The crystallographic orientations of echinoderm skeletal elements can supplement standard morphological comparisons in the exploration of echinoderm evolution. At a coarse scale, many echinoderms share a crystallographic pattern in whichcaxes radiate away from the axis of pentaradial symmetry. Within this common pattern, however,caxes of different taxa can differ dramatically in their degree of variability, angles of inclination, and relationships to the external morphology of skeletal elements. Crystallographic data reflect a variety of taxon-specific influences and therefore reveal different information in different taxa. In echinoids, orientations ofcaxes in coronal plates correlate well with high-level taxonomic groupings, whilecaxes of apical plates record modes of larval development. In blastoids,caxes of radial plates have a structural interpretation, with thecaxis oriented parallel to the orientation of the surface of the radial plate during its initial growth stages. In crinoids,caxes do not correlate with taxonomic group, plate morphology, or developmental sequence, but instead correlate with relative positions of skeletal elements on the calyx. Although their full potential has yet to be explored, the varied crystallographic patterns in echinoderms have been used to clarify skeletal structure, characterize developmental anomalies, and infer homologies of skeletal plates both within specimens and between groups. A axes are less constrained in their orientations thancaxes and offer less promise of revealing novel paleobiological information.
Retinotopic organization of primary visual cortex in glaucoma: Comparing fMRI measurements of cortical function with visual field loss
