In the first broad survey of skeletal crystallography in fossil crinoids, we examine 10 Ordovician species representing five orders and apply crystallographic 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 according to the position of each plate on the calyx. Plates lower on the calyx have axes more inclined toward the stem attachment than are axes from plates higher on the calyx. Although most specimens display this general pattern, exact orientations vary widely between species with no discernible relationship to phylogeny. Furthermore, the topological pattern of variation does not 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 history implies that crystallographic data will be of limited use to high-level phylogenetic studies within crinoids. Neither does skeletal crystallography strongly favor any of several competing interpretations of homologies among major crinoid calyx plates. Crystallographic data are informative, however, for some minor skeletal plates. Brachial plates have c axes that roughly parallel the surface of the plate, whereas interbrachial plates have perpendicular c axes, suggesting that distinct generative processes produce these plates. Anal plates have orientations similar to interbrachials, suggesting similar developmental mechanisms.

Although c axes have regular orientations relative to plate morphology within a specimen, a axes show extensive intraspecimen variability with respect to plate morphology.

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