6533b858fe1ef96bd12b6a8f

RESEARCH PRODUCT

LES VARIATIONS DU NIVEAU MARIN SUR LE BASSIN DE PARIS AU BATHONIEN-CALLOVIEN. IMPACTS SUR LES COMMUNAUTÉS BENTHIQUES ET SUR L'ÉVOLUTION DES ORNITHELLlDÉS (TEREBRATELLIDINA)

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description

The study deals with the biological impacts of sea-Ievel changes on evolution of a Jurassic benthic fossil group (Ornithellida, Brachiopoda). The different time and scale at which the shifts in marine environments occur leads to the recognition of three interacting dynamic systems: environmental, communities, populations dynamics. The analysis is thus divided into a three steps hierarchy: sequence stratigraphy, analysis of the diversity of brachiopod assemblages, and analysis of morphological variations within omithellid populations. Sequence stratigraphy of the Dogger marly and carbonate successions in the Paris basin provides accurate correlations of depositional environments from shallow water carbonate platform to a deeper open marine domain. It also demonstrates the major events of sea-Ievel variation. The main result is the distinction of 2 types of depositional sequences (sequence of platform drowning vs sequences of platform building) depending on the rate of carbonate production vs water depth. The environmental changes involved have strong repercussions upon marine benthic community structures which are taxonomically replaced. The deep marine faunas act as a stable set of populations which periodically invade the previously emergent platform. These remplacements allow to characterize every sequence (especially the TST component) by a specifie brachiopod association. The associations occur within biostratigraphical marker-beds which extend over the Anglo-Paris basin as demonstrated by outcrops and weIl-log correlations. The second step of analysis deals with the structure -in terms of diversity - of major brachiopod shell-beds. Three structures (mono, oligo, polyspecific) are defined by descriptive classification and factor analysis. They corresponds to 3 time-averaging degrees of a few decades, a few thousands to a few 10's of thousands of years, respectively. The condensation allows taphonomic feedback processes which emphasize the epibenthic fauna and especially brachiopods. A model is proposed integrating renewals of benthic communities structure linked with sea-Ievel changes. The last part involve the impacts of su ch ecological and historical changes on ornithellid populations (Digonella, Obovothyris, Ornithella). Modelling of Digonella shells shows the existence of morphological constraints depending on the respective rates of change of shell biconvexity vs shell outline. The constaints provide a means to decipher heterochronic processes which resuIt in the population variation. The heterochronies intra-population are read between 2 poles (acceleration vs retardation) as a quantitative morphotypic composition which may be compared to the community dynamics. As a general pattern, the greater the taphonomic feedback processes, the greater the retardation component within a Digonella population. Those processes ensure the availability of substrates for larva settlement and also for permanent fixation. The history of ornithellids shows repeated exploitations of the conjonctions of historical circumstances and ecological responses produced as a resuIt of platform drownings. The analysis is thus justified by the coherence of the changes shown at the population, community and environment levels.