A Morphometric Analysis of the Highly Variable Clypeasteroid, Periarchus Lyelli

The Late Eocene echinoid, Periarchus lyelli (Conrad, 1834), known for its wide geographic range, high abundance, and specific stratigraphic range, is an ideal example of an index fossil. However, due to its highly variable test morphology, many have questioned if this classification is erroneous, and if, in fact, this sand dollar is actually two or three distinct species that have been misclassified as a single species. P. lyelli, as a species, is typically described as subcircular (or slightly domed) in cross-sectional outline with a nearly flat oral surface (Kier, 1980). However, numerous examples can be found where the cross-sectional outline is highly domed or bell shaped, sharply pointed, or in some specimens flattened with little vertical relief.

A preliminary study on this subject has been performed on specimens from Mississippi, North Carolina and South Carolina (Williamson, 2006), showing significant separation in test shapes from these three states. Continuing the initial research, this study will analyze the test shapes of P. lyelli over its entire North American geographic distribution, along the gulf coast and southern Atlantic coast of the United States. Echinoid test analysis will incorporate multivariate statistical analysis techniques such as Elliptical Fourier Analysis, Procrustes Analysis, and Thin Plate Spline Analysis. In addition to physical examination, substrate analysis will be performed to determine if differences in test shape correlate to differences in lithofacies and paleoenvironmental differences.

The correct classification of index fossils is extremely important for accurate stratigraphic and geologic identification. An examination of the highly variable clypeasteroid, P. lyelli will not only provide insight to the precise classification of the sand dollar, but also the evolution of new architectures and the difference between interspecies variation and intraspecies variation