FAUNAL TRENDS AND SEA-LEVEL CHANGES - BIOGEOGRAPHIC PATTERNS OF KIMMERIDGIAN AMMONITES ON THE WESTERN-EUROPEAN SHELF

The Kimmeridgian series of the northern Aquitaine margin is representa tive of the marine sedimentation on the Western European Shelf. It has been used to demonstrate a fundamental relationship between relative sea-level changes and the biogeographic dynamics of the Kimmeridgian a mmonites. This synthesis, based on comparative sedimentological and pa leobiogeographical studies, shows that the shelf was settled by submed iterranean and/or subboreal ammonites during transgressive phases and maximum sea-level rises. Endemic lineages differentiated during the se a-level highstands and the beginning of lowstands. Correlative with th e long-term sea-level rise, endemic elements have progressively taken a prominent role within the Kimmeridgian ammonite faunas of the shelf. The patterns of faunal changes have been deduced from studies of ammo nite lineages from distinctive biogeographic origins: Rasenioides, Lit hacosphinctes, Orthaspidoceras and Gravesia. Compared with subboreal a mmonites, it seems that the submediterranean species were more toleran t of changing environments and adapted to new environments more easily . Therefore most of the endemic lineages which settled the Western Eur opean area originated in submediterranean faunas.