Chemical Differences among the Shells of Two Euryhaline Species of Fossil Ostracoda (Crustacea): A Preliminary Study

Abstract

Microfossils were extracted from a Florida Bay sediment core, and brackish and marine environments were interpreted on the basis of fossil ostracode and foraminiferid assemblages. A total of 48 hand-picked specimens of two species of euryhaline ostracodes (Cyprideis salebrosa and Peratocytheridea setipunctata) were chemically analyzed for Ca, Mg, Sr, and Fe concentrations to determine the effect of salinity on bulk skeletal chemistry. Results indicate that adult specimens of the two species have similar Mg and Sr concentrations, but Fe is more concentrated in the shells of C. salebrosa. There are no differences in trace element concentrations in adult specimens from brackish or marine sections of the core. Nodose and non-nodose instars of P. setipunctata contain similar concentrations of Sr, but greater concentrations of Mg and Fe than conspecific adults or adults of C. salebrosa. The enrichment of Mg and Fe in instars may be the result of rapid shell growth rate. Rapid carapace calcification may represent an adaptive strategy for survival that is maintained throughout the ontogeny of an individual. Ostracodes (Cambrian-Recent) are microscopic bivalved crustaceans ubiquitous to most water bodies. After hatching from an egg, an ostracode grows by molting (ecdysis). As a result of this life cycle, a series of carapaces graded in size and shape are shed in the sediment. Each ostracode can potentially molt 7 to 8 times prior to development of the genitalia that characterize the adult stage. When fossilized, these shells provide a record of ontogenic development and abundant specimens for paleontological analysis primarily in the fields of paleoecology and biostratigraphy. The use of suites of fossil ostracodes for interpretation of paleoecology stems from their well documented sensitivity to diverse environments in the Recent and the extensive geologic ranges of some ostracode taxa. Ostracodes abundantly occupy fresh, brackish, and marine waters, and their fossilized carapaces can provide evidence to support paleoenvironmental