Two common Upper Ordovician crinoids, Xenocrinus and Dendrocrinus, had distinctive columns that showed marked contrasts of both form and function. Xenocrinus baeri (Meek) had a tetragonal facet geometry but functioned in the normal manner for a symplectially articulated column. Column flexure was by bending, the four directions of curvature being limited by the tetragonal arrangement of crenulae. In contrast, columns of Dendrocrinus casei Meek, with a characteristic pentastellate symmetry, were able to twist, a functional adaptation not previously reported from crinoids; this was controlled by the unusual geometry of the crenularium. This suggests that some or all crinoid columns may be subject to twisting stresses, perhaps associated with changes in the current direction, that crenulae may resist passively.Functional morphology, column, crinoids, Xenocrinus, Dendrocrinus