THE EVOLUTIONARY RELATIONSHIPS OF EARLY ECHINODERMS: REINTERPRETATIONS BASED ON THE AMBULACRAL SYSTEMS OF EUMORPHOCYSTIS

Evolutionary relationships of early echinoderms are poorly understood, largely because of a misunderstanding of homologous skeletal elements. New data from undescribed specimens of Eumorphocystis multiporata, a Middle-Late Ordovician diploporitan from the Bromide Formation of Oklahoma, provides insight into the phylogenetic relationships of blastozoans and crinozoans. Eumorphocystis bears composite arms of axial and extraxial components encasing a coelomic extension, which has previously been ascribed as a unique feature of crinoids. Eumorphocystis and early crinoid arms both mount to radial plates and are composed of three layers: uniserial brachial plates on the outer portion of the arm derived from the thecal wall (extraxial), biserial floor plate forming the food groove (axial), and ambulacral cover plates protecting the food groove (axial). The brachial and floor plates encapsulate a coelomic cavity that cores the arm and presumably the radial vessel. Neither the triplate arrangement of early crinoid arms nor spaces for coelomic cavities has been previously observed in blastozoan echinoderms, although there is some similarity with the middle Cambrian Dibrachicystis in which a coelomic canal has been reported. There are some major constructional differences between Eumorphocystis and protocrinoid arms including: the proximal food grooves of Eumorphocystis are developed on alternating biserial floor plates, whereas in protocrinoids, the proximal food grooves are subtegmal and cannot be observed. The coelomic foramen is also positioned differently between the two groups: the coelomic foramen of Eumorphocystis directly perforates the thecal wall and the coelomic canal of protocrinoids is positioned at the edge of the tegmen where the arms insert.  Phylogenetic analyses of early echinoderms, including data from Eumorphocystis, indicate that Eumorphocystis shares a sister taxon relationship with early crinoids. This study demonstrates that it is of upmost importance to understand the homologous skeletal elements of early echinoderms if evolutionary relationships are to be uncovered. These new data from Eumorphocystis, and subsequent phylogenetic analyses, strongly indicate that crinoids are nested within blastozoans.