MORPHOLOGICAL TRENDS IN AN OLIGOCENE CASSIDULOID ECHINOID FROM FLORIDA: TESTING SHAPE RELATIONSHIPS WITH LITHOFACIES AND PALEOGEOGRAPHY

Echinoid tests have many specialized shapes for adaptation to various marine environmental conditions. Such test shape variations are present in higher taxonomic groups of the echinoids such as the clypeasteroids, spatangoids, holectypoids, and cassiduloids. Our study examines one common species of cassiduloid in the Suwannee Limestone (Oligocene) of Florida, Rhyncholampas gouldii, that exhibits modest morphological variability through its spatial range in the state.

We chose this species because it is present throughout most of the areal distribution of the Suwannee Limestone from the northern areas such as near Live Oak, FL to near the unit's southern extent of exposure south of Lakeland, FL. The Suwannee Ls ranges from mudstone to grainstone facies that frequently contain finely comminuted bioclasts. We selected three geographically separated localities to test for shape differences between populations of R. gouldii. The sample localities include quarries near Live Oak in Suwannee County (a northern site), the Lansing Quarry in Hernando County (west-central FL), and the former Terramar Quarry south of Lakeland in Polk County (our southernmost site).

Biometric data were gathered from 194 specimens (n=72, Live Oak; n=49, Lansing; n=73, Terramar) for the following morphological traits: test length, width, height, peristome position, periproct length and width, and periproct position. Univariate and multivariate statistical tests were run on the biometrics, including regression, anova, and cluster analyses. The populations from each locality are different from one another at a statistically significant level. A shape trend exists, with the echinoids proportionally taller in the south where the carbonate facies are finer grained (muddier) and the echinoids in the northern locality are proportionally shorter (less “peaked”) in the coarser-grained facies of that site. A possible explanation is the echinoids with higher tests (i.e., a steeper or more conical shape) may have an adaptive advantage in avoiding sediment accumulation over their petaloid ambulacra, thereby maintaining functional capabilities for their tube feet. Our ongoing analysis of shape patterns in the cassiduloids of Florida may further clarify the functional morphology advantage of test shape in this and other species of echinoids.