FIRST-YEAR GROWTH RATES AND SIZE OF TURRITELLINE GASTROPODS ACROSS THE U.S. ATLANTIC COAST PLIO-PLEISTOCENE EXTINCTION EVENTS

The Plio-Pleistocene regional extinction of marine fauna in the western Atlantic is now widely believed to have been mainly a result of decreasing primary productivity, although other factors may also have been involved. Previous work has pointed to a multi-phase extinction with at least two pulses. To investigate further, our ongoing study (e.g., Lavarreda et al., 2007; Monarrez et al., 2008; Petsios et al., 2009) has used stable isotope sclerochronology to determine the growth rates of Pliocene and Pleistocene turritelline gastropods from Florida and North Carolina.

Decreasing productivity has previously been linked to a decrease in overall shell size and growth rate in some fossil and Recent turritelline gastropods from Chile and California. Changes in growth rate and shell size may thus reveal differences in the timing and causes of the western Atlantic extinction. Preliminary work estimates 25 to 30 distinct Turritella species from Florida and North Carolina in the units studied; 11 of these species have thus far been analyzed for growth rate information, standardized to the first year of growth. Preliminary results reveal a complex pattern: before the onset of extinction growth rates and shell size were highly variable (from 470 mm to 59 mm of spiral growth in the first year, 640 mm to 120 mm total spiral length). The first “pulse” of extinction included the disappearance of the largest species (>450 mm spiral length), but variability in growth rates remained (440 mm to 58 mm of spiral growth in the first year). This pulse saw a 64% extinction rate, with 7 of the original 11 species becoming extinct. During the second “pulse”, all species with >100 mm spiral length became extinct (n=3 of 4, 75% extinction rate), leaving only a small, short-lived species, T. subannulata. Comparison of two individuals of this species reveals no significant difference in growth rate before and after the extinction event (100 mm spiral growth in first year).

The growth rate variability present in Plio-Pleistocene Turritella suggests that the simple model predicted from the Chile and California studies may not completely apply here. Survival appears to have been size-related, but not necessarily controlled by growth rates. To more fully document the nature of growth rate changes, isotopic analysis of the remaining species is in progress.