MARINE MICROMOLLUSKS OF THE PINECREST BEDS: AN UNDERTREATED YET PRODIGIOUS LATE PLIOCENE FAUNAL ELEMENT; THE CASE OF THE FAMILY TORNIDAE

Among Cenozoic malacofaunas globally, that of the Pinecrest beds, Tamiami Fm. in southern Florida, has been heralded as one of the best known and species-rich (Allmon et al., 1993). Yet the same authors lamented the strikingly disproportionate lack of characterization of an element in this assemblage, the micromollusks.

We have been investigating the sediments exposed by the mining operations of SMR Aggregates’ Phase 10 in Sarasota County, Florida since July, 2013. Bulk matrix and spoil were processed by washing and serial sieving, and then culled under stereomicroscope. Because of the lack of previous attention, our focus was on micromollusk remains, here defined as <5.50 mm in average maximum adult shell dimension. Better-preserved material of each apparent species-level taxon in this general size range was studied using a ZEBS EVO MA 10 scanning electron microscope (SEM). Linear dimensions were determined by the instrument’s software.

At present, we recognize 260 species-level taxa of micromollusks in this exposure, which appears to be confined to the late Pliocene. This level of diversity greatly exceeds that previously known from the Pinecrest beds, and significantly outstrips dedicated collections of circumscribed Holocene Florida coastal areas. After the pyramidellids, the family Tornidae is the most speciose family in this assemblage, comprising over 17% of species richness of the micromollusk component. Considering this richness and the substantial improvement in Holocene tornid systematics (e.g., Rubio et al. 2011), we analyzed the systematics, biodiversity, and evolution of this group.

Recorded tornid species richness in Florida varies over the last 3 Ma, with 52 species (49 <30 m depth) recorded from the Holocene (Rubio et al., Idem); 28 from the lower Pleistocene Upper Caloosahatchee Fm., 26 from the lower Pleistocene Lower Caloosahatchee Fm., 16 from the Upper Pinecrest beds, and, prior to our work, none from Lower Pinecrest beds (Campbell, 1993). The latter figure is now raised to 42, indicating a rich faunule. These 42 species represent 12 genera. The Florida Holocene has the same count, and 11 occur in common, one fossil genus (9%) is absent from the Western Atlantic. At the species level, 27 of the 42 fossil taxa (64%) have become extinct, but most of the latter have clearly-related Holocene congeners.