Abstract
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.
Highlights
The Great Ordovician Biodiversification Event (GOBE) resulted in a significant increase in lower-level diversity across the entire globe, as well as across phyla (Webby et al 2004)
The Holocystites Fauna is an enigmatic grouping of diploporitan echinoderms that, historically, has been restricted to the middle Silurian of North America
Silurian Holocystites Fauna to Ordovician diploporitans, the discovery of these diploporitan faunas within reefal facies on Anticosti Island suggests that diploporitan echinoderms may have taken similar biogeographic migration pathways to the midcontinental United States as those previously described in other taxa such as brachiopods, trilobites, and some crinoids (e.g., Jin 2001; Ausich and Deline 2012; Wright and Stigall 2013; Bauer and Stigall 2014)
Summary
The Great Ordovician Biodiversification Event (GOBE) resulted in a significant increase in lower-level diversity across the entire globe, as well as across phyla (Webby et al 2004). The Holocystites Fauna is an enigmatic grouping of diploporitan echinoderms that, historically, has been restricted to the middle Silurian of North America (with a potential exception of a holocystitid from middle Silurian Australia; see Jell 2011) Whereas this https://mc06.manuscriptcentral.com/cjes-pubs fauna has been well studied in terms of its systematics (e.g., Paul 1971, Frest et al 2011, Sheffield and Sumrall 2015, 2017) and taphonomy (e.g., Thomka et al 2016), very little information about their evolutionary and biogeographical history exists. Member of the Ellis Bay Formation, Upper Ordovician, uppermost Hirnantian from the outcrop belt on the northern portion of the island (Fig. 4) In this member, https://mc06.manuscriptcentral.com/cjes-pubs stromatoporoid−coral−calcimicrobial reefs are built up on the eastern part of the Island (Copper 1976, 2001) and they extend discontinuously across Anticosti Island at the same temporal and stratigraphic horizon. This diploporitan-reef association likely reflects the presence of stabilized substrates https://mc06.manuscriptcentral.com/cjes-pubs associated with the buildups that are required by the obligate encrusting strategy of Holocystites
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