MORPHOMETRIC ANALYSIS OF CENOZOIC CASSIDS (MOLLUSCA: GASTROPODA)

Cassid snails are predators of echinoids, leaving behind distinct drill holes in prey tests. Drill holes are being increasingly used to explore predator-prey interactions between cassids and echinoids in the fossil record. Although traces resembling cassid drill holes are common in the fossil record, cassids are an understudied group of gastropods and their taxonomy is poorly resolved for many genera, regions, and time intervals. The ongoing Echinoid Associated Traces (EAT) project examines traces of predation by cassids and parasitism by eulimid snails on post-Paleozoic echinoids to better understand the evolutionary history of biotic interactions. A more thorough understanding of the distribution and classification of cassids would therefore augment paleoecological studies of cassids.

To delineate cassid morphotaxa, we carried out a morphometric analysis using common Cenozoic cassid morphotaxa reposited in the Invertebrate Paleontology Division, Florida Museum of Natural History, Mississippi Museum of Natural Science, and the Palaeontology Collection, National Museum of Natural History, Paris, France. Using a combination of linear and semi-qualitative character variables, 9 linear measurements and 6 additional variables were collected for 130 specimens representing 9 qualitatively defined morphotaxa. Distinctive morphotaxa could not be fully differentiated by Principal Component Analysis (PCA) using 8 continuous linear variables. However, ordination plots suggested that differences in morphological variability and body size exist among morphotaxa and/or through time.

Expanded analysis (Non-metric Multidimensional Scaling; NMDS) including additional variables delineated the qualitative morphotypes, indicated variable disparity across morphotaxa and tentatively suggested body size trends through time. Results suggest that the combined use of continuous ratio variables and semi-qualitative variables enhance our ability to delineate morphogroups in the ordination space. Analyses using additional specimens would help to assemble a more comprehensive morphospace of Cenozoic cassids, improve the quality of alpha-taxonomic data on cassids included in the EAT Database, and assess preliminary patterns suggestive of taxon/time specific trends in disparity and body size.