PLIO-PLEISTOCENE TAMIAMI FORMATION (PINECREST BEDS) MARINE MICROMOLLUSKS: AN UNDERTREATED YET PRODIGIOUS FAUNAL ELEMENT

Among Cenozoic malacofaunas globally, the Tamiami Formation (Pinecrest beds) in southwestern Florida has been heralded as one of the best known (Allmon et al., 1993). Yet the same authors lamented the strikingly disproportionate lack of characterization in this assemblage of the micromollusks.

We investigated these Plio-Pleistocene sediments, exposed by shell mining operations of SMR Aggregates, in Sarasota County, Florida. The material studied was primarily bulk sampled from Phase 10, but a minor component came from earlier stages of the mining project, specifically Phases 6 and 9, housed in the IP-FLMNH collections.

Bulk matrix and some spoil were processed by standard methods of washing, and serial sieving. The material was then dried and picked under stereomicroscope. Our primary focus was directed at micromollusk remains, here defined as < 5.5 mm in average maximum adult shell dimension. Selected specimen(s) of each apparent species-level taxon were then studied using a ZEBS EVO MA 10 scanning electron microscope (SEM). Linear dimension(s) were determined using the instrument’s software.

At present, we have recognized approximately 220 species of micromollusks from these exposures. This level of biodiversity greatly exceeded that previously known from the Tamiami Formation (Pinecrest beds) and outstrips similar dedicated micromollusk collections of Holocene Florida coastal areas (Lee, 2009 and unpublished).

Generally, the taxonomic composition of the Pinecrest beds microfauna exhibits a pattern similar to that found in Olsson and Harbison’s (1953) North St. Petersburg monograph, with a dominance of species richness in the gastropod families Tornidae (40 species; 18% of total micromollusk species), Pyramidellidae (33; 15%), Eulimidae (18; 9%) Caecidae (17; 8%), Turridae s.l. (12; 6%), Cerithiopsidae (8; 4%), Rissoinidae (7; 4%), and Triphoridae (4; 2%) plus the bivalve groups generally assigned to the Galeommatidae (19; 9%).

However, species richness within certain families, e.g., Tornidae (greater) and the spongivorous Cerithiopsidae + Triphoridae (lesser) differs considerably from known Holocene Florida faunas. These disparities are likely reflective of ecological factors, e.g., warmer waters in the Pliocene and greater availability of sponge prey in the Holocene.