HABITATS, MARINE LIFE, AND HISTORY OF A TRANSGRESSIVE BARRIER-ISLAND (OR SPIT)-LAGOON SYSTEM: PALEOECOLOGY AND FATE OF THE INCREDIBLY DIVERSE TROPICAL BIVALVE FAUNA OF THE PLIOCENE PINECREST FORMATION OF FLORIDA

The Pliocene Pinecrest fauna of the Sarasota region lived in a marine climate that was marginally tropical: warmer than that of Sarasota Bay today. The units described here represent facies of a transgressive barrier-island(or spit)-lagoon complex. A diverse lucinid fauna and other key indicators reveal that the principal shell bed represents the floor of a marginal marine lagoon with extensive sea grass meadows. Lag deposits of small strombids (“rollers”) formed along the inner shore. Below and above this central lagoonal unit are bioherms of the oyster Hyotissa. These were low intertidal structures fringing both sides of the lagoon. Generally higher in the intertidal zone on both sides of the lagoon were clusters of the “corkscrew” gastropod Vermicularia, now usually positioned at the top and bottom of the lagoonal sequence. Environmental reconstructions, a uranium-helium date of~3.0 my, guide fossils, and substantial faunal overlap show that the Pinecrest units above these lagoonal units are coeval with them. They represent back-barrier, tidal channel, and inlet deposits. A black, organic-rich sand containing fossils of large mammals, shore birds, and a brackish molluscan fauna formed in marshes on the back of the barrier island/spit. Meter-scale tidal channels draped with mussels were incised into the black unit. Other key features include: tidal inlet storm deposits with layered and imbricated, articulated open-ocean scallops; the margin of a tidal inlet channel filled with shell debris; and a huge death assemblage of cormorants (hurricane victims) in sloping beds of reworked black marsh sands deposited in the lagoon as a washover fan by hurricane-driven waters that breached the barrier. Only about 30% of Pinecrest bivalve species survive today. The fact that all the survivors range northward to Texas or Cape Hatteras whereas all the purely tropical species are gone indicates that ice age cooling caused the excess extinction. My inevitably incomplete fossil collections from the small Pinecrest Lagoon contain more bivalve species than, according to the exhaustive review of Mikkelson and Bieler, have ever been found in any of six nearshore and shallow offshore areas, each thousands of square kilometers in size, in the region of the Florida Keys . The modern marine bivalve fauna of Florida is clearly impoverished.