Quantifying shell patterning helps identify species of Trionychidae

Softshell turtles (Trionychidae) display characteristic pits and ridges, or "sculpturing," on the bony carapace. Variation in sculpturing pattern may be useful in classifying fossilized shell fragments. Although past attempts could discern qualitative differences in certain best-case scenarios, many early taxonomic uses of sculpturing traits have been reevaluated as unreliable in the face of intraspecific variation. The potential of sculpturing to contain consistently reliable, quantitative, taxonomically informative traits remains underexplored. Here, we revisit this idea by quantifying trionychid shell patterning with topographic measurement techniques more commonly applied to nonhomologous quantification of mammalian teeth and geographic surface topography. We assess potential sources of variation and accuracy of these metrics for species identification. Carapaces of extant specimens used in this study included members of the species Apalone ferox, Apalone spinifera, and Amyda cartilaginea and were obtained from the herpetology collections of the Florida Museum of Natural History. 3D scans of shells were systematically sampled to create digital "fragments." These fragments were quantified using three topographic measurements: Dirichlet Normal Energy (DNE), Relief Index (RFI), and Orientation Patch Count Rotated (OPCR). A nested MANOVA suggests there is significant variation at the species, individual, and carapace location levels of analysis. Linear discriminant analysis correctly predicts a sample's species identity from DNE, RFI, and OPCR 75.2% of the time. These promising results indicate that topographic measures may provide a method for identifying shell fragments that are currently identifiable only as Trionychidae indet. Future work should explore this approach in additional species and account for ontogenetic changes.