ENVIRONMENTAL STRATIGRAPHY, PALEOECOLOGY, AND FATE OF THE HUMONGUS PINECREST BIVALVE FAUNA OF SARASOTA, FLORIDA

Numerous lucinid species in life position in the central facies point to a lagoonal seagrass habitat; scattered colonial coral heads are also present. Immediately above and below this facies are banks of Hyotissa oysters with many other attached bivalve taxa. These bordered the lagoon subtidally. Above and below them are colonies of Vermicularia gastropods that occupied the intertidal zone on both sides of the lagoon. A black, organic-rich unit above the lagoonal facies, with many species of mammals, shore birds, and brackish-water mollusks, formed in a marsh along the back of the barrier island. Distal tidal channels here were draped with mussels of the genus Perna. A large channel in the black unit marked by a visible margin and filled with shell hash represents a migrating inlet. Also associated with these upper units are deposits produced by storms that breached the barrier. They contain articulated open-ocean scallops, some of which are imbricated or form layered beds. Associated well-sorted sands yield a sparse fauna of rapid-burrowing species with living populations or analogs adapted to shifting sands along tidal inlets. Finally, a group of fossil cormorants, many largely articulated, have been found in the marsh deposit. Their death has been attributed to red tide poisoning, but in fact they were preserved in the angled beds of a washover fan consisting of black sediments blasted into the lagoon by a storm. The birds were blown from their perches and buried.

There was little of peninsular Florida to provide runoff to deliver mud or depress salinity: thus, the great bivalve diversity and growth of large coral heads. The fact that all surviving species of this tropical bivalve fauna (30%) today range into temperate regions indicates that climatic cooling caused the heavy extinction of the rest.