It is well known that echinoderm skeletal elements behave optically as single calcite crystals, but one implication of this observation, that skeletal units have crystallographic axes with characterizable orientations, has received little attention. This paper examines patterns of crystallographic axis orientations in 43 blastoid species and assesses their usefulness to the study of homology among echinoderm skeletal elements, phylogeny both within blastoids and at higher taxonomic levels, and biomineralization processes.
Calcite decoration and optical goniometry techniques reveal intraspecific and interspecific patterns of variation in the orientations of c and a crystallographic axes in the three major sets of blastoid thecal plates. Within species, basal, radial, and deltoid c axes all show consistent orientations, clustering near the meridional plane that bisects each plate. In contrast, a axes of most plates have random orientations, although for a few plates a axes are clustered rather than random. While c axes from different species all fall about the meridional plane bisecting each plate, inclinations within this plane vary, so crystallographic data can provide new characters for phylogenetic analysis.
In assessing homologies among deltoid plates, axes of posterior deltoids show little correspondence with those of regular deltoids, possibly suggesting a nonhomologous origin. Superdeltoids and epideltoids have commingled c axis orientations and may be homologues.
Consistent c axes within species suggest molecular or cellular control of biomineralization during development, but the occurrence of both random and constrained a axes, depending on the plate examined, implies heterogeneities in this control affecting different plates on the theca.