Testing Ordovician Chitinozoan Biostratigraphy Using Sequence Stratigraphy: Evidence from Anticosti Island, Canada

The Ordovician was a period of major climate state changes, from greenhouse to icehouse, and of palaeobiodiversity radiations and declines. Dating Paleozoic rocks remains challenging, yet it is a first-order prerequisite for understanding causal links between climate change and biosphere evolution. Chitinozoan biostratigraphy has become a widely accepted tool for long and short distance chronocorrelation in the Ordovician, despite limited knowledge of the palaeogeographic controls and ecological preferences of these organisms. This contrasts with other biostratigraphic tools, for which palaeo-ecological variation is known to be a critical factor controlling the degree of synchroneity of the lowest and highest occurrences of taxa, and thus the precision expected from a biostratigraphic framework. Anticosti Island, part of the St. Lawrence carbonate platform at the tropical low palaeolatitudinal (15-20°S) eastern margin of Laurentia, preserves an extensive Upper Ordovician succession and represents an excellent natural laboratory for testing for palaeo-ecological influences on chitinozoan distributions. Previous studies have shown that the crickmayi/florentini-concinna biozone boundary correlates with the Vauréal/Ellis Bay Formation boundary at the western offshore carbonate-dominated end of the island, but Hercochitina crickmayiextends throughout the so-called Ellis Bay Formation in the eastern nearshore mixed siliciclastic carbonate settings of Anticosti Island. We use a robust sequence-stratigraphic framework as the chronostratigraphic template against which the chitinozoan data are evaluated. In addition, our new chitinozoan dataset, combined with published legacy data, strengthens biostratigraphic and geochemical correlations between western and eastern parts of Anticosti Island around the Katian/Hirnantian boundary, the exact position of which remains debated due to the absence of the Hirnantian marker graptolite Normalograptus extraordinarius. This study extends the chitinozoan biostratigraphic framework across western (Anse aux Fraises-Pointe Laframboise sections) and eastern (Rivière aux Saumons-Ruisseau Macaire and Rivière Prinsta sections) Anticosti Island. Based on independent sequence-stratigraphic timelines, palaeo-ecological parameters (e.g., distance to the inferred shoreline, transgressive-regressive tracts, depositional environments, and lithofacies) can be quantitatively evaluated for their influence on chitinozoan occurrences. Our results suggest that chitinozoan biozones may not be strictly synchronous but are partly controlled by palaeo-ecological parameters, which adds nuance to how chitinozoan biostratigraphy is to be used in correlating Ordovician successions.