Réponses populationnelles aux conditions du Milieu (Exemples pris chez les ostracodes d'aquitaine entre le Stampien et le Burdigalien)

Abstract

The microfauna of Ostracods at the Oligo-Miocene boundary in Aquitaine was studied by Bekaert (1990) and Ducasse & Rousselle (1991). The analysis of intraspecific populations makes it possible to reconstruct the evolutionary history of about 15 species and to show that it indicates changes in the environment. Emphasis is being put on the specificity of evolutionary and/or adaptative behavior of 4 species belonging to the Cyamocytheridea and Loxoconcha genera. 1/ When they face up the same environments two species belonging to different genera may response in a comparable way: such is the case of C. punctatella ( Bosquet, 1952 ) and L. aequapuncta Deltel, 1964 which changed ecologies in the same way: substitution of morphs which have different ecological needs. C. punctatella “punctatella” morph and L. aequapuncta “beginner” morphs lived in marine coastal waters and died out in the end of Stampian. The “carbonneli”, “reversa” morph of C. punctatella and “derived” morph of L. aequapuncta appeared in the Upper Oligocene and lived in brackish water, in the lower Miocene. 2/ In the same environments two species belonging to different genera may response in a different way: in the Lower Miocene, in marine coastal waters, L. puctatella ( Reuss, 1850 ) and C. strigulosa ( Reuss, 1850 ) are always found together. The latter was very polymorphic; it became abundant and its polymorphism was maximum in the Uppermost Oligocene; two of its morphs died out at the Oligocene-Miocene boundary; its size increased during the Lower Miocene. L. punctatella remainded monomorphic; its size decreased slightly through the Aquitanian-Burdigalian boundary. Therefore observations accounting for evolution paleoecology must be done under the taxonomic level of the species.