A PRELIMINARY MORPHOMETRIC ANANLYSIS OF THE MORPHOLOGICALLY VARIABLE CLYPEASTEROID PERIARCHUS LYELLI

The Late Eocene clypeasteroid echinoid Periarchus lyelli Conrad, 1834, is found in the Southeastern and South-Central United States, from Texas through North Carolina. Its wide geographical range, high abundance, and narrow stratigraphic zonation make this “sand dollar” an ideal index or marker fossil. Although the distribution and gross morphology of P. lyelli are well known, much controversy exists as to whether P. lyelli represents one species, consists of two sub-species, or two distinct species.

Within the Eocene Castle Hayne Limestone of North Carolina and the Santee Limestone of South Carolina, P. lyelli has a sub-circular test, slightly domed petaliferous area, and nearly flat oral surface. (Kier 1980). This is not the case in the coastal plain region of Alabama and Mississippi. Here, within the unconsolidated sands and marls of the Moodys Branch Formation, P. lyelli has a distinctive sharply conical petaliferous area, and a very thin outer margin. To describe the difference between the two morphologies Ravenel (1844) erected a separate sub-species, Periarchus lyelli pileusinensis. Sedimentary geologists familiar with the Cenozoic geology of the Gulf Coastal Plain have long used P. lyelli and P. lyelli pileusinensis as index fossils, treating each as separate species.

In order to determine if there is a valid morphological basis for taxonomically dividing P. lyelli, modern morphometric techniques were used to analyze test shape. We performed an Elliptical Fourier analysis followed by PCA on the lateral cross-sections of 72 specimens obtained from the Castle Hayne Limestone, Santee Limestone, and Moodys Branch Formation. The resulting analysis yields three clusters, with specimens from the Moodys Branch Formation forming a distinct cluster, while specimens from the Castle Hayne and Santee Limestone cluster closer together than those from the Moodys Branch Formation. This preliminary study illustrates that P. lyelli found within the unconsolidated sands and marls of the Moodys Branch Formation are morphologically distinct from specimens found within the carbonates of the Castle Hayne Limestone and Santee Limestone and confirms that P. lyelli pileusinensis warrants sub-species, or possibly species status.