MORPHOLOGICAL DISPARITY AND VARIABILITY OF THE FOSSIL GASTROPOD HYSTRIVASUM (FAMILY TURBINELLIDAE)

Hystrivasum (family Turbinellidae) is an extinct genus of charismatic fossil gastropod commonly found in Neogene deposits of Florida. More than 10 species have been described, with multiple species co-occurring at the same localities or within the same stratigraphic units. It was observed that it is difficult to distinguish many of these species from each other using the morphological traits used to describe them. In this study, we applied geometric morphometrics to explore shell shape variation in Hystrivasum, advance our understanding of interspecific and intraspecific morphological variability, and supplement information provided by the qualitative traits used previously to delineate these species. The taxonomy of many Neogene mollusks is not well resolved. Studies aimed at resolving the taxonomy of these mollusks may inform paleoecological studies and result in improved estimates of spatial and temporal trends in biodiversity, regionally and globally.

For this pilot study, we focused on five Hystrivasum species: H. horridum, H. locklini, H. olssoni, H. squamosum, and H. schinerae (n=5 specimens for each species). The specimens are from five localities of the upper Pliocene portion of the Tamiami Formation and two localities of the lower Pleistocene Caloosahatchee Formation. All studied specimens are reposited in the Invertebrate Paleontology Division at the Florida Museum.

Specimens were photographed in apertural view and four fixed landmarks and 40 resampled curve semi-landmarks representing overall shell shape were extracted per specimen using the software Slicer. A generalized Procrustes superimposition was conducted, and the results were visualized using a Covariance based Principal Component Analysis (PCA) run using tangent coordinates. All analyses were carried out using R 4.1.1.

Preliminary results suggest that specimens from the same species group together within the morphospace defined by the landmarks targeted in this study. However, there is some overlap among species. In addition, species vary in terms of intraspecific morphological variability. Future analyses based on an expanded dataset should help us to advance our understanding of the interspecific and intraspecific variability within the genus and improve species delimitation within the genus Hystrivasum.