PROTOCONCH ENLARGEMENT IN WESTERN ATLANTIC TURRITELLINE GASTROPOD SPECIES FOLLOWING THE CLOSURE OF THE CENTRAL AMERICAN SEAWAY

The closure of the Central American Seaway (CAS) around 3.5 Ma significantly altered the marine environment of the Western Atlantic (WA) in a variety of ways, altering salinity, temperature, productivity, and circulation patterns. The end of communication of nutrient-rich Pacific upwelling waters combined with decreased local upwelling due to changes in circulation patterns, and possible decrease in riverine nutrient input combined to cause drastic shifts in the WA biota. The suspension-feeding turritellines appear to have been among the hardest hit marine clades in the regional extinction, supporting the hypothesis that nutrient decline was a dominant factor.

While the high level of extinction of turritellines has been well documented, the selectivity of their extinction has not been examined in detail. Protoconch size can be used as an approximation of maternal nutrient apportionment, with larger protoconchs indicating larger egg sizes, and therefore less reliance on planktotrophy before settlement. We examined the size of protoconchs in living turritellines from the WA and tropical eastern Pacific (TEP) in a molecular phylogenetic context, determining changes in protoconch size implied by the phylogeny. Pre-CAS closure data for the WA were derived from Oligocene and Miocene fossils from Panama and Venezuela. These data were compared with post-closure WA protoconch sizes to indicate potential extinction selectivity.

Post-CAS closure WA protoconchs are larger than both post-CAS closure Pacific, and pre-CAS closure WA protoconchs in both maximum diameter and diameter/volutions ratio (Mann-Whitney U). Continuous character mapping on the phylogeny indicates that protoconch size increased in the WA lineages in both of the geminate species pairs examined. Post-CAS closure Pacific protoconch values were wide ranging relative to both pre- and post-CAS closure WA values, but mean and median values between pre-closure Atlantic and Pacific values were highly similar. This indicates that extinctions were selective against highly planktotrophic WA species, supporting the primacy of nutrient decline in the associated Pliocene WA extinctions relative to other potential factors.