We report the discovery of a rich assemblage of latest Katian and Hirnantian scolecodonts (polychaete jaws) from a new Ordovician–Silurian boundary outcrop in the Reinu quarry, northern Estonia. The recovered polychaete fauna contains at least 40 species attributed to 11 families. Many common taxa appear to range from the latest Katian to the Hirnantian and into the Rhuddanian, indicating the resilience of jawed polychaetes to the end-Ordovician mass extinction. The Reinu assemblage is similar to the coeval faunas known from other Baltic sections, as well as from Anticosti Island, Canada, although with some specific features. The study revealed the highest abundance of scolecodonts in the Ordovician of Baltica, with ca 5400 maxillae per kilogram of rock recorded in the Siuge Member of the Ärina Formation, Porkuni Regional Stage, early Hirnantian.
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