THE ROLE OF HOLORTHURIANS IN QUANTIFYING ECHINODERM MORPHOLOGY AND INTERPRETING BODY PLAN EVOLUTION

Large-scale evolutionary patterns are most commonly explored by studying phylogenetic relationships as well as biodiversity. Both of these methods focus on patterns of speciation, which is an important, but limited view of evolutionary change. To gain a broader view of macroevolutionary patterns and the origination of novel body plans, we can examined trends in disparity. Disparity is largely understudied (especially at higher taxonomic-levels), because of the level of detail required to accurately quantify morphology. Echinoderms are an ideal phylum to study body plan evolution, given their ecological, taxonomic and morphological diversity particularly in the Paleozoic when echinoderms had a class-level diversity five times that of today.

Quantifying echinoderm body plans is a difficult task given the large array of morphological features within the phylum. Holothurians (Sea Cucumbers) are both important and particularly difficult to include given their unique features. Despite their long evolutionary history their fossil record is limited owing to the reduction of the skeleton to miniscule spicules, which disarticulate rapidly after death. However, rare body fossils of holothuriods are known, including the exquisitely pyritized Paleocucumaria, from the Devonian of Germany. Using Paleocucumaria as a guide we incorporated holothurian characters into a previously constructed character suite used to quantify morphology in Early Paleozoic Echinoderms. In a preliminary analysis, five fossil holothurians were added to a previous compilation of 111 Paleozoic echinoderms and a new morphospace was produced. Unsurprisingly, holothurians occupy a position closest to echinoids and ophiocistioids based on their overall body construction as well as characters such as armored tube feet. In addition, some ophiocistoids have reduced and perforated ossicles as can be seen in holothuroids. Within the holothuroids, stratigraphically younger taxa are closer to other classes of echinoderms, which could indicate convergent evolution in holothuroids. Alternatively, this could represent a taphonomic artifact in which better-preserved individuals share more characters and, therefore, plot closer to echinoids and ophiocistoids.