COMPARING THE PALEOECOLOGY OF MOLLUSCAN ASSEMBLAGES FROM THE PLIO-PLEISTOCENE NASHUA FORMATION OF NORTHERN FLORIDA

Timing and causes of the Plio-Pleistocene molluscan extinctions are not yet widely understood and contributing factors may vary geographically. Most work has focused on southern Florida and the Caribbean; here we examine the Nashua Formation from northern Florida. This unit is primarily a fine grained, shell-rich quartz sand, similar to other fossiliferous units farther south. Samples analyzed in this study were collected from spoil piles at three different sites within the East Coast Aggregates quarry near Hastings, FL. One sample from each site was analyzed in 2018 by students at Georgia State University. Two additional samples were studied in 2021 (one each from Site 2 & 3) with another sample soon to be processed (Site 1). Samples were sieved and shells greater than 5mm retained. Identifiable shells with a visible umbo (bivalves) or apex (gastropods) were picked and then separated as “whole” (consisting of >85% of the total shell) vs. fragmented individuals. Diversity, abundance, ecology, and drilling predation were studied.

Site 2 yielded 304.5 bivalve individuals (19 genera) and 156 gastropod individuals (15 genera). Site 3 yielded 262 bivalve individuals (19 genera) and 36 gastropod individuals (9 genera). Diversity was similar across both samples with overlapping 95% confidence intervals on rarefaction curves. Mulinia dominated bivalves in both samples (S2 = 37%, S3 = 61%); Oliva was the most abundant gastropod (S2 = 12%, S3 = 4%). Anadara was common in Site 2 (8%), but not in Site 3 (3%). The proportion of individuals represented by suspension feeders was greater in Site 2 compared to Site 3 (87% vs. 68%) in part due to differences in carnivorous gastropods (S2 = 9%, S3 = 30%). Drilling frequency based on complete drillholes was similar in both samples (S2 = 7%, S3 = 6%); prey effectiveness based on incomplete drillholes was 19% in both samples. Material analyzed in 2018 yielded more specimens resulting in increased representation of genera for bivalves and gastropods. Sample processing is not yet fully complete; more work is needed to compare samples studied in 2018 and 2021 from these sites. Future efforts will additionally include another sample from Site 1 to compare across the years. Overall, more research on the Nashua Formation is needed to better assess faunal changes in northern Florida across the Plio-Pleistocene.