Systematics and biostratigraphy of Chesapecten and Carolinapecten (Mollusca: Bivalvia: Pectinidae) in the upper Miocene and Pliocene "Lower Tamiami Formation" of southwestern Florida

Abstract

The position of the Miocene-Pliocene boundary in the highly leached marine sediments of the southwestern Florida Peninsula has been debated for more than a half century. A systematic revision of well-preserved calcitic shells of fossil scallops in the genera Chesapecten and Carolinapecten in these deposits and comparisons to known Miocene and Pliocene taxa in the Mid-Atlantic Coastal Plain indicate that the boundary lies between the Murdock Station Member of the Peace River Formation in Charlotte County, Florida, and the basal Units 11 and 12 exposed in pits near Sarasota. Both genera display stratigraphically successive morphologies that permit the construction of pectinid interval zones that should prove useful in future studies of upper Miocene and Pliocene leached deposits in the Florida Peninsula. In Charlotte County the Chesapecten succession begins with Ch. middlesexensis hunterae n. ssp., followed by Ch. middlesexensis bayshorensis n. ssp.; the Carolinapecten succession begins with Ca. corpulentus n. sp. followed by Ca. murdockensis n. sp. The latter is subdivided into three successive chronological subspecies: Ca. murdockensis druidwilsoni n. ssp., Ca. murdockensis murdockensis n. ssp., and Ca. murdockensis parawatsonensis n. ssp. Chesapecten and Carolinapecten in Charlotte County are accompanied by other calcitic fossils that are strong indicators of a late Miocene age, including broad-ribbed Ecphora in the E. gardnerae complex and non-auriculate oysters in the Mansfieldostrea compressirostra complex. In the basal units of the Sarasota pits in Sarasota County, the first Chesapecten to appear is Ch. quinarius n. sp., a form previously confused with Ch. jeffersonius (Say) or Ch. septenarius (Say). The new species occurs with Ch. madisonius sarasotensis n. ssp. and Carolinapecten eboreus watsonensis (Mansfield) in a fauna that is Pliocene, including Ecphora quadricostata and auriculate oysters in the Mansfieldostrea compressirostra complex. Contrary to previous studies, the early Pliocene species Ch. jeffersonius (Say) is not present in these areas, and the Murdock Station Member of Charlotte County does not correlate biostratigraphically with Unit 11 in the Sarasota area.