The Homology and Phylogeny of the Diploporita (Blastozoa: Echinodermata)

Evolutionary relationships of extinct echinoderms are poorly understood, especially within stem-bearing blastozoans, a large group of echinoderms with unique respiratory structures and feeding brachioles. They were highly experimental in their body plans and very unlike echinoderms today (e.g., sea urchins). Many of the blastozoan subgroups recognized in recent classifications do not represent clades (natural associations of organisms derived from a single ancestor); they are either grades of organization or groups united by superficially similar features. Consequently, these ‘traditional’ groupings cannot be used to analyze evolutionary questions, such as biogeography or rates of evolution. This problem is highlighted within the diploporitan echinoderms, a blastozoan group united by superficially similar double pore (diplopore) respiratory structures, which appear to encompass multiple independently evolved lineages.

Major diploporitan groups show wide variation in body wall morphology, feeding apparatus, and attachment structures (i.e., stems and holdfasts). Although the diploporitans have been defined by the diplopore structures, recent evidence indicates that they may have evolved in multiple blastozoan groups. Furthermore, other features of the body of diploporitans (e.g., size and shape of the attachment structure) are likely dictated by environmental factors. To date, diploporitans have not been analyzed in a rigorous phylogenetic context and their relationships are uncertain.

To test diploporitan monophyly, taxa were analyzed to identify homologous elements across diploporitans and other closely related blastozoans using the Universal Elemental Homology scheme that has been utilized across multiple early echinoderm clades. This included identifying homologous elements between certain diploporitan blastozoans and early crinoids, echinoderms thought to be rooted within blastozoans.

Morphological data were coded to create a character taxon matrix. Phylogenetic relationships were assessed utilizing maximum parsimony and maximum likelihood; support for the resulting relationships was assessed using bootstrap and Bremer support. Results of the phylogenetic analysis indicate that the diploporitans include at least three distinct lineages rooted within Blastozoa and that crinoids are also rooted within blastozoans. A posteriori testing of elements within the phylogenetic framework supported homology and not homoplasy, which contradicts previous arguments. The reconstructed evolutionary relationships of the diploporitans will provide a framework to explore biogeographic patterns and morphological trends in the future.