TAPHONOMY OF A MOLLUSCAN ASSEMBLAGE FROM THE CALOOSAHATCHEE FORMATION (PLIO-PLEISTOCENE) TYPE AREA, LA BELLE, FLORIDA

Despite the richness of the Plio-Pleistocene Caloosahatchee Formation, relatively little work has been done on the taphonomy of the shell beds. We examined the taphonomy of a bulk sample from the type area, 3.5 km west of La Belle, Florida (collected in 1955 by Squires and Heaslip and housed at the American Museum of Natural History). The sample included >1400 gastropods and >7000 bivalve specimens representing >50 species. In general, preservation is good; some specimens retain original luster (particularly Macrocallista) and one articulated Tellina specimen exhibited original coloration.

Four taxa, representing different morphologies and life modes, were selected for detailed taphonomic analysis: the relatively deep burrowing, thin-shelled Dosinia elegans; the shallow-burrowing, robust-shelled Chione latilirata; the chemosynthetic, infaunal lucinid Stewartia; and epifaunal pectinids (multiple species). The occurrence of the following taphonomic variables was noted: fragmentation, encrustation, non-predatory boring, edge rounding, and alteration outside the pallial line. In addition, taphonomic grades were assigned to each specimen.

Based on life mode, we expected infaunal taxa to be preserved better than the epifaunal pectinids. However, taphonomic grade of pectinids was significantly better than for infaunal taxa (92% of shells at grades 1 and 2, compared to 72%, 65%, and 43% for Dosinia, Chione, and Stewartia respectively). Pectinids also had relatively low degrees of encrustation, edge rounding, and non-predatory boring. However, they were also significantly more fragmented than all other taxa. None of the infaunal species differed significantly from each other in taphonomic grade. As expected, the thick-shelled Chione specimens lacked fragmentation; they also had significantly lower values of nonpredatory boring and alteration of the area outside the pallial line. The other infaunal taxa showed high amounts of nonpredatory boring (50-55%). All three infaunal species had moderate amounts of edge rounding (24%, 26%, and 48%). These results suggest a parautochthonous mode of accumulation in which amount of transportation outside the habitat was low, but shells were repeatedly reworked, thus experiencing episodes of exposure alternating with burial.