EXPLORING MORPHOLOGIC TRENDS IN EARLY PALEOZOIC ECHINODERMS

Morphological disparity has been proposed to have peaked in the Cambrian with a large degree of experimentation with novel body plans. Following this apex, disparity contracted, which likely was a result of extinction along with genetic and ecological canalization. Testing this pattern has been difficult in that quantifying morphology becomes more challenging at higher taxonomic levels. Echinoderms are an ideal phylum to test this pattern with a zenith in class-level diversity in the Ordovician that is five times higher than today.

To examine morphological disparity in echinoderms during the Early Paleozoic a new discrete morphological character set was constructed as part of the Assembling the Echinoderm Tree of Life Project. This character set contains 289 characters describing morphology from across different body regions within echinoderms. A preliminary study of 38 genera from 15 classes was coded from museum and literature sources and examined using Principal Coordinate Analysis.

The resulting morphospace consisted of three major regions: pelmatozoans, homalozoans, and edrioasteroids along with several taxa occupying medial positions (e.g. Helicocystoidea, Cyclocystoidea and some Eocrinoidea). Both the homalozoan and edrioasteroid zones were almost fully occupied by the Cambrian and stayed consistent in the Ordovician. The pelmatozoan (stalked echinoderms) region substantially expanded during the Ordovician from the small area occupied in the Cambrian by eocrinoids. Further sampling is needed to quantitatively evaluate the degree in which disparity has changed during this interval and whether the high levels of disparity achieved in the Ordovician were maintained throughout the Paleozoic.