TESTING FOR ECOPHENOTYPIC VARIATION ACROSS A PALEOENVIRONMENTAL GRADIENT IN A SMALL MACTRID BIVALVE (YORKTOWN FORMATION, PLIOCENE, VIRGINIA)

Studies of invertebrate marine organisms have shown changes in shell morphology and size correlate with paleoenvironmental change. If paleoenvironmental conditions determine the morphology of a species then change will quantitatively affect morphology even at a fine resolution. Mulinia congesta (Conrad), an opportunistic shallow infaunal siphonate suspension-feeding bivalve, was collected from the Rushmere and Morgart's Beach Members of the Pliocene Yorktown Formation in southeastern Virginia. The section is an unconformity-bounded sequence of thin shallow marine material deposited in less then 500 Ky. The Rushmere is a coarse-grained shelly deposit, which grades into the much finer Morgart's Beach deposits, corresponding to a paleoenvironmental transition from higher to lower energy.

The different facies have different characteristic faunal assemblages, but M. congesta is common throughout the section and collected at 30-cm intervals over a three-meter high section. Each valve was digitally imaged and x-y coordinates for fifteen landmark were recorded. Procrustes method was used to simultaneously fit the landmark points for 330 specimens. This procedure derives shape coordinates that are invariant in respect to variations in size, rotation, and position of specimens. Tangent coordinates were used to derive principle component ordination to compare the overall shape differences between shells from two different facies, and size was estimated using shell length and centroid size.

The comparison of size of 288 specimens from Rushmere and 42 specimens from Morgart's Beach shows a decrease in maximum shell length and centroid size through time. However, Wilcoxon 2-sample test shows that the median decrease in size is insignificant, (p=0.24 for centroid size; p=0.17 for length). In the PC1 and PC2 morphospace derived from Procrustes analysis, the two groups show perfect overlap. Even when sensitive geometric methods are employed, there is no quantifiable change in shape or size through the section. The preliminary results suggest that despite changes in paleoenvironmental conditions and related changes in faunal composition, shell morphology and size of M. congesta remained unchanged.