Using paleocommunities to frame evolutionary and paleoecological studies: An example from the Fort Thompson Formation (Pleistocene) of Florida

G.M. Daley, A.M. Bush and D.H. Geary 2001
Geological Society of America Abstracts with Programs 33.

High-resolution paleoecological analysis of an outcrop of the Pleistocene Fort Thompson Formation was conducted at the Caloosa Shell Quarry near Ruskin, Florida. This study was designed to create a paleoecological framework for future evolutionary and paleoecological studies. A massive shell bed was sampled using bulk replicate sampling of small stratigraphic intervals in several stratigraphic sections. The data set derived from the molluscan remains was analyzed using principle components, factor, and cluster analyses to define fossil assemblages. Guild analyses and comparison to modern analogs of the ancient species were used to determine the most likely set of paleoenvironmental and paleoecological conditions at the time of deposition.

Multivariate analyses indicate that each sample contained one of two distinct faunal assemblages. An assemblage dominated by the bivalve Carditamera floridana and other grass-dwelling mollusks was recognized from the lower zone of the outcrop. Samples from the upper zone contained an assemblage dominated by the burrowing clam Chione cancellata as well as other grass and sand dwelling mollusks. The lower zone assemblage had lower paleoecological complexity, and a higher abundance of the opportunistic, salinity-tolerant mactrid Mulinia congesta, indicating less stable paleoenvironmental conditions.

These results indicate that there were two distinct sets of conditions in effect at different times during deposition. The earlier fauna indicates the presence of sea grass, but also environmental instability, perhaps due to fluctuating salinity. Later, a richer fauna combining sea grass-dwelling organisms with sand-loving taxa developed.

Morphometric analysis of Mercenaria campechiensis (which lived under both paleoenvironmental regimes) indicates morphological differences between samples from different times. Preliminary autecological and synecological analyses of other taxa also indicate differences between the two times.