PHYLOGENY AND SYSTEMATICS OF THE BIVALVE GENUS CALLISTA

Despite the diversity and abundance of venerid bivalves in modern marine environments, their phylogenetic and evolutionary history has received little attention until recently. This study focuses on the systematics and phylogeny of the subgenera classified within the genus Callista (Poli 1791; subfamily Pitarinae). A number of fossil and modern taxa have been classified historically within this genus, including Callista s.s., Costacallista, Macrocallista, Microcallista, Chionella, and Megapitaria; however, past authors have noted that these subgenera may represent polyphyletic form taxa. The goal of this study is to determine whether morphologic data differentiate these subgenera using both parsimony and morphometric analyses. The ingroup for the parsimony analysis included three to five species from each of the Callista subgenera. We established polarity using an outgroup of seven genera within the Pitarinae (including Pitar and Saxidomus) and within other venerid subfamilies (including Mercenaria, Tapes, and Chione). Data were collected from the literature and museum specimens, which were sampled from a variety of geographic areas and stratigraphic intervals and compared to type specimens whenever possible. Over ninety shell characters (with 2-5 character states each) were coded based on hinge dentition, ornament, adductor muscle scars, and pallial line geometry, for both fossil and modern species. Parsimony was used to select among trees and both strict and majority-rule consensus trees were constructed. The resulting phylogeny was used to reassess subgeneric classification within Callista. Outline methods were also used to quantify morphological differences between species classified within Callista s.s. and Macrocallista. The results of this study are preliminary and will form the foundation of an extensive project investigating pitarine systematics and phylogeny.