Abstract

Nicholsodiscus anticostiensis new genus and species (Mill Bay Member, Vauréal Formation, Rawtheyan, Upper Ordovician) is described from Anticosti Island, Quebec, Canada. Nicholsodiscus anticostiensis n. gen. and sp. is known from two complete specimens preserved in situ with the cupule-bearing side facing toward the bedding surface. Sedimentological, petrographic, and trace fossil evidence suggest that this bedding surface had the consistency of a hardground during the lifetime of the cyclocystoids. This provides the first unequivocal evidence that cyclocystoids lived with their cupule-bearing side (ventral) toward the substratum. A cupules-down orientation results in the ambulacral grooves facing the substratum. Such an orientation excludes suspension-feeding from being a possible feeding mode of the Cyclocystoidea as discussed by Henderson and Shergold (1971).

A third Anticosti Island cyclocystoid specimen of unknown affinities is described from the same locality. Furthermore, a re-examination of the holotype of “Cyclocystoidesraymondi Foerste for purposes of comparison with Nicholsodiscus anticostiensis n. gen. and sp. provided evidence that it is a member of Zygocycloides, albeit with missing interseptal plates.

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