323-2 New Crinoids from the Upper Ordovician (Katian) of Anticosti Island, Québec, Canada: Implications for early Paleozoic Faunal Turnover

Abstract:

The transition from the Ordovician to the Silurian represents one of the most significant intervals of ecological and evolutionary restructuring in Paleozoic marine ecosystems. Widespread faunal turnover during this interval has long been linked to environmental perturbations associated with the Late Ordovician mass extinction (LOME), the first of the "Big Five" mass extinction events in Earth's history. However, the timing, magnitude, and underlying mechanisms of taxonomic and ecological change remain insufficiently understood, in part because relatively few fossiliferous successions preserve a continuous record spanning the extinction interval and the Ordovician–Silurian boundary. Among marine invertebrates, crinoids experienced a particularly pronounced macroevolutionary transition, characterized by substantial changes in taxonomic composition, ecological structure, and patterns of clade dominance that ultimately shaped middle Paleozoic crinoid communities. Determining whether these changes were driven directly by the extinction, or began prior to the LOME, is essential for understanding the evolutionary dynamics of this critical interval in crinoid evolutionary history. 

Anticosti Island (Québec, Canada) preserves one of the most complete and fossiliferous sedimentary successions spanning the Late Ordovician through the Early Silurian, providing an exceptional natural laboratory for investigating biotic responses before, during, and after the LOME. Continued study of the island's exceptionally preserved fossil crinoid fauna has already improved our understanding of both pre-extinction diversity and post-extinction recovery, and continues offering new opportunities to evaluate the rise of middle Paleozoic crinoid assemblages. 

Here we describe a new genus of monobathrid camerate and a new genus of flexible cladid from the Katian-aged Vauréal Formation of Anticosti Island. These taxa belong to clades that underwent major evolutionary radiations following the LOME, subsequently becoming important components of Silurian faunas and, more broadly, the middle Paleozoic crinoid evolutionary fauna. Their occurrence in pre-extinction Katian strata demonstrates that these lineages originated before the onset of the primary extinction pulse and were already diversifying within Late Ordovician ecosystems. Taken along with other recent discoveries, this suggests that the macroevolutionary transition traditionally attributed to post-extinction recovery may instead reflect evolutionary processes initiated prior to the LOME, with the extinction event acting to enhance, rather than solely generate, the composition and ecological dominance of subsequent crinoid communities.

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