CONVERGENCE IN DISCOIDAL OUTLINE OF SAND DOLLARS FROM GEOGRAPHICALLY ISOLATED REGIONS: A GEOMETRIC MORPHOMETRIC ASSESSMENT OF THE GENERA DENDRASTER AND ARACHNOIDES

Clypeasteroids, or sand dollars, have developed a unique discoidal/circular morphology along the perimeter curvature or ambitus. The distinctive round and flattened morphology appears adaptive to their shallow water, burrowing, life habitat. Currently, there is little understanding how on this morphology is obtained within some clypeasteroids. This analysis applies geometric morphometric analysis to fossil clypeasteroid Dendraster ashleyi from the western United States coastline compares with Arachnoides placenta from Taiwan, and examines the following: 1) The application of geometric morphometric methods to fossil clypeasteroid data using the genus Dendraster; 2) Quantifying morphological, ontogenetic, and developmental variation between the clypeasteroid genera Dendraster and Arachnoides; 3) Quantifying how circular or discoidal morphology is developed in the examined genera; 4) Quantifying lateral and posterior morphological variation within and between the examined genera; and 5) Assessing how regional endemicity due to geographic isolation may effect morphological and ontogenetic variation between and within the assessed Clypeasteroids.

This study begins to quantify how this distinctive discoidal morphology developed evolutionarily in clypeasteroids, specifically Dendraster and Arachnoides, and how morphological variation during developmental ontogeny compares between clypeasteroids. Results demonstrate strongest ontogenetic trends occur in the aboral/oral profiles for the landmark and semilandmark analyses of ambitus morphology and curvature shape change. Other morphologic traits including the petaloid structures, as well as the lateral and posterior profile morphologies do not readily demonstrate apparent ontogenetic trends. Interspecific variation or potentially morphological variation due to environmental controls appears to play a larger role in observed morphological variation for these traits. This analysis also demonstrates the utility of geometric morphometric analysis for clypeasteroid genera. This comparative analysis builds a foundation for broader analysis on the development of circular/discoidal morphologies in clypeasteroids.