Meeting Abstract
Although there are about 250 living species of sand dollars (Clypeasteroida: Echinoidea), there are also more than 700 fossil forms. The richness of this record makes possible detailed exploration of evolutionary change. The sand dollar family Mellitidae, common on beaches today throughout the subtropical and tropical Americas, is also well-represented in Pliocene fossil deposits along the southeastern seaboard of the U.S. The family is characterized by holes that develop in the body skeleton (test). These holes, or lunules, pass completely through the test from top to bottom, and arose as adaptations to hydrodynamic forces in wave-swept environments. All mellitids have a lunule in each of the five rays (ambulacra), plus a single lunule in the posterior inter-ray (interambulacrum). In living members of the genus Mellita, the anteriormost ambulacral lunule is absent. Because mapping the occurrence of novel features in fossils can contribute to knowledge of speciation, we set out to: i) determine distribution and diversity of fossil Mellita; ii) determine when and where the anterior lunule was lost; iii) correlate systematics and evolutionary biology of Mellita with past environmental conditions to better understand how biodiversity varies with changes in factors contributing to modern biodiversity. With morphometrics, we aimed to determine if two Pliocene Mellita, M. aclinensis and M. caroliniana comprise a single species, and explore the possibility that a thick, round-lunuled, undescribed form constituted a new species. Morphology, phylogenetics, biogeography and stratigraphy of all mellitid genera (Mellita, Leodia, Encope), were examined in a synthesis of the evolution of the family.
