Bivalves from the Middle Ordovician Sarka Formation (Prague Basin, Czech Republic)

A rich bivalve fauna from the Middle Ordovician (Sarka Formation, early and mid Darriwilian) of Bohemia shows close affinities to Middle Ordovician bivalves from Spain (Iberian Peninsula) and France (Armorican Massif). Twelve species and nine genera (one new) are described: Praenucula applanans (Barrande, 1881), Praenucula bohemica (Barrande, 1881), Praenucula dispar (Barrande, 1881), Concavodonta ponderata (Barrande, 1881), Pseudocyrtodonta ala (Barrande, 1881), Pseudocyrtodonta incola (Barrande, 1881), Tatula petula gen. et sp. nov., Redonia deshayesi Rouault, 1851, Babinka prima Barrande, 1881, Coxiconchia britannica (Rouault, 1851), and the oldest pteriomorphids in the Prague Basin Modiolopsis sp. and Cyrtodonta sp. Ctenodonta, widely used as cumulative name for all praenuculids, was not recorded in the Middle Ordovician of Bohemia. Remarks on the characters of the Protobranchia shell are discussed, the value of the orientation of the teeth to the umbo or out from the umbo for higher systematic has been overestimated in the past. Accessory muscle scars in the Protobranchia help to move with foot, not only by retraction and protraction but also by elevation; some of them hold a visceral sac. The Recent Protobranchia mostly show simpler type of taxodont teeth in comparison with the Ordovician Protobranchia. Palaeoecological aspects of all the species are shown, infaunal burrowers dominate in the lithofacies of the black shales of the Sarka Formation. The bivalve association is dominated numerically by heterodonts, subdominant are several species of protobranchs and two pteriomorphids. The palaeogeographic distribution of the Middle Ordovician bivalves is discussed. Some of the Middle Ordovician bivalves are widely distributed, they reach Baltica and also the Laurentian margins and probably had to have planktotrophic larvae. Clear preference of the heterodonts for high latitude is confirmed by the dominant heterodonts (six species) in the bivalve association of the Middle Ordovician Sarka Formation.