Biostratigraphy and paleoecology of the Pleistocene invertebrates from the Leisey Shell Pits, Hillsborough County, Florida

Abstract

A Pleistocene macro-invertebrate assemblage from two Leisey Shell Corporation pits located 1 km inland from Tampa Bay along the Gulf Coast of Florida, is examined. The assemblage is dominated by marine mollusks and consists of 98 species of bivalves, 113 species of gastropods, 3 polyplacophorans, and 3 scaphopods. Other invertebrates identified include 1 poriferan, 3 cnidarians, 16 bryozoans, 1 annelid, 9 arthropods, and 1 echinoderm. The fossil material was collected at Leisey Shell Pit 1 and Leisey Shell Pit 3 from three superposed marine shell beds (and spoil) in each pit, The strata containing the molluscan faunas ofthe two lower most shell horizons (lower shell bed and bone bed) are placed in the Bermont Formation and contain approximately 3% extinct marine species each. The uppermost unit (upper shell bed) is most similar to the late Pleistocene Fort Thompson Formation and contains approximately 1% extinct species of marine mollusks. At both Leisey pits the base of the lower shell bed contains dark. organic lenses or layers with freshwater and estuarine mollusks and freshwater and terrestrial vertebrates. The remainder of the lower shell bed appears to have been deposited under normal marine conditions in relatively quiet waters from 3 to 6 m in depth. The bone bed at each pit (Leisey 1A and 3A) consists of a thin lens containing an extremely rich concentration of terrestrial, freshwater, and nearshore marine vertebrate fossils, in addition to the rich, shallow-marine invertebrate fauna with abundant freshwater gastropods and some terrestrial gastropods. The molluscan assemblage indicates deposition in a shallow (intertidal to -2 m), protected marine bay or lagoon, containing abundant marine grass beds and having a large river flowing into it. The upper shell bed at the Leisey Pits has the least diverse molluscan assemblage, having also been deposited in shallow water (intertidal to -2 m) under relatively high energy conditions possibly in a tidal channel.