The Ordovician brachiopod genus Cyrtonotella: taxonomy and distribution in the Baltic Basin

The taxonomy of the Baltic Ordovician orthoidean brachiopods of the genus Cyrtonotella Schuchert & Cooper, 1931 is discussed, two neotypes are suggested and emended descriptions of species and subspecies are presented. This taxonomic revision of Cyrtonotella-like brachiopods highlights their differences from the family Nanorthidae Havlíček, 1977 in which they have been included up to now. The well-preserved Baltic representatives of the genus Cyrtonotella display several morphological features, which are unique to this group of brachiopods and may have a family-level importance. The studied brachiopods belong to the shallow-water faunal associations of the Baltic Basin, including its easternmost parts (the Moscow Basin). These brachiopods disappear during the faunal crisis at the Sandbian–Katian transition. The global distribution of Cyrtonotella is poorly known due to taxonomic problems. New data on the Baltic species, which specify their stratigraphic and spatial distribution in the Baltic Basin, is a step towards reducing the taxonomic confusion among Cyrtonotella species.