EVOLUTIONARY PATHWAYS OF THE CRINOID ORAL REGION

From the ancestral state of the crinoid oral region (moveable ambulacral cover plates, fixed orals around the mouth, and undifferentiated peristomial cover plates in a single layer with 2-1-2 ambulacral symmetry), the crinoid oral region evolved high morphological disparity. In general, tegmen geometries varied; plates of the oral region were maintained with the mouth on the tegmen surface or the mouth became subtegmenal; and they either maintained a 2-1-2- symmetry, had a five-fold symmetry, or plating became irregular. Evolutionary outcomes for the ambulacral cover plates were either movable or fixed, differentiated or undifferentiated, and present or absent. True orals that formed the mouth frame were either all visible, only Oral 1 was visible, or all orals were covered. The peristomial cover plates were either differentiated or undifferentiated; moveable or fused; or indistinguishable from other oral region plating. Some oral regions were reduced to spiculate integuments. Each crinoid clade had independent oral region histories. However, despite the fact that each clade had characteristic tegmen morphologies in qualitative tegmen morphospace, similar morphologies evolved iteratively among different clades. Oral region size and complexity were the primary morphological constraints controlling the evolutionary history of oral region plating and geometry.