Hirnantian (latest Ordovician) localities containing echinoderm fossils are rare; the few that have been discovered primarily contain disarticulated crinoid ossicles. Therefore, relatively little is known about echinoderm evolutionary dynamics across the Late Ordovician – early Silurian boundary, especially noncrinoid echinoderms. New diploporitan echinoderms, Holocystites salmoensis and an unidentified holocystitid, from reefal facies of the Upper Ordovician Ellis Bay Formation of Anticosti Island provide a critical data point concerning diploporitan biogeography and evolutionary pathways undertaken during the Ordovician and Silurian. These fossils also provide a crucial link in understanding the ancestry of the Silurian Holocystites Fauna, an unusual diploporitan fauna from the middle Silurian of North America, whose origination dates back at least 15 million years earlier than previously thought with the discovery of taxa described here. New fossil data such as these stress the importance of uncovering new localities from underrepresented times and places in Earth’s history, so that these evolutionary transitions can be better understood.

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