COMPARISON OF SHAPE BETWEEN MODERN AND FOSSIL SPECIES OF CHIONE SPP.: STASIS OR EVOLUTION?

Outline shape is a key morphological character in the analysis of bivalves and has been applied with success in various studies devoted to documenting evolutionary change. Bivalves of the chionine lineage are well studied and present an interesting controversy. When attempting to define fossil species, the documented range of shape variability of C. elevata from modern environments mirrors that seen in fossil specimens as demonstrated by Stanley and Yang (1987). Therefore, they concluded that, in terms of its shape, Chione represents ~4 Ma of stasis ranging from the Pliocene to the Recent. Though this initial hypothesis may have been an oversimplification of observed shape trends (Bunce et al., 2008), it was later re-examined and overturned by Roopnarine (1995) who also split modern Chione spp. from the Southeastern US and Caribbean provinces into two separate species (C. elevata, and C. cancellata respectively) (Roopnarine and Vermeij, 2000). The current study represents on-going research that examines shape evolution by focusing on a single genus, Chione, from both spatial and temporal perspectives beginning with the clade’s first abundant fossil occurrence in the Miocene (Chipola Formation) to the Recent which encompasses the species: Chione chipolana, C. erosa, and C. elevata. Data consist of left valves only and were obtained from photographs using the computer program ImageJ to extract digitized shell outlines. The digitized outlines were processed using standardization and elliptical Fourier techniques described in Claude (2008) using the statistical package R. Based on the current results (n = 733), modern environments are indistinguishable when compared via PCA ordination and each displays a high degree of variability. Coupling these data with the fossil specimens shows a large amount of overlap in morphological variability within the lineage which is in agreement with prior conclusions drawn by Stanley and Yang (1987) and further suggests that stasis in shape may have lasted ~18 Ma. Future plans for research are to include specimens of C. cancellata from the Carribbean and to further evaluate observed trends using the statistical techniques outlined by Hunt (2006) to determine whether they represent stasis, random walks, or gradual change.