PHYLOGENETIC IMPLICATIONS AND FUNCTIONAL INTERPRETATION OF GLYPTOCYSTITOID RHOMBIFERAN STEM MORPHOLOGY

Glyptocystitoid rhombiferans are a clade of extinct echinoderms that are characterized in part by a specialized stem. Primitively, the proximal stem has alternating inner and outer columnals and the distal stem has narrow, elongate columnals. Stem features have traditionally been deemed conservative, yet analysis reveals new morphologies with phylogenetic implications.

Vagrant forms, including Cheirocrinids and Pleurocystitids, show the primitive condition of large outer columnals with top and bottom indentations that accommodate inner columnals. Articulating surfaces have large, hinge-like spiraling pivot points allowing the stem to bend abruptly over a span of a few columnals. Inner lumen diameter of both columnal types is equal. Sessile callocystitids and echinoencrinitids show dual-projecting, outer columnals with an upward projecting collar in contact with, and nesting, the inner columnal, as well as a latero-distally projecting skirt which overlies and rests upon the collar projection of the underlying columnal. These forms show greatly reduced spiraling pivot points and, in callocystitids, outer columnal lumena are of narrower diameter than inner columnals. This morphological shift is associated with transition from a more flexible proximal stem used for locomotion, to a proximal stem used largely for thecal reorientation to maximize feeding from prevailing currents. In contrast, distal stems are homeomorphic, and vagrant forms and echinoencrinitids have extremely narrow lumena. However, sessile callocystitid distal columnals show relatively large diameter lumena and, in some forms, have symplectial articulations. These distal stem adaptations may be associated with a sessile lifemode and attachment by a holdfast, and are reminiscent of morphologies observed in paracrinoids.

Changes in stem morphology are congruent with current phylogenetic hypotheses for glyptocystitoid rhombiferans: (1) inner and outer columnals of the proximal stem, and the small distal lumen diameter, are pleasiomorphic for Glyptocystitidae; (2) a modified proximal stem with projecting collar and peripheral skirt is synapomorphic for Echinoencrinitidae plus Callocystitidae; (3) large distal stem lumen and small diameter outer columnal lumen are synapomorphic for Callocystitidae.