TESTING FOR EVOLUTIONARY STASIS IN PLEISTOCENE AND RECENT CHIONE ELEVATA FROM FLORIDA

Patterns of morphologic change in the fossil record are key to understanding the tempo and mode of evolutionary in deep time. Whether or not evolution occurs primarily in bursts after long periods of morphological stasis, as predicted by the theory of punctuated equilibrium, can be tested by characterizing the morphology of a taxon in different slices of time. Here we test whether or not evolutionary stasis can be quantifiably recognized using morphometric data collected from a well-defined and easily identified clam species, Chione elevata (Say, 1822).

Fossil specimens were collected from the Fort Thompson Formation and Bermont Formation (Pleistocene, Florida), and three samples were collected from Recent beaches on Sanibel and Captiva Islands. The Pleistocene fossils were sieved from bulk sediment samples from the Caloosa Shell Quarry in Hillsborough County. The Recent beach specimens were surface-collected near the high tide mark in a small area for each beach. The shallow-marine environments of the Recent localities are similar to the interpreted paleoevironments of the fossil deposits.

Each specimen was imaged using a 35 mm camera suspended above the specimens, which were oriented using a bullseye bubble. For each specimen, a total of 14 morphological landmarks were sampled using ImageJ software. The morphological data was then subjected to Procrustes analysis, Principle Components Analysis, and other multivariant analyses to determine (1) whether there was allometry in the samples, as well as (2) whether the amount of variability within samples was greater than the variability between samples through time.