CARNIVOROUS CASSIDS (MOLLUSCA: GASTROPODA) FROM THE UPPER EOCENE OCALA LIMESTONE OF FLORIDA

Cassid gastropods, predators that feed on echinoids, leave behind distinct circular, cylindrical penetrations-drill holes in their prey’s tests. Cassid predation on present day echinoids is well documented and in recent decades paleontologists increasingly use drill holes preserved in prey tests to study predatory interactions between cassids and echinoids archived in the fossil record.

The ongoing Echinoid Associated Traces (EAT) project attempts to explore drill hole records on post-Paleozoic echinoids to investigate predation by cassids as well as parasitism by eulimid gastropods. As a part of this project, the Echinoid Associated Traces Database (EAT-D) is developed using data from predation traces to better understand associated biotic interactions.

Fossil echinoids are abundant in Florida’s upper Eocene Ocala Limestone and holes resembling modern drill holes by cassids are commonly seen in these echinoids. Unlike the echinoids, cassids are rarely found in the Ocala Limestone. Within the past four decades, only 80 specimens of known cassid fossils have been recorded from this formation. All are moldic and most are incomplete. We aim to resolve the taxonomy of the cassid snails using specimens reposited in the Invertebrate Paleontology Division, Florida Museum, a finding that will improve the taxonomic resolution of Paleogene data compiled in the EAT-D.

RTV silicone rubber peels were made as 3D replicas of the external moldic specimens. Morphological variations in peels and fossil specimens were observed and linear measurements were recorded using selected specimens. There is high morphological variation amongst the specimens and based on the density and morphology of axial ribs, there appears to be three distinct morphotypes. Morphotype 1 resembles Phalium globosum Dall, 1890, the only described cassid from the Ocala Limestone, morphotype 2 (with widely spaced fewer axial ribs), and morphotype 3 (with densely arranged numerous axial ribs) resemble modern Semicassis sp. There is high variability observed in specimens belonging to morphotype 2 in terms of rib density and rib morphology. We are still in the preliminary stages, but expect that morphometric analyses using a larger number of specimens will help resolve the taxonomy of predatory cassids from Florida’s Eocene deposits.