant of preservation quality than original shell mineralogy. In addition, variable carbonate saturation levels within settings of the Western Interior Seaway varied enough to both preserve nacreous ammonite shells in some settings while dissolving bimineralic bivalve shells in other settings.