Abstract

Ostracods from the upper part of the Valisettes Formation (Frasnian), from the Matagne Formation (Idem) and from the base of the Senzeille Formation (Famennian) exposed in the Neuville railway section (southern side of the Philippeville Massif within the Dinant Synclinorium) have been studied in order to advance the knowledge of the late Devonian mass extinction. The ostracod assemblage present in the upper part of the Valisettes Formation belongs to the Eifelian ecotype, and is dominated by filter-feeding metacopid ostracods indicative of an hypoxic trend related to an increase in the water depth. The overlying Matagne Formation is characterised by opportunistic species belonging to the myodocopid ecotype indicative of more severe hypoxic water conditions. The ostracod assemblage recognised in the base of the Matagne Formation is comparable to the assemblage found in the Upper Kellwasser Horizon of the Schmidt quarry parastratotype (Kellerwald, Germany), and the sartenaeri, splendens and sigmoidale Zones of the biozonation on entomozoid ostracods are also recognised. The splendens Zone – sigmoidale Zone transition which corresponds to an important change in the entomozoid fauna in relation to the late Devonian mass extinction must be fixed definitively into the late Frasnian. Only Palaeophilomedes neuvillensis Casier, 1988, and Ovatoquasillites avesnellensis (Lethiers, 1973) survived the extinction in the Neuville railway section. P. neuvillensis is perhaps the sole true nekto-planktonic myodocopid present in the section, and the pre-adapted Lazarus species O. avesnellensis is known to have lived in very shallow marine and semi-restricted environments. We can conclude that the surface waters and the shallow marine environments were probably not disturbed during the late Devonian mass extinction. A figure, summarises the succession of events at the Frasnian – Famennian transition on the southern border of the Philippeville Massif.

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