Respiratory Structure Morphology, Group Origins, and Phylogeny of Eublastoidea (Echinodermata)

Evolutionary relationships of Paleozoic echinoderms have fostered significant debate over the past century. Many early echinoderms have complexly plated bodies with a variety of morphologies, very unlike modern echinoderms (e.g., sea urchins, sea stars). A major clade, Blastozoa, has been subdivided based on the occurrence of specific respiratory structures but these groups have yet to be fully assessed in a quantitative framework. Phylogenetic inference provides a quantitative means to assess trait evolution, respiratory structure modification, or clade origination. Herein, we assess respiratory morphology, evolution, and group origination of Eublastoidea.The respiratory structures (hydrospires) of eublastoids have been used to separate major subgroups within Eublastoidea but have only been examined externally. Previously only assessed by 2D serial sections of specimens, my research provides 3D detailed anatomical models of these internal structures. Rendering in 3D allows for detailed morphological analysis and functional morphology simulations. Our findings suggest separation by the external expression of hydrospires results in a misleading understanding of evolutionary history.These insights into eublastoid respiratory structures shed light on an ongoing debate regarding the origins of blastoids. Origins of blastoids are unclear and a series of ancestors has been proposed. A single species of blastoid Macurdablastus uniplicatus, was recorded from the Late Ordovician with the next undisputed species in the middle Silurian. We reassessed anatomy and evolutionary relationships through detailed morphological examination, synchrotron imaging, and phylogenetic analyses. Results from our subsequent phylogenetic analyses suggest Macurdablastus is not a true blastoid (eublastoid) but is include in the broadly defined Blastoidea that includes coronoids, eublastoids, Lysocystites.The revival of the term Eublastoidea to include species with recumbent ambulacra and hydrospires provided the basis to explore the evolution of this long-lived clade. Following and expanding upon a proposed homology scheme for echinoderms, I produced a comprehensive character matrix for the external and internal morphology of eublastoids. The phylogeny was used to reassess eublastoid classification and as a framework to address the validity of group separation via the external expression of hydrospires. This work provides the first complete assessment of echinoderm respiratory structures and detailed reassessement of eublastoid morphology.