PLEISTOCENE ESCALATION IN STROMBID GASTROPODS OF FLORIDA AND A POSSIBLE CATALYST ROLE FOR GLACIAL ‘SUPER-EL NIÑO’ CONDITIONS

There is circumstantial evidence that warm, productive, and competitive environments conducive to escalation prevailed in the Pliocene western Atlantic. This time supported large-bodied suspension feeders and a diverse fauna of abundant and highly specialized predators. Their decline during an end-Pliocene regional mass extinction event has been taken as evidence that such conditions ceased to exist by the Pleistocene with the closing of the Central American Seaway and the onset of Northern Hemisphere Glaciation.

Here, we test the hypothesis of escalation in strombid gastropods (S. alatus complex) of the Florida Plio-Pleistocene. We examined shells collected from the Pliocene Pinecrest Beds and the Pleistocene Bermont Formation and measured shell repair frequency (a measure of selection for shell armor) and traits known to confer an adaptive advantage against durophagous predators.

Our preliminary data reveal unexpected escalation in anti-predatory traits of strombid gastropods (S. alatus complex) during the post-Pliocene of Florida. Repair frequency nearly doubled (12.5% to 20.8%) in adult Strombus from the Pliocene to the Pleistocene. Anti-predatory trait evolution in Pleistocene Strombus also occurred, including increases in: (1) adult mean size and lip thickness (79 to 87 mm shell length; 3.9 to 4.5 mm thickness at the shoulder of the body whorl); (2) percent of individuals with knobs on the last body whorl (4.2% to 16.7%); (3) maximum number of knobs on the last whorl (from £2 to £9); and (4) growth rates, as characterized by oxygen stable isotope sclerochronology (20% faster in the Pleistocene). Our initial observations of modern Florida Strombus suggest that Pleistocene escalation is likely an ongoing process.

These data are contrary to the expectation that conditions in the Pleistocene western Atlantic were less conducive to escalation than before. Either warm, productive, and competitive conditions are not necessary for adaptive improvement, or our current understanding of the Pleistocene paleoenvironment is inadequate. We speculate that Pleistocene glacial intervals, which were characterized by global ‘super-El Niño’ conditions and increased dust-fall, may have had a ‘fertilizing-effect’ on coastal marine communities of Florida.