A MORPHOMETRIC ASSESSMENT OF SHAPE CHANGE IN THE MIOCENE BIVALVE ANADARA DARIENSIS ACROSS PALEOENVIORNMENTAL AND GEOGRAPHIC GRADIENTS

The Gatun Formation of central Panama is an important baseline for studies assessing how the closure of the Central American Seaway impacted the marine biota of that region. Ongoing research is addressing how communities of marine organisms within this formation change through space and time. This study addresses biotic change through space and time using the alternative method of looking at patterns in the morphology of a single species found widely throughout the Gatun formation in addition to other Caribbean and eastern Pacific localities. The extinct bivalve Anadara dariensis was widespread throughout the Caribbean and eastern Pacific during the middle-late Miocene. Visual inspection suggests that specimens from different geographic regions and different time periods display minor but discernable variation in shape. We use a range of methods (outline analysis, linear measurements, and landmarks analysis) to analyze shape changes among over 200 specimens of Anadara dariensis from a range of Caribbean and eastern Pacific fossil localities, including those of the Gatun Formation. We focus on landmark analysis, for which landmarks can be readily identified from the dentition, umbo, and muscle scars. The position and shape of muscle scars may capture shape variation relevant to burrowing depth and sediment type. The majority of the variation in shape among Anadara dariensis specimens appears to be related to the enlargement of the posterior muscle scar. Some localities are characterized by larger muscle scars while others are characterized by smaller muscle scars, with a general trend of muscle scar size decreasing through time. The differences in shape inferred in this analysis may be an ecophenotypic or population level response to changing environmental conditions (e.g. water depth and sediment type) or changes through time (the Gatun accumulated over an interval of 3 m.y.) among these localities.