Published online by Cambridge University Press: 08 April 2016
True muscular articulations, as opposed to ligamentary ones, are reported here for the first time in the biserial arms of advanced inadunate crinoids (Poteriocrinina). The topography of biserial articular surfaces indicates clearly derivation from a uniserial muscular articulation with two muscle scars, a transverse ridge, and other structures considered typical of this articular mode. Biserial muscular articulations extend from the arm base to within a few brachials of the arm tip, although the articulations are more pronounced and better developed proximally. The inadunate facets are compared with those of the biserial arms of camerates, which are shown to be non-muscular and probably were capable of only limited movement. The arm movements of inadunate biserial arms are interpreted to have been ones where alternating muscle bundles on either side of the arm induced twisting motions of the arms so that a planar filtration fan mode of feeding was possible, in contrast to the parabolic feeding fan or other modes of feeding that were probably performed by other Paleozoic crinoids.