Latest Frasnian and earliest Famennian (Late Devonian) bivalves from the Montagne Noire (France)

Abstract

Twenty one species of bivalves included within 13 genera are described from the latest Frasnian and earliest Famennian (Late Devonian) of the Montagne Noire, France. Twelve new species and 4 new genera are described(Cabricardium sedolor n. gen., n. sp.;C. archi n. gen., n. sp.;Disarnella helena n. gen., n. sp.;D. spyridion n. gen., n. sp.;Lunulacardium petrboki n. sp.;L. passula n. sp.;Metrocardia myrmedon n. gen., n. sp.;M. rouxiana n. gen., n. sp.;M. frydai n. gen., n. sp.;Karkulum rubrum n. gen., n. sp.;K. toros n. gen., n. sp.;Cheiopteria feisti n. sp.). They form at least four bivalve-dominated communities of theCheiopteria Community Group which are analogous to the communities of this group known from the Silurian of North Gondwanan and Perunican Europe. Analyses of the bivalve-dominated communities show that in the region of the Montagne Noire near the Frasnian and Famennian boundary, unique restricted conditions developed for the explosive adaptive radiation of the bivalves after the Kellwasser Event (late Frasnian) in the sedimentation environment of the black calcareous shale and micritic limestone sedimentation. The bottom waters, which were not deep (just below wave base), were temporarily ventilated by surface currents, as is documented by current oriented empty bactritid shells, by temporary mass extinction of juvenile bivalves unable to survive longer periods of hypoxia, by the lack of trilobites, by the limited occurrence of brachiopods and by the drift of bivalve, ostracode, brachiopod, bactritid and goniatite larvae into the region of the Montagne Noire. Similar facies of black shale with nodules or intercalations of dark micritic to biodetrital bituminous limestone with bivalve-dominated communities which developed after the Kellwasser Event are known also in the late Frasnian and earliest Famennian of Morocco, the South Urals and Poland.