UNDERSTANDING SHAPE VARIATION IN THE ORDOVICIAN MITRATE ECHINODERM ENOPLOURA USING OUTLINE MORPHOMETRICS

Enoploura is an extinct genus of mitrate stylophoran echinoderm found predominantly in the Ordovician of Ohio, Kentucky and Tennessee. These organisms are controversial because much of their anatomy has not yet been adequately documented. Articulated specimens of mitrates are rare, so variation of the skeletal plates is poorly understood. However, this group has highly conservative plating such that each animal has exactly the same skeletal plates in the same positions in the theca - a condition that is rare among echinoderms. Consequently, disarticulated thecal plates can be used to investigate patterns of shape variance across the theca using outline morphometrics. A large collection of disarticulated plates recovered from locality of the middle Upper Ordovician (Maysvillian) Bull Fork Formation near Cincinnati, OH provides a unique opportunity to collect novel data concerning Enoploura plate shape. From washed residues, plates were picked by hand, sorted into like morphological groups and identified by morphological comparison to complete material. From these collections, specimens were selected for analysis by choosing only well-preserved and unbroken plates. No effort was made to discriminate plates by size thereby retaining some ontogenetic information. Photographs were made of as few as 6 specimens when limited by available material, and as many as 115 with abundant material, with an average of 42 among all 15 plates. Certain plates along the thecal margin were also excluded from the analysis because their strongly 3-D shape was inappropriate for this study given the 2-D nature of the analysis. Images were used to collect points from around the boundary. The first eight elliptical Fourier descriptors of the plate outlines were calculated and then used as a measure of shape. Finite mixture models were fit and then compared using Akaike’s Information Criterion, corrected for sample size (AICc). Results suggest unimodal morphology for the species. Bootstrap methods indicate that the variance in shape is greatest in plates near the mouth. These differences in variance are discussed in relation to developmental plasticity, with plates near the mouth “absorbing” variance of other body plates as they surround a strictly developmentally controlled mouth.