SYMMETRY PATTERNS IN SELECTED FOSSIL AND RECENT MELLITID ECHINOIDS: HOW MUCH SHAPE VARIATION REALLY EXISTS?

One of the unifying characteristics in echinoderms is the pentameral symmetry present in these organisms. Furthermore, in some groups of mobile echinoderms, bilateral symmetry is superimposed on the pentameral symmetry pattern. Irregular echinoids such as the clypeasteroids exhibit this bilateral symmetry pattern and normally it is easy to distinguish. However, the bilateral pattern may not be as perfect as expected when “paired” skeletal structures are examined closely. In 1962, Van Valen defined three styles of variation from true bilateral symmetry (in paired structures) to include directional asymmetry, antisymmetry, and fluctuating asymmetry. In our study, we examined paired structures in fossil and recent mellitid echinoids to explore the style(s) of symmetry present in the selected species.

Biometric data were collected for up to 64 morphological traits on three species of mellitid echinoids from Florida. We studied two fossil taxa from the Tamiami Formation (Pliocene), Mellita aclinensis and Encope tamiamiensis, and one modern species, Mellita tenuis, collected from a living population in the Gulf of Mexico. The number of individuals measured varied by species, but ranged from 68 to 120 for the selected traits. The trait of primary focus is the petaloid ambulacrum length for the left-right pairs (i.e., ambulacrum IV and II, and ambulacrum pair V and I of the Lovén system). To test for statistically different ambulacrum length values in left-right pairs, a T-test was used to judge symmetry or asymmetry in petaloid ambulacra (IV-II, and V-I). If asymmetry was present, an interpretation of directional asymmetry, antisymmetry, or fluctuating asymmetry was completed.

Our results showed each species exhibited directional asymmetry to the left for petal length as well as petal width. These results corroborate results from a previous study completed for selected modern clypeasteroids by Lawrence et al. (1998). Although our study only has one species in common with theirs (Mellita tenuis), we see the pattern of directed asymmetry in petaloid ambulacral length for sand dollars was true in the past environments just as it is in modern environments. We are expanding this analysis to add more morphological features on more species to try understand the cause of the subtle asymmetry in species of irregular echinoids.