QUANTIFYING ECHINODERM MORPHOLOGY

Quantifying morphology often illuminates evolutionary trends not seen in studies of phylogeny or biodiversity. This is particularly true during the Early Paleozoic with widespread experimentation in new body plans and features. These vastly different body forms also make quantifying morphology especially difficult at higher taxonomic levels. Using a new character suite constructed as part of the Assembling the Echinoderm Tree of Life Project we attempted to quantify morphology within Early Paleozoic Echinoderms This suite included 289 discrete morphological characters encompassing the entire organisms including the oral surface, theca, feeding appendages, respiratory structures, and pelma. We included 38 genera from 15 classes in the study using literature sources and examination of museum specimens and explored the morphologic patterns using Principal Coordinate Analysis (PCO).

Preliminary results reveal three major groups within morphospace: stalked echinoderms (pelmatozoans), homalozoans, and edrioasteroids. Multiple genera show intermediate forms between these groups such as the Cyclocystoidea, Helicocystoidea, and some Eocrinoidea (e.g. Sineocrinus and Gogia). Several classes overlap significantly in morphospace suggesting multiple examples of convergent evolution. Rhombiferians and edrioasteroids both show a large degree of morphologic diversity with instances of younger clades deviating away from the centroid of their class into the morphologic space of another. This pattern is shown by Caryocrinites encroaching on crinoids and Streptaster approaching the morphology of a Helicoplacoidea. Overall, this approach yields meaningful results describing the large array of morphologies present within echinoderms. These trends will be further explored by including more taxa in the study and expanding the character suite to included other major clades of echinoderms, specifically the asterozoans and echinoids.