A Morphological and Phylogenetic Examination of the Miocene and Pliocene Bivalve Genus Chesapecten

The scallop genus Chesapecten, characterized by its strong ribbing and large size, originated in the early Miocene and was extinct by the late Pliocene. It was highly abundant in the mid Atlantic region of the U.S. and an important component of marine ecosystems. The genus is comprised of nine species and is widely thought to represent an example of gradual evolution. The limited geographic and stratigraphic ranges of each species make them important indicators for the Coastal Plain deposits in which they are found. However, because of high variability within species and hybridization between species, they can be difficult to identify based solely on morphology. This study quantitatively defined the species of Chesapecten through: (1) a morphological assessment of late Miocene and early Pliocene species, (2) an examination of genus-wide trends in morphology through time, and (3) a phylogeny of the genus. Specimens for the morphological assessment were collected from Cobham Wharf (Surry County, Virginia) and included all three species present at that location (C. middlesexensis, C. jeffersonius, C. madisonius), while those used for the phylogeny were obtained from the collections at the Virginia Museum of Natural History (Martinsville, Virginia) and included eight defined species. A portion of the cliff at Cobham Wharf was bulk-sampled at half-meter increments, yielding seven total samples with about 12 specimens each. Approximately 96 museum specimens were selected for the analysis. Analyses involved collecting 10 morphologic landmarks defining shell shape from each field specimen and approximately 40 discrete and continuous characters describing shell shape and ornamentation from each museum specimen. Landmark data were Procrustes transformed to eliminate variation based on size, rotation, and location. From this, principal components analysis was used to produce a morphospace. Phylogenetic data were used to reconstruct a parsimony-based phylogeny and bootstrapping allowed us to assess overall robustness of tree topology. Morphological results indicate that some features display trends through time and can be used to distinguish species. Phylogenetic results produced an evolutionary sequence that differs from the stratigraphic sequence, suggesting a more complex explanation of the evolutionary history of Chesapecten than anagenesis