La limite Oligocène-Miocène en Limagne : changements fauniques chez les mammifères, oiseaux et ostracodes des différents niveaux de Billy-Créchy (Allier, France)

The lacustrine sequence of Billy-Créchy (Allier) yielded four superposed fossiliferous levels that can be precisely dated for the first time and represent the upper part of the Oligocene and the basis of the Miocene. The age of the localities is given by mammals and particularly by the Rodent and Lagomorph faunas: three are ascribed to the Paleogene mammalian levels MP 29-MP30 and the upper one to the Neogene MN 1. The Paleogene faunas evidence a high degree of similarity with those of the Oligo-Miocene Lower Freshwater Molasse of Switzerland and the localities Créchy 1 and 2 correspond particularly well to the Swiss locality Brochene-Fluh 53, to which a magnetostratigraphic calibration ascribed an age between 24.1 and 23.6 Ma, the date of the Oligo-Miocene boundary being fixed at 23.8 Ma. Changes in the Rodent composition and abundance in these faunas document the impact of the Oligo-Miocene crisis, the uppermost fauna being very impoverished. The avifaunas differ from those of the Aquitanian localities of the Saint-Gérand-le-Puy area by the presence of two forms which are no longer found in the more recent deposits. These birds are mainly aquatic and indicate the proximity of shallow to moderately deep lakes, along the edge of which the Flamingo-related Palaelodidae were nesting. In Ostracod assemblages - numerous along the sequence but in most cases crushed or completely encrusted -, small-sized specimens are abundant, well-preserved, and among them Cypridopsis sp. is dominant; they are represented by adults and juveniles: they indicate shallow-water environmental conditions and unstable environment. Among large specimens, freshwater and brackish species occur; all of them are broken, coated by CaCO3: they belong to different biocenoses and attest some transport.