EVOLUTION OF SHAPE AND SIZE IN CHIONE SPP. MIOCENE TO RECENT

Outline shape is a key morphological character for most bivalves and has been applied with success in studies of evolutionary change. Recent examination of shell outlines in Neogene bivalves (Bunce et al., 2008) has suggested that the conclusion of morphologic stasis in the chionines as proposed by Stanley and Yang (1987) was potentially an oversimplification of the shape trends seen in this clade. Whereas these prior studies have excluded either the Miocene (Stanley and Yang, 1987) or the Recent (Bunce et al., 2008) and examined multiple lineages, this study represents ongoing research that examines shape evolution by focusing on the shape variability in a single genus, Chione, from both spatial and temporal perspectives beginning with the clade’s first-known fossil occurrence (Miocene Chipola Formation) to the Recent which encompasses the species: Chione chipolana, C. erosa, and C. elevata. For analysis, automatically digitized shell outlines were extracted from photographs using the computer program ImageJ to obtain digitized XY coordinates. The digitized outlines were then statistically analyzed by elliptical Fourier techniques described in Claude (2008) using the statistical package R. Prior to analysis, shape similarity for left and right valves was confirmed for Recent specimens using both PCA of the Fourier harmonics and caliper measurements of height and length ensuring that both left and right valves could be used by mirroring the left valve. Based on the current results (n = 475) there are a few observations that belie the hypothesis of stasis. First, Miocene C. chipolana (n = 60) display the least amount of shape variability in PCA ordination. Second, valve eccentricity is lowest in the Miocene, peaks in the Pliocene, shows a sharp decrease from Pliocene to Pleistocene, and slightly increases from the Pleistocene to the Recent. In addition, changes in size show a sharp increase from the Miocene to the Pliocene followed by an overall decline in size into the Recent, a trend previously reported by Roopnarine (1996). These trends will be further evaluated using the techniques outlined by Hunt (2006) to determine whether they represent stasis, random walks, or gradual change.