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

Abstract

A rich bivalve fauna from the Middle Ordovician (Šárka 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: <i>Praenucula applanans </i>(Barrande, 1881), <i>Praenucula bohemica </i>(Barrande, 1881), <i>Praenucula dispar </i>(Barrande, 1881), <i>Concavodonta ponderata </i>(Barrande, 1881), <i>Pseudocyrtodonta ala </i>(Barrande, 1881), <i>Pseudocyrtodonta incola </i>(Barrande, 1881), <i>Tatula petula </i>gen. et sp. nov., <i>Redonia deshayesi </i>Rouault, 1851, <i>Babinka prima </i>Barrande, 1881, <i>Coxiconchia britannica </i>(Rouault, 1851), and the oldest pteriomorphids in the Prague Basin <i>Modiolopsis </i>sp. and <i>Cyrtodonta </i>sp. <i>Ctenodonta</i>, 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 Šárka 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 Šárka Formation

The youngest representatives of the genus Ribeiria Sharpe, 1853 from the late Katian of the Prague Basin (Bohemia)

“…Unlike the rostroconchs, the bivalves diversified significantly during the Middle Ordovician and Late Ordovician (e.g. Babin 1993;Polechová 2013) and the diversity during the Ordovician increased and reached almost 200 genera (Cope & Kříž 2013) in comparison with rostroconchs and their 26 genera. Ribeirioids never exceeded the diversity of bivalves even in the Early Ordovician, when the bivalves were rare and restricted to the Gondwana and peri-Gondwana terranes.…”

“…They can also hold the muscular floor of the visceral sac (Heath 1937;Polechová 2013). The function of side muscles in Ribeirioida, also considered as infaunal burrowers, could be similar.…”

Editorial preface: IGCP 596, Mid-Paleozoic climate and biodiversity: Proceedings of the Opening Meeting in Graz, Austria (September 19–24, 2011)

“…A detailed taxonomic study of the Middle Ordovician bivalves from Prague Basin (Czech Republic) is provided by Polechová (2013). The author describes a diversified bivalve fauna consisting of twelve species in nine genera indicating the palaeoecological aspect of each species.…”

The bivalve Cuneamya from the Late Ordovician of Bohemia

“…Kříž (1995) recognised the presence of the genus Coxiconchia in the Middle Ordovician strata of Bohemia and Kříž (1997) briefly summarised the palaeoecology of the Ordovician bivalves. In the last decade, bivalves from selected stratigraphical intervals were revised (Kříž and Steinová 2009, Steinová 2012and Polechová 2013. Cuneamya catilloides (Barrande in Perner, 1903) from the Late Ordovician is a peculiar species originally assigned to gastropods which requires special attention.…”