NEW ORDOVICIAN-AGE HOLOCYSTITES FROM ANTICOSTI ISLAND, QUEBEC

The Holocystites Fauna of North America is an enigmatic group of middle Silurian diploporitan blastozoans that provides a rare window into Silurian-age echinoderm communities. Much taxonomic confusion exists in this group that stems from intraspecific variation of their highly plastic thecae and inconsistencies in the identification of plates surrounding the peristomial opening by various authors. Despite extensive study for more than a century, the evolutionary and biogeographic history of this clade remains very poorly understood. This lack of understanding largely relates to a poor availability of Upper Ordovician (Hirnantian) through Lower Silurian (Rhuddanian) echinoderm material to study. Such faunas may bear close relatives of the Holocystites Fauna, thereby illuminating their origin.

However, new data from an Upper Ordovician (Hirnantian) fauna from reefal facies of the Ellis Bay Formation collected on Anticosti Island (Quebec, Canada) has expanded our understanding of the phylogenetic, temporal, and biogeographic patterns of the Holocystites Fauna. A previously undescribed holocystitid taxon, Holocystites n. sp., was recently discovered within the reefal facies of the Ellis Bay Formation, extending the range of holocystitids from only the middle Silurian into the Upper Ordovician. This taxon fully embodies the features diagnostic of Holocystites, including: humatipore respiratory structures, reduced ambulacral length ending in large appendage facets, an oral area consisting of two plate series (oral and facetal), and a slightly clockwise rotation of the oral plates with respect to the ambulacra. All specimens of this new Holocystites taxon have relatively squat, globular thecae and tumid plates; these specimens also have an extra plate between the D and E ambulacra that has not been noted in any other species of Holocystites. One specimen in the collection has an elongated theca with relatively flat plates; however, this individual is interpreted as a juvenile ontogenetic stage of the globular species with tumid plates.