CHARACTER CHOICE AND MORPHOLOGIC EVOLUTION OF EARLY PALEOZOIC INADUATE CRINOIDS

The mathematical description of morphology is highly influenced by the methodology used, particularly the choice of discrete characters. We examined the effects of character choice on the evolutionary patterns of Middle Ordovician through the Early Silurian crinoids, focusing on inaduates (disparids and cladids). To do this, we compare a previously examined dataset with a new data set compiled as part of the Assembling the Echinoderm Tree of Life Project (AEToL) of approximately 200 Early Paleozoic crinoids. The AEToL dataset is seventy percent larger and has additional characters describing the stem, cup, arms, and particularly the oral surface. Both datasets were analyzed using principal coordinate analysis and disparity through time was calculated. There is a clear difference in the morphospace produced by the two datasets in terms of the features that define the primary axes. The previous study focuses on the infrabasal circlet while the AEToL dataset emphasizes the differences between inaduate and camerate crinoids. Although the resulting morphospaces differ, the underlying structure of the analysis is still maintained in regard to the distances between individual crinoids. Both data sets show similar patterns in disparity through time during the Ordovician. In the Silurian, the two data sets differ, with the previous study showing an increase in disparity while the AEToL character set shows a continuous decrease throughout the early Silurian. This difference in the Early Silurian is largely caused by a change in morphologic position of the inaduate crinoids, especially the position of the mylodactalids. The smaller role of the mylodactyids is caused by the addition of new characters in the AEToL dataset, which has a lower percentage of the stem characters that made them unusual in the previous study. Analysis of exclusively inaduates using the AEToL character set produces a morphospace showing a clear separation between disparids, cladids, and flexibles. The disparids occupy a larger area of the morphospace than cladids, which is expected given the unique morphologies within the Calceocrinidae, Anomalocrinidae, and Myelodactyalidae. The AEToL morphospace also groups large-bodied cladids (e.g. Carabocrinus and Paleocrinus) near hybocrinids based on the shared morphology of their oral surfaces.