Early Pleistocene freshwater bivalves (Mollusca: Unionidae) from the Leisey Shell Pits, Hillsborough County, Florida

Abstract

Unionid valves are reported from the early Pleistocene Bermont Formation at the Leisey Shell Pits, Hillsborough County, Florida. Collected from the base of Leisey Shell Pit 3 were numerous valves of Utterbackia sp. cf. U. imbecillis (Say 1829) and Megalonaias boykiniana (Lea 1840) and from the base of Leisey Shell Pit 1 several valves of Elliptoideus sloationus (Lea 1840) were collected. A matched pair of Megalonaias valves was also obtained from spoil at Leisey Shell Pit 1 and is most likely from the same horizon as the Elliptoideus. These southern Florida unionid remains represent the oldest Cenozoic freshwater bivalve fauna in the eastern United States. The fossil occurrence of Megalonaias and Elliproideus, 320 kilometers south of their modern range limit in the panhandle of Florida points to a major range contraction of these unionid species in peninsular Florida since the Pleistocene.