Skeletal microstructure of helens, lateral spines of hyolithids

In hyolithids the skeleton consists of four elements: a conch, an operculum and a pair of long, logarithmically curved 'spines' called helens. These last elements are rarely preserved, and have therefore remained poorly known and enigmatic. We have studied three-dimensionally preserved helens of the species 'Hyolithes' lanceolatus, from the Permian of New South Wales, Australia, and 'Hyolithes' groenwalli, from the Cambrian of Bornholm, Denmark. Helens were massive calcareous elements. Their original microstructure, herein reported for the first time, consisted of concentric lamellae surrounding a narrow elongated core, which may have been rich in organic matter. This concentric pattern resulted from the successive accretion of shell material at the proximal, internal portion of helens. This growth model is in accordance with helen morphology and with the presence of a characteristic surface sculpture of overlapping lamellae.