Two beds containing large numbers of turritellid gastropods (Family Turritellidae) occurring in the densely fossiliferous Upper Pliocene Pinecrest Sand of Florida formed as a result of upwelling and consequent high biological productivity, together with some degree of physically mediated time averaging. Analyses of size-frequency distribution and shell surface condition, combined with isotopic data on chronological age of individual shells, water temperature and upwelling intensity, suggest that both beds formed relatively quickly, probably in less than 100–200 years. The upper bed, occurring within Petuch's (1982) unit 2 (2.5-2.0 Ma) and containing abundant Turritella apicalis Heilprin, appears to have formed largely as a result of upwelling; the lower bed, occurring in upper unit 6/7 (3.5-2.5 Ma) and containing abundant Turritella gladeensis Mansfield, appears to have formed over a longer period, as a result of upwelling, increased time-averaging, and perhaps cooler overall water temperatures. This study highlights the potential to isolate and examine separately some of the biological and physical factors affecting shell bed formation, and especially to address the role of biological productivity in this process. □Pinecrest Beds, Pliocene, upwelling, turritellid gastropods, taphonomy.