THE PHYLOGENY OF THE DIPLOPORITA

The phylogenetic relationships of Paleozoic blastozoan echinoderms are poorly understood and many of the traditionally ascribed groups are likely polyphyletic. Diploporitans, diagnosed as those blastozoans with double pore (diplopore) respiratory structures, have never been placed within a rigorous phylogenetic framework and their highly variable morphologies of the feeding ambulacral system, attachment structures, and even the group-defining diplopores suggest that they do not represent a natural clade.

Phylogenetic analysis utilizing maximum parsimony presented here spanned a wide range of morphologies present in diplopore-bearing blastozoans. Non-diplopore bearing echinoderm taxa were also included in the analysis to test diploporitan monophyly. The resulting analysis infers a polyphyletic grouping for diplopore-bearing blastozoans, which suggests that diplopore respiratory structures have evolved more than once within the echinoderm tree of life. The analysis also indicated that some diplopore-bearing echinoderms share a sister relationship with crinoids (e.g., Eumorphocystis) and that crinoids are nested within Blastozoa. To test the original hypothesis that Diploporita is monophyletic, we performed a constraint analysis to compare the original groupings of Diploporita with results presented in this study. The results of the constraint analysis indicate that a single diplopore-bearing clade is significantly less parsimonious than multiple diplopore-bearing clades and that a systematic revision of diplopore-bearing echinoderms is necessary to correct the polyphyly of Diploporita. This study further refines the relationships of Paleozoic blastozoan echinoderms and provides a framework with which to begin exploring how global long term trends might have affected evolutionary trends of these organisms. Further work to constrain how many times diplopore-type respiratory structures evolved throughout the echinoderm tree of life requires denser sampling, particularly of poorly understood taxa.