A REEVALUATION OF EVOLUTIONARY STASIS BETWEEN THE BIVALVE SPECIES CHIONE EROSA AND CHIONE CANCELLATA (BIVALVIA, VENERIDAE)

Demonstrating stasis in the fossil record has proven to be problematic with respect to both data collection and analysis. A previous approac h is morphometric analysis of lineages sampled temporally and geograph ically. A hypothesis of stasis is apparently supported if morphologica l distances between descendent species and ancestral species are no gr eater than those between geographically distant samples of the descend ent species. Evidence presented in this paper conflicts with such inte rpretations for at least one bivalve lineage, Chione erosa-Chione canc ellata, of the late Neogene of tropical America, The direction and mag nitude of morphological variance were quantified between two geographi cally distant groups of C. cancellata from the Recent, and compared to Pleistocene samples of C. cancellata and Pliocene samples of C. erosa . The results indicate that, although the magnitude of intraspecific g eographic variation is as great as interspecific temporal variation, t he species are morphologically discrete groups. The direction of morph ological variance is as important as its magnitude, and interpretation s overlooking this point are at best equivocal.