ORAL REGION HOMOLOGIES IN EARLY PALEOZOIC PELMATOZOANS

Pelmatozoans include crinoids and various blastozoans, all of which possess a pelma or stem. The monophyly of this grouping has previously been questioned, but a new analysis of oral characters shared among these taxa suggests they form a monophyletic group, regardless of the origin(s) of the pelma. Based on comparative anatomy of the axial skeleton, the following oral region characters are judged to be homologous among early Paleozoic crinoids, coronoid blastoids, glyptocystitid rhombiferans, eumorphocystid diploporans, and eocrinoids: a mouth frame with an identical (or nearly) arrangement of oral plates, moveable peristomial (mouth) and ambulacral cover plates, 2-1-2 ambulacral symmetry, hydropore position, and rigid attachment of the oral surface to the underlying calyx. Collectively, these functionally-integrated characters appear to have evolved slowly because of the relative complexity of food processing, thus preserving symplesiomorphic homologies across clades. Derived characters separating these clades include recumbent vs. elevated ambulacra, presence-absence of ambulacral floor plates, brachioles vs. arms, location of the periproct, etc. Phylogenetic hypotheses based on oral region characters suggest crinoids are nested in a larger clade including coronoids and glyptocystitids, with eumorphocystids and eocrinoids as less inclusive sister clades. A previous hypothesis of a more direct relationship between crinoids and edrioblastid edrioasteroids is not supported based on analysis of oral region characters.