Skeletal crystallography and crinoid calyx architecture

In the first broad survey of skeletal crystallography in fossil crinoids, w e examine 10 Ordovician species representing five orders and apply crystall ographic data to questions of crinoid phylogeny, homology, and development. Orientations of c crystallographic axes in the large calcite crystals that form the skeletal plates of the crinoid calyx vary systematically accordin g to the position of each plate on the calyx. Plates lower on the calyx hav e axes more inclined toward the stem attachment than are axes from plates h igher on the calyx. Although most specimens display this general pattern, e xact orientations vary widely between species with no discernible relations hip to phylogeny. Furthermore, the topological pattern of variation does no t correlate with the order of addition of plates to the calyx during growth . Lack of a phylogenetic signal among diverse crinoids early in the clade's h istory implies that crystallographic data will be of limited use to high-le vel phylogenetic studies within crinoids. Neither does skeletal crystallogr aphy strongly favor any of several competing interpretations of homologies among major crinoid calyx plates. Crystallographic data are informative, ho wever, for some minor skeletal plates. Brachial plates have c axes that rou ghly parallel the surface of the plate, whereas interbrachial plates have p erpendicular c axes, suggesting that distinct generative processes produce these plates. Anal plates have orientations similar to interbrachials, sugg esting similar developmental mechanisms. Although c axes have regular orientations relative to plate morphology with in ii specimen, a axes show extensive intraspecimen variability with respec t to plate morphology.