MORPHOLOGICAL ANALYSIS OF ECHINOLAMPAS APPENDICULATA, RHYNCHOLAMPAS CAROLINENSIS AND EURHODIA RUGOSA OF THE CASTLE HAYNE (EOCENE) FORMATION

This study investigated the morphological relationships of Echinolampas appendiculata, Rhyncholampas carolinensis and Eurhodia rugosa from the Castle Hayne (Eocene), NC locality. Specimens were analyzed quantitatively using a truss consisting of 18 distinct measures. Principle component analysis (PCA) of these data indicated that the three groups are morphologically distinct. The analysis also showed that the groups shared parallel growth trajectories, but that initial shapes were different. This implies that heterochrony, changes in the timing of development, may have occurred amongst these groups in a way similar to what has been described in other fossil echinoid species (McKinney 1984 & 1986). The major traits in which the groups varied were periproct height and width, peristome length, and the length of the 2nd and 3rd ambulacrum. Additionally, the data indicated Echinolampas appendiculata exhibited the most variability for the majority of the traits analyzed among the three groups. Living members of the three genera live at fairly distinct water depths, and substrates and feed on particles of somewhat different sizes, suggesting a partitioning of the environment into sub-environments by these groups. In light of this and phylogenetic data, we speculate that Rhyncholampas carolinensis and Eurhodia rugosa initially diverged from two closely related stocks whose similarity in morphology and niche selection drove increased competition. This competition caused them to speciate to occupy narrower niches, so as to reduce competition, driving the morphological changes discussed above. Meanwhile, because of its phylogenetic distance E. appendiculata did not diverge, and remained a generalist, thus reducing competition between itself and the other two cassiduloids.