MORPHOLOGIC PATTERNS AND EVOLUTION OF THE NEOGENE SCALLOP, CHESAPECTEN, ALONG THE MIDDLE ATLANTIC COASTAL PLAIN, USA

The genus Chesapecten containing seven species is a morphologically complex and distinct group of Pectinidae restricted to the Miocene and Pliocene along the Middle Atlantic Coastal Plain (MAPC), USA. The distinguishing morphological characteristics of Chesapecten include biconvexity of valves with greater convexity of the left valve relative to the right valve, strong ribbing and prosopone on the exterior of individual valves, equal rows of plicae and lirae on the exterior of individual valves, and reduced cardinal crura. Successional species of Chesapecten are separated by gaps in sediment deposition in the Cenozoic fossil record. Therefore, these species serve as valuable guides to discern the stratigraphy, environmental conditions, and geological history of the Miocene and Pliocene along the MAPC. Moreover, we can use shells of Chesapecten to study patterns of morphological change to assess variability within species, among species, and between transitional species. Is the morphological variability among species of Chesapecten the same (i.e. variability within species) or distinct (i.e., variability among populations)? Here we assess the morphological variation among C. middlesexensis, C. jeffersonius, and C. madisonius using 51 characters to test whether the highly variable pairs of transitional forms (i.e., C. middlesexensis-C. jeffersonius and C. jeffersonius-C. madisonius) occupy different areas in morphospace.