Skeletal morphology and paleontological significance of the stem of extant Phrynocrinus nudus A. H. Clark (Echinodermata: Crinoidea)

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Abstract
The stem (column plus attachment structure) of the extant, Pacific bourgueticrinine crinoid Phrynocrinus nudus A. H. Clark is described, and some notes provided on the crown. The attachment structure is a simple, terminal discoidal holdfast, typical of an obligate encruster of hard substrates and probably derived neotenously from the attached larval stage of an ancestral isocrinid. The column is long and composed of robust columnals. Unlike isocrinids, the mesistele, rather than the proxistele, is the most flexible part of the column. The proximale columnal has an inflexible articulation with the basals; a new proximale grows irregularly between these ossicles. Articulation between the proximale and columnal 2 is synostosial, but all more distal articulations are synarthrial. Proximally and in the dististele, articular facets are rounded, but for much of the column facets are strongly elliptical with offsets approaching 90° in many columnals. However, offset of fulcra is variable between columnals, indicating that it is not a consistently useful criterion for separating species in the bourgueticrinines.