THREE-DIMENSIONAL MORPHOMETRICS OF AN OLIGOCENE ECHINOID: AN ADVANCED METHOD FOR ANALYZING ONTOGENETIC MECHANISMS IN EVOLUTION

The advent of new technology allowing for three-dimensional digital figures to be taken via laser scanning provides great opportunity for progress to be made in the study of echinoid morphology. Echinoid tests are relatively simple in shape and allow for highly-detailed scans to be taken and studied. From these scans, homologous landmarks can be chosen with precision and compared across samples to explore several variables. Such variables may include: ontogenetic change, species discrimination, phenotypic plasticity, and evolutionary change. This study examines morphologic variation of Rhynocholampas gouldii in the southeastern United States with samples taken from three locations in Florida. This species has a test shape that shows geographic variation associated with ecological adaptation to local substrate conditions (e.g., sediment size). For this quantitative study, each test was scanned using a Nextengine 3D Laser Scanner, which produced a digital copy of the sample. From this, three-dimensional coordinates of a series of homologous landmarks that fully describe the organism were analyzed using landmark analysis software packages. Our analysis indicates that R. gouldii shows significant variation in shape change among different geographic subpopulations. Furthermore, this shape change can be detected at different ontogenetic stages (test sizes) and related to specific adaptations in the paleoenvironmental substrate. Specifically, there is a trend toward higher test shapes with muddier substrates. We illustrate this using 3-D morphometric techniques to identify shape changes that would not otherwise be amenable to study.