A MORPHOMETRIC STUDY OF ERISOCRINUS (CRINOIDEA) USING ARCGIS

High disarticulation rates of cladid crinoids leave a rarity of complete specimens in the fossil record, resulting in a lack of large collections available for morphological studies. Cladid crinoids have among the highest disarticulation rates and are commonly preserved in shales in the upper Paleozoic, where they are subject to distortion caused by sediment compaction. Erisocrinus, a Pennsylvanian cladid crinoid, has been found to be preserved in abnormally high numbers in crinoid Lagerstätten such as the Barnsdall Formation in Oklahoma and LaSalle Limestone (Bond Formation) in Illinois.

A morphological study of Erisocrinus utilizing heads-up digitizing methods to create three-dimensional models through ArcGIS® is ongoing. In opposition to the standard method of using pre-determined landmarks found on each specimen, all of the sutures between the plates of the crown are traced from high-resolution, 2-dimensional photographs; topological restraints that prevent overlap of the digitized lines are put into effect. Perimeters and areas of each plate (represented as individual polygons) are automatically calculated through ArcGIS®. The digitized plates of each specimen can be compared with other models in a series in order to determine ontogenetic changes. These 3-dimensional models allow for a growth study that reveals phenotypic characteristics that would not appear in a traditional growth study. Using this new method to study the morphology of a cladid will add paleobiological information not easily found due to the high disarticulation rate in the subclass.

The effects that sediment compaction had on the cup of Erisocrinus, a problem highlighted in the thin-plated area at the base of the cup near the stem attachment (basal concavity), is also addressed using the ArcGIS® method. Through this method, we are attempting to reverse the effects of compaction to reconstruct the original body shape. Because basal concavity is a key character in many cladid groups, our method, if successful, could have widespread applicability.