Semicassis globosum (Mollusca: Gastropoda: Cassidae) from the upper Eocene Ocala Limestone of Florida with redescription and discussion of its extreme morphological variability

ABSTRACT

This study aimed to resolve the taxonomy of cassid gastropods from the upper Eocene Ocala Limestone based on specimens collected from Florida Museum of Natural History locality AL004. In 1890, Dall described a specimen from this formation, collected in a nearby county as Phalium globosum (subfamily Phaliinae). However, further collection of cassids from AL004 suggested the presence of more than one morphologically distinct taxon. Eocene cassids are particularly interesting because that is presumably the time when cassids experienced rapid diversification, which has implications for the evolutionary history of echinoid-cassid predator–prey interactions. Based on 75 specimens from AL004, it appears that the specimens interpreted as belonging to two or more morphologically distinct taxa are more likely members of a single species with extreme morphological variability, redescribed here as Semicassis globosum. Results of morphometric analyses using nine qualitatively defined cassid morphotaxa (subfamily Phaliinae) suggest that S. globosum is characterised by high intraspecific variability in shell morphology that may reflect high ecophenotypic variability or elevated intraspecific genetic variability. This study highlights the challenges associated with delineating species status using low numbers of specimens, as a dearth of diagnostic material may lead to taxonomic “over-splitting,” thereby resulting in an inflated estimate of biodiversity.

KEYWORDS:

• Semicassis globosum
• Cassidae
• Florida
• Ocala Limestone
• Eocene

Acknowledgments

Larry Rogers and Sandy Owens kindly provided RWP with access to FLMNH-IP locality AL004 during the past 40 years. Field assistance to RWP at FLMNH-IP AL004 was provided by John Arthur, Roger and Linda Farish, Rodney Feldmann, Jeremy Green, George Hecht, John Jaeger, Alex Kittle, Adiel Klompmaker, Will Ochoa, Craig Oyen, David Ramey, Sean Roberts, Kevin Schindler, Nicole Scroggins, Carmi Thompson, and Kathy Weedman. Digital images of cassids were taken by Sean Roberts (FLMNH). George Phillips (Mississippi Museum of Natural Science), Didier Merle and Jean-Michel Pacaud (National Museum of Natural History, Paris, France), David Dockery (Mississippi Geological Survey, Jackson, Mississippi), Thomas Waller and Mark Florence (Smithsonian National Museum of Natural History, Washington, D.C) permitted visits to collections under their supervision. We thank John Slapsinsky for providing access to specimens at IZ-FLMNH, and Kristopher Kusnerik for assisting ST at the NMNH-Paris. Helpful discussions with Harry G. Lee (FLMNH) were greatly appreciated as were formal reviews by Geerat Vermeij and two others. ST’s doctoral advisory committee: Mark Brenner (Department of Geological Sciences, UF), Gustav Paulay (FLMNH), and Larry Page (FLMNH) provided feedback on the study. Fieldwork funding for RWP was generously provided by Barbara Toomey and the FLMNH McGinty Endowment. Research funding was supplied in-part by NSF EAR SGP-1630276 and EAR SGP-1630475. Additional funding to ST was provided by the Lelia and William Brayfield Scholarship Award, the Southwest Florida Fossil Society, and the R. Jerry Britt, Jr. Paleobiology Scholarship (FLMNH). This is University of Florida Contribution to Paleobiology 865.

Disclosure statement

No potential conflict of interest was reported by the author(s).

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Funding

This work was supported by the National Science Foundation, USA [EAR SGP-1630276,EAR SGP-1630475].