A RE-EVALUATION OF HOLOCYSTITES (DIPLOPORITA: ECHINODERMATA)

Holocystites Hall 1864 is a clade of Silurian-age diploporitan echinoderm species that is especially common in North America. A new analysis shows that Holocystites is similar to other diploporitans that lack ambulacral floor plates, and much of the confusion about these fossils stems from an over interpretation of their morphological features. Currently, the group is diagnosed as those diploporitans that bear six peri-orals (oral plates O1-O6 that border the peristome of most advanced blastozoans), and eight facetals (the most proximal circlet of thecal plates that bear brachiole facets) that bear five ambulacral facets. One of the facetals is O7 and is associated with the hydropore and gonopore. The genus is also separated from other diploporitans by the presence of humatipores, special diplopores through which respiration takes place. However, within recognized Holocystites species, there is wide morphological variability that likely represents more distant evolutionary relationships than is suggested by their systematic treatment. Because the genus was a very successful group, with a wide biogeographical range and a large number of species across North America and Eurasia, it is imperative that the characteristics that define the group and discriminate holocystitid clades be better understood.

Collections of Holocystites species housed in various museum collections were examined to determine the suite of characters that unify this clade. These characteristics are limited, as very few specimens preserve any information concerning the attachment structures, feeding ambulacral systems, anal plates, or gonopores; further, no known specimen has yet been found with preserved brachioles. Species are discriminated based on the size and placement of the brachiole facets with respect to the oral plates and facetals, positioning and nature of the hydropore and gonopore with respect to the oral plates and facetals, and plating about the anal opening.

The understanding of the characters that support this clade will help to better interpret other holocystitids and aid in the inference of holocystitid phylogeny. It will also help address their unusual biogeographic patterns and help address the overall phylogenetic pattern of diploporitans that, in its current understanding, is likely polyphyletic.