Perforocycloides nathalieae new genus and species, an unusual Silurian cyclocystoid (Echinodermata) from Anticosti Island, Québec, Canada

Abstract

Cyclocystoids are a poorly known, rare, extinct class of bi-facially flattened, disc shaped echinoderms, ranging from the Middle Ordovician to the Early Carboniferous. Articulated cyclocystoids are relatively common in the Ordovician but are rarer in younger strata. Here we describe Perforocycloides nathaliae new genus and species, from the early Silurian of Anticosti Island, Québec, Canada, the first articulated cyclocystoid from the Silurian of North America. This taxon is distinguished from other cyclocystoids by the number of variably sized marginal ossicles, the lack of interseptal plates, and the novelty of pores located in the distal part of the sutures between adjacent marginals on the dorsal surface. These dorsal intermarginal sutural pores led to canals which penetrated the contiguous area of the lateral surface of the marginals and emerged on the ventral surface between the cupules of adjacent marginals. These dorsal intermarginal sutural pores/canals appear to be unique to Perforocycloides and whilst their function is speculative, they provided some form of communication between the dorsal disc and the distal side of the ventral marginals/cupules. Perforocycloides most closely resembles the Ordovician-Silurian genus Zygocycloides, suggesting that this genus may have diversified more widely during the Silurian than previously reported. A review of global Silurian cyclocystoid distribution suggests taxa were geographically confined and that greatest diversity appears to have been located within Baltica. However, it also demonstrates our current limited knowledge. No specimens have been recorded from Gondwana (e.g. Africa, Australia, South America), Siberia, and North and South China, nor are any specimens known confidently anywhere from Přidolí strata.

Recovery of Laurentian cyclocystoids following Late Ordovician extinctions (Brassfield Formation, Llandovery; southwestern Ohio)

“…This is among the oldest reported Silurian cyclocystoids from North America and is the only North American Llandovery cyclocystoid that is preserved with a complete rim of marginal ossicles. Zygocycloides Smith and Paul, 1982 is most similar to Nicholsodiscus Glass et al, 2003 (Katian) and Perforocycloides Ewin et al, 2019 (Llandovery, Telychian), both from Anticosti Island, Québec. Cyclocystoids (Ordovician to Mississippian) survived Late Ordovician extinctions, and this discovery documents that this echinoderm clade was part of shallow-water, marine paleocommunities during the initial post-extinction transgression onto the Laurentian platform. UUID:…”

“…Similarly, this Brassfield cyclocystoid has 15 marginal ossicles. Perforocycloides Ewin et al, 2019 has 17–20, and Zygocycloides Smith and Paul, 1982 has 18–20. All other cyclocystoids have more, with Polytryphocycloides having as many as 60 marginal ossicles.…”

“…Cyclocystoids (phylum Echinodermata, subphylum Echinozoa) are one of the most enigmatic clades of Paleozoic echinoderms. Despite an Ordovician–Mississippian range, cyclocystoids are extremely rare echinoderms (e.g., Smith and Paul, 1982; Ewin et al, 2019), and their paleoecology has been debated regularly (e.g., Nichols, 1969; Smith and Paul, 1982; Glass et al, 2003; Sprinkle et al, 2015; Reich et al, 2017; Ewin et al, 2019). Much of their stratigraphic range is known from only isolated marginal ossicles.…”

“…Much of their stratigraphic range is known from only isolated marginal ossicles. Ordovician specimens with complete rings of marginal ossicles are well documented from Avalonia, Baltica, and Laurentia (Smith and Paul, 1982), and this record has recently expanded both temporally and geographically (e.g., Reich et al, 2017; Ewin et al, 2019).…”

“…14), but relatively little is known about their recovery during the Llandovery. Although reports of Llandovery isolated marginal ossicles exist (Frest et al, 1999; Hints et al, 2022), only two Llandovery cyclocystoids with a complete ring of marginal ossicles have been described previously, including Polytryphocycloides davisii (Salter in Salter and Billings, 1858) from the Mulloch Hill Sandstone in Scotland (Smith and Paul, 1982) (upper Rhuddanian; Donovan et al, 2012, p. 155) and Perforocycloides nathalieae Ewin et al, 2019 from the Cybele Member, Jupiter Formation (Telychian) of Anticosti Island, Québec. In this report, a third cyclocystoid with a complete ring of marginal ossicles, Zygocycloides ?…”