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2 Acta Palaeontologica Polonica

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Early Frasnian ostracods from the Arche quarry (Dinant Synclinorium, Belgium) and the Palmatolepis punctata Isotopic Event

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Casier J.-G. , Olempska E.

Acta Palaeontologica Polonica

2008

tom 53

nr 4

Ostracods from the Arche quarry at Frasnes are analysed. Twenty−seven species are recognised in the Chalon Member and in the very base of the Arche Member of the Moulin Liénaux Formation. Three new species: Scrobicula gracilis, Microcheilinella archensis, and Bairdia (Rectobairdia) chalonensis, and one subspecies Plagionephrodes laqueus praelaqueus, are proposed. The fauna is in the Favulella lecomptei Zone based on metacopid ostracods and belongs to the Eifelian Mega−Assemblage. Ostracods are indicative of a regressive trend from a moderately deep poorly oxygenated marine environment below fair weather wave base to very shallow well oxygenated and agitated environments. Comparison of the ostracod fauna present in the Arche quarry with faunas described from the Frasnes railway section and from the Lion quarry shows that ostracods did not suffer a crisis during the Palmatolepis punctata Conodont Zone and close to the Early–Middle Frasnian boundary.

Ostracods and facies of the Early and Middle Frasnian at Devils Gate in Nevada: Relationship to the Alamo Event

70%

Casier J. G. , Berra I. , Olempska E. , Sandberg C. , Preat A.

Acta Palaeontologica Polonica

2006

tom 51

nr 4

813-828

In order to document the Alamo Event and to investigate its influence on shallow−marine environments, we undertook a study of ostracods, conodonts, and analysis of the sedimentology of the lower member of the type Devils Gate Limestone. Six major carbonate microfacies (MF1–MF6) ranging from open−marine environments below storm wave base to pre−evaporitic supratidal lagoons were recognized. The sedimentological study detected no important sedimentological changes during the Alamo Event; only an influx of detrital material and lithoclasts indicate that an unusual event had occurred. Ostracods are generally rare or absent in the lower member of the Devils Gate Limestone, and only 2,000 carapaces, valves and fragments were extracted; from these some 26 taxa were identified. Two new species, Voronina? eureka and Serenida dorsoplicata are proposed. The ostracods belong to the Eifelian Mega−Assemblage and their distribution was influenced by strong salinity variations. Because of the rarity and low diversity of ostracods and conodonts in samples collected from the lower part of the lower member of the Devils Gate Limestone it is not adequate to demonstrate conclusively an extinction event close to the Alamo Event Bed. Nevertheless the greater abundance and diversity of ostracods above this bed seems to indicate that the Alamo Event did not result in significant extinction of ostracod taxa in this shallow water setting. The ostracod fauna present in the lower member of the Devils Gate Limestone suggests faunal exchanges between Nevada and the Russian Platform via the Western Canadian platform.