MODERN ECOLOGY AND ANCIENT STEMMED ECHINODERMS: REVIEWING RHOMBIFERAN STEM MORPHOLOGY THROUGH THE LENS OF ORGANISMAL ECOLOGY

Among Palaeozoic echinoderms, glyptocystitoid rhombiferans remain enigmatic. Prior studies have focused primarily on thecal plating and phylogeny; consequently, relatively little attention has been given to the stem and its’ functional and ecological implications. The generalized morphology of the stem bears a proximal portion consisting of heteromorphic inner and outer columnals and a distal homoeomorphic portion of barrel-shaped columnals. The proximal portion is motile with spiralling pivot points conferring rotational movement, allowing precise positioning of the theca. The distal portion is stiffer and functions primarily as a supportive base. As with modern benthic macrofauna, these morphologies are primarily an effect of substrate composition. This generalized glyptocystitoid model allows high mobility and flexibility for inhabiting both hard and soft substrates. Variation resulting from specialization is shown in this group, particularly among Callocystitidae.

In many callocystitids like Lepadocystis, the distal portion of the stem has a single element encrusting holdfast. Although motility is lost, anchoring to a hardground or other calcified filter feeders is beneficial as a method of competitive exclusion through permanent territories. Similarly, Lepocrinites has an enlarged fused fusiform distal end, which instead is used to autonomously anchor in variable soft sediments. This stem model is not a true cemented holdfast and thus has better flexibility in function to suite the requirements of its’ fluctuating habitat and allowing reattachment if dislodged. In Brockocystis the lower portion of the proximal stem is elongated and fused into a cup-like structure. This structure could have served as a wedge or anchoring structure without sacrificing motility, as with the Lepadocystis and Lepocrinites models.

Further analysis of thin sections and SEM studies of microstructure are needed. Through time glyptocystitoids have alternated between motility and sessile life modes. As a result, function of the stem has been both as an anchor and a support. Applying ecological principles in interpretations of fossilized stem structures is valuable when considering callocystitids as an opportunistic macrofaunal group adapting to increasingly complex marine ecosystems.