THE MORPHOLOGY OF ERISOCRINUS TYPUS: A STUDY USING ARCGIS©

Cladid crinoids have among the highest disarticulation rates of all Paleozoic crinoids, but Erisocrinus typus has been shown to be an exception. A large number of specimens were collected from the crinoid Lagerstätte in the Barnsdall Formation (Upper Pennsylvanian) exposed near Copan, Oklahoma, including a growth series consisting of eight complete crowns ranging in size from 1.1 to 7.5 cm.

A digital growth study using these eight crowns of E. typus was performed using standard heads-up digitization methods in ArcGIS©. The sutures between all the plates of the crown were traced from high-resolution, two-dimensional photographs. Topological constraints that were put into effect prevented the digitized lines from overlapping and facilitated conversion into polygons. The perimeters, areas, and other measurements of these polygons, represented as individual plates, were automatically calculated by the software.

Features identified through traditional methods can be evaluated more accurately by using ArcGIS© such as the proportional area of the basal plates, a feature upon which different species of Erisocrinus have been established. Selected plates can be followed through the eight specimens of the series to produce graphs showing ontogenetic changes in size and shape. Plate addition can also be tracked using this method. Ongoing studies of the systematics of the genus will benefit from close analysis of the changes in the basal plates and other growth features used as defining characteristics in the numerous Erisocrinus species that have been described.

A previous study of the ontogeny of this species concluded the growth of the cup to be isometric. However, results from this study concerning the relative rates at which plates are changing size and shape show that E. typus grew anisometrically. The anisometric growth of the cup can also be traced from the upflared infrabasals present in the juvenile stage to the distinct basal concavity seen in the adult.

The methods utilizing ArcGIS© allow for a much more accurate growth study, as compared to one that can be performed via traditional methods. Because of the rarity of complete cladid specimens that can be used in growth studies, this study will have far-reaching implications in better determining the ontogeny and systematics of cladid crinoids.