Copper and Jin—Ordovidian–Silurian brachiopod evolution, extinction, and recovery
variants in this study. Pending a broader investigation into the internal structures of Cryptothyrella and Elkanathyris n. gen., the smooth form that co-occurs with the ribbed Elkanathyris pallula n. gen. n. sp. is assigned provisionally to Hyattidina sp. under open nomenclature.
Conclusions
An abundant and well-preserved suite of athyride brachiopods is present in the Late Ordovician (Hirnantian only) and early Silurian (Llandovery) sequence of Anticosti Island. Their stra- tigraphic distribution provides clues as to the change-over in shelly communities crossing the Ordovician-Silurian mass extinction boundary. Marked are rapid evolution of Hindella species during deposition of the Hirnantian Ellis Bay Formation, and their disappearance at the top of the Ordovician, alongside the last occurrence of the orthide genus Hirnantia. Hindella is replaced by the athyride Koigia in the Rhuddanian, a smaller genus that is locally abundant along with the new Early Silurian shelly fauna of Zygospiraella, Becscia and Viridita (Jin and Copper, 2010; Copper and Jin, 2014). In the Aeronian- Telychian, athyrides diversified further into the meristelline and whitfieldelline subfamilies that mark the Telychian through Wenlock in Laurentia. Detailed serial sections using acetate and butyrate peels are
reconstructed in three dimensions to demonstrate the nature of the calcified skeletal supports of the lophophore in these early athyrides. This sheds a new, and different, light on their rise in the Silurian. For the first time we note: (a) Hindellide brachidia lack a skeletal connection between the spiralia and the dorsal
hinge crura (this is thus unlike what is normally shown in figures), (b) the umbonal blades of the brachidium and crura are bent at their tips into a hook-like structure (new dis- covery), and (c) the jugum of hindellides is a simple arch, either rounded or angular. Using the crura, and brachidium, we modify and simplify the existing taxonomy of early athyrides, combining such genera within the Hindellidae Schuchert, 1894. The evolution of such early athyrides provides a stratigraphically useful tool that explains the westward migration of pentameride, rhychonellide, and atrypide shelly communities in the early Silurian equatorial belt of Laurentia and Baltica.
Acknowledgments
We jointly thank NSERC’s Discovery Grant Program (Natural Sciences and Engineering Research Council) for its long-term support of stratigraphic work on Anticosti Island. J. Dougherty and M. Coyne provided access and curation for the Billings collection (Geological Survey of Canada, Ottawa), in charge of the curation of the Anticosti collections, now In Ottawa, made by us since 1966. Specimens of Cryptothyrella were kindly provided for study by W. Ausich (Ohio State University) and B. Hunda (Cincinnati Museum Center, Ohio). Stratigraphic information on the Cryptothyrella occurrences in Ohio and New York was provided by C.E. Brett (University of Cincin- nati) and M. Kleffner (Ohio State University). The constructive comments of journal reviewers R.-Y. Li and F. Alvarez and journal editor B. Hunda greatly helped improve the presentation
1145
and discussions. This paper is a contribution to the International Geoscience Programme (IGCP) Project 653—The onset of the Great Ordovician Biodiversification Event.
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