FUNCTIONAL MORPHOLOGIES OF THE COLUMNS OF UPPER ORDOVICIAN XENOCRINUSAND DENDROCRINUS

Two common Upper Ordovician crinoids, Xenocrinus and Dendrocrinus, had distinctive columns that showed marked contrasts of both form and fun ction. Xenocrinus baeri (Meek) had a tetragonal facet geometry but fun ctioned in the normal manner for a symplectially articulated column. C olumn flexure was by bending, the four directions of curvature being l imited by the tetragonal arrangement of crenulae. In contrast, columns of Dendrocrinus casei Meek, with a characteristic pentastellate symme try, were able to twist, a functional adaptation not previously report ed from crinoids; this was controlled by the unusual geometry of the c renularium. This suggests that some or all crinoid columns may be subj ect to twisting stresses, perhaps associated with changes in the curre nt direction, that crenulae may resist passively.