Ecology of Devonian ostracods: application to the Frasnian/Famennian boundary bioevent in the type region (Dinant Synclinorium, Belgium)

Abstract

Three mega-assemblages and six assemblages of ostracods numbered 0 to V are recognised in the Devonian. They are indicative of lagoonal environments, semi-restricted environments and marine environments above and below fair-weather wave-base or storm wave-base. The water energy, the oxygenation and the temperature of marine waters seem to be the most important factors controlling the distribution of ostracods in the Devonian. These ostracod assemblages and mega-assemblages are also recognised in the Silurian and the Carboniferous. The ecological study of ostracods in the southern border of the Dinant Synclinorium, the type region for the definition of the Frasnian and Famennian stages, proves that the Late Devonian mass extinction is related to a period of hypoxy of marine waters followed and probably linked to a regression close to their boundary. The hypoxy is marked by the occurrence of the Myodocopid Mega-Assemblage, and its acme corresponds to the splendens Zone/sigmoidale Zone boundary of the parachronology based on Entomozoid ostracods. The sea-level fall is attested by the occurrence of ostracods indicative of semi-restricted water conditions in several sections, and particularly at Sinsin where stacked valves due to very small waves related to the lap are observed. Thus, the study of ostracods shows that intrinsic factors linked to the global tectonic regime are the most important factors explaining the Late Devonian mass extinction. The Frasnian/Famennian historic boundary of Gosselet (Annales de la Societe geologique du Nord, 4, 303–320, 1877) corresponding to the recovery of the mega-fauna after the acme of this mass extinction marks in reality the return to true marine water conditions after the semi-restricted marine episode. The Matagne Fm, the Senzeille Fm and the position of the F/F boundary in the Synclinorium of Dinant are discussed.