ORIGINAL PAPER An updated look at the taxonomy, stratigraphy, and palaeoecology of the Devonian bivalve genus Ontaria Clarke, 1904 (Cardiolidae, Bivalvia) Judith Nagel-Myers 1 Received: 8 July 2020 /Revised: 22 January 2021 /Accepted: 4 March 2021 # Senckenberg Gesellschaft für Naturforschung and Springer-Verlag GmbH Germany, part of Springer Nature 2021 Abstract The genus Ontaria Clarke, 1904 includes a group of small, almost circular bivalves that occur abundantly in the Mid- and Late Devonian pelagic facies. These bivalves are characterised by their straight hinge line, small and, central beak, as well as commarginal rugae (if present). Based on an extensive review of type material and additional samples as well as a comprehensive literature review, this study identifies three valid species Ontaria suborbicularis Hall, Ontaria concentrica von Buch, and Ontaria iniquistriata Beushausen. Thirteen Ontaria species are either not valid or have been synonymised, and three species that were previously included in Ontaria do not belong to this genus. Representatives of Ontaria first occur in the late Eifelian and become abundant in the Frasnian and Famennian. Ontaria last occurs in the late Famennian before the Devonian/ Carboniferous boundary. This genus has been identified as a representative of the family Cardiolidae previously reported only from the Silurian, making Ontaria the first report of a Devonian cardiolid taxon. Ontaria inhabited low-energy, generally well- oxygenated, off-shore environments. The overall shell morphology, small size, and the presence of a small siphon supports the idea that Ontaria lived semi-infaunal in/on muddy substrates. Keywords Bivalves . Taxonomy . Stratigraphy . Palaeozoic . Devonian . Ontaria . Cardiolidae Introduction Bivalves are commonly found in the deeper water faunas of the Mid- and Late Devonian and the fine-grained, siliciclastic sedimentary rocks and limestones deposited on the outer shelf preserve a low diverse invertebrate fauna (e.g. Wendt and Aigner 1985 ) frequently including bivalves (e.g. Becker 1993a; Kaever et al. 1980; Paeckelmann 1979). Unfortunately, these taxa have been largely overlooked be- cause of poor preservation and an associated lack of taxonom- ic, stratigraphic, and palaeoecological data for these groups and the genus Ontaria Clarke, 1904 is no exception. Ontaria is a small, almost circular bivalve with a straight hinge line developing characteristic strong commarginal ornamentation. These bivalves are easily identifiable and occur abundantly, sometimes covering bedding planes. Ontaria has been reported from the Middle and Upper Devonian of Germany (e.g. Becker 1993a), France (Babin 1966), Morocco (Nagel 2006), the Ural mountains (e.g. Kondiain et al. 1967; Tsinkoburova 2015), Western Australia (Teichert 1943), and North America (e.g. Cooper 1967; Kirchgasser 1983). The re-examination of most of the type material from Germany and North America results in three valid Ontaria spe- cies and fourteen taxa identified as synonyms, many of which were described over a century ago based on a single or partial specimens (Table 1). This study outlines the stratigraphic range of Ontaria as far as currently known and offers a first palaeoecological interpretation of their presumed life habit. Ontaria has often been confused with the Frasnian taxon Paracyclas Hall, 1843. Paracyclas is comprised of small, some- what circular bivalves that develop strong commarginal orna- mentation. But in Paracyclas, the posterior dorsal margin is curved and transitions smoothly into the posterior shell margin (e.g. LaRocque 1950) while Ontaria’s dorsal margin is straight and abruptly angles down into the posterior margin. Paracyclas umbo is prosogyrate and located in front of the centre of the This article is a contribution to the special issue “The Rhenish Massif: More than 150 years of research in a Variscan mountain chain”. * Judith Nagel-Myers jnagel@stlawu.edu 1 Geology Department, St. Lawrence University, 23 Romoda Drive, Canton, NY 13617, USA Palaeobiodiversity and Palaeoenvironments https://doi.org/10.1007/s12549-021-00491-2