Uppermost ordovician bivalves from the Prague Basin (Hirnantian, Perunica, Bohemia)

Abstract

Twelve species (four new) and 9 genera of bivalves are described from the uppermost Hirnantian, Upper Ordovician of the Prague Basin, Bohemia: Praenucula dispar (Barrande, 1881), Praenucula abrupta sp. nov., Sluha kosoviensis (Barrande, 1881), Nuculites aff. planulatus Conrad, 1841, Myoplusia contrastans (Barrande, 1881), Myoplusia obtusa (Barrande, 1881), Myoplusia sp., Metapalaeoneilo dromon sp. nov., Praeleda compar (Barrande, 1881), Mytilarca mareki sp. nov., Modiolopsis pragensis sp. nov., and ?Sphenolium cf. parallelum Ulrich, 1894. From the coarse, storm generated sandstones representing the additional regressive event, when the shelf was channelled, and coarse material transported from the shore in the late Hirnantian, the low diversified, almost monospecific Modiolopsis pragensis Community, of the Modiolopsis Community Group, was described. It indicates restricted living conditions. 11 bivalve species form, together with 25 species of brachiopods, 5 species of gastropods, and undescribed conulariids, hexactinellids, trepostomate bryozoans, annelids, hyolithids, orthocone nautiloids, rare ostracods, phyllocarids, blastoids, cystoids, crinoids, dendroids, graptolites, and chlorophytes (receptaculitids) the redefined Hirnantia sagittifera-Sluha kosoviensis Community. It represents the most diversified community of the Hirnantia Community Group known in the World. We suppose that the Hirnantia sagittifera-Sluha kosoviensis Community occupied the well-ventilated environment of the soft bottom carbonate silts with high organic content. The community was most probably autochthonous, with minimal transport as is indicated by the common preservation of shells with conjoined valves (articulate brachiopods, semi-infaunal bivalve Mytilarca mareki, and infaunal bivalves). The bivalves Mytilarca mareki and Metapalaeoneilo dromon sp. nov., Nuculites aff. planulatus, and ?Sphenolium cf. parallelum may have originated in the Baltica carbonate platforms and the equatorial regions of Avalonia and Laurentia and support the ideas about the position of the Kosov Province in the temperate-to-subtropical zone (between 30° to 45° S). For the analogous and homologous communities we described the Hirnantia Community Group as a substitute for the term "Hirnantia fauna". The Hirnantia Community Group was most probably extending from the circumpolar sphere into the temperate to tropical zones in proximity to the carbonate platforms. In Bohemia and elsewhere it occurs just a few metres below the Ordovician-Silurian boundary and represents the evidence of the environmental recovery during the uppermost Ordovician transgression following the Upper Ordovician global glaciation.