Reassessment of Arenig and Llanvirn age (early Ordovician) brachiopods from Anglesey, north-west Wales

The distinctness of the Celtic biogeographic province in late Arenig and early Llanvirn time is confirmed by reassessment of its brachiopods from Anglesey. Of the eighteen brachiopod species in the Arenig Treiorwerth Formation, two orthaceans are placed in new monotypic genera: Treioria chaulioda sp. nov. and Ffynnonia costata (Bates); the four other endemic genera, together with Orthambonites, Productorthis, Tritoechia, and Rugostrophia, constitute an assemblage very similar to that found in volcaniclastic rocks of the northern Appalachians which has also been assigned to the Celtic province. The seven brachiopod taxa identified in the Llanvirn Bod Deiniol Formation include Baltic and Scoto-Appalachian forms also found together in these North American rocks. Although geological evidence supports separation of the Celtic province areas from those of the Scoto-Appalachian province by the lapetus Ocean, there is no similar evidence to indicate a comparable separation between the Celtic province areas and those of the Anglo-French province such as the Shelve district.