EXPLORING EUBLASTOID MORPHOSPACE OCCUPATION BY INVESTIGATING UNDERLYING SKELETAL ELEMENTS

Morphology is a primary source of data used to examine changes in taxa throughout time and space. 3D geometric morphometrics provides a mechanism for the quantification of variation in shape and proportion through the comparison of homologous features. Eublastoidea, a Paleozoic echinoderm clade with a conservative body plan, is an ideal group for morphometric analysis because their plate junctions are homologous and easily identifiable on all species.

Historically, Eublastoidea has been grouped by generalized shape types, which have been used as characters to estimate the evolutionary history of the group. Herein, we investigate whether the varying proportions of skeletal elements that produce the gross thecal morphology are distinguishable in morphospace. In addition to analyzing individual plate circlets and their ratios to one another, we also investigated a previously studied vault:pelvis ratio and whether it shows clear patterns in morphospace occupation. We used 58 specimens (46 species) from the University of Michigan Museum of Paleontology in this study.

Our results suggest that none of the described generalized shape types are isolated in morphospace. Additionally, the plate circlet ratios, specifically the deltoid to total height and deltoid:radial height groupings, show patterns and trends in morphospace. Distinction, or isolation, in morphospace is a strong indicator of taxonomic separation since morphology is a by-product of phylogenetic relationships. Our results demonstrate that the examination of individual skeletal elements and their proportions to one another are more informative than taxonomically general shape descriptions. Eublastoidea taxonomy is based on morphology, and as such, it is important for us to better understand how the underlying skeletal elements may play a role in morphospace separation.