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The Givetian platform of the Ardennes Massif records several alternations between a siliciclastic-carbonate ramp and a carbonate shelf. Usually these depositional contexts are considered as a major disruption implying a perturbation of many ecological parameters. We established the impact of these variations on the biodiversity structure through the study of the trophic organisation. Thanks to a previous microfacies analysis, 550 levels of the Mont d'Haurs section in Givet and ten associated environments were precisely defined. Seven palaeotrophic levels are recognized from micropalaeontological data, including benthic, planktonic, heterotroph and autotroph organisms. The spatial and temporal distributions of these levels have been analysed through means of multivariate analyses. The statistic results show that the distribution of the palaeotrophic levels during periods characterized by a mixed ramp is not significantly different than during carbonate shelf influences. These environmental modifications do not affect the community-type. These results support recent studies performed on different benthic communities occurring in the Givetian of the Ardennes Massif. Indeed, trilobite and ostracod faunas of this period appear more affected by global environmental changes as the Kačák (uppermost Eifelian) and Taghanic (late middle Givetian) events. Moreover, along the proximal-distal transect on the platform, the reef constitutes the most singular environment. However, contrary with the Frasnian reefs of the Ardennes Massif, which consist of carbonate mud mounds laying on a deep mixed ramp, there is no trilobite community that appears restricted to Givetian reefs. Thus global bioevents during the Middle Devonian have a more important control on the biodiversity than the platform morphology. These results support the view that at wide scales (spatial and temporal), the biodiversity responds more positively to ecological disturbances. [ABSTRACT FROM AUTHOR]

During the Devonian the sedimentation on the continental shelves of Ardenne Massif and Boulonnais has changed from a mixed siliciclastic-carbonate ramp (Eifelian), through a carbonate barrier reef (Givetian) and then to a detritic influx with local mud-mounds (Frasnian). Here we analysed the faunistic dynamics of the trilobite associations through the changing environment. We used multivariate analyses (clustering and ordering) to discriminate the trilobite associations within 67 different samples. Three previously known communities and one new were recognised: the Eifelian Mixed association, the Givetian Dechenella association and the two Frasnian Bradocryphaeus and Scutellum-Goldius associations. These trilobite faunas present a progressive ecological specialisation. The Mixed association occurs both in the ramp or carbonated (local reef developed on the ramp) facies without any significant difference in its composition. The Dechenella fauna occurs preferentially close to barrier reefs, but can also survive during short periods of detrital input. The two Frasnian communities show a strong relationship with their environment. The Scutellum-Goldius association is only found in reef systems, whereas the Bradocryphaeus flourishes exclusively in lateral facies. [ABSTRACT FROM AUTHOR]

Numerous exuviae of three Dechenella species (D. givetensis, D. ziegleri and D. calxensis) from the Middle Devonian (Givetian) of NE France (Ardenne Massif) provide the opportunity to identify the evolutionary modifications of ontogeny of the three Dechenella species and to elaborate a conceptual framework of developmental shape changes. First we used biometric and morphometric approaches to characterize shape modifications. Then we computed ontogenetic trajectories by multivariate regression of geometric shape variables on centroid size in order to compare them. Finally, we compared parallelism between trajectories and rates of development relative to size. These analyses demonstrate a significant difference in the cranidial developmental trajectories of D. givetensis and D. ziegleri indicating an allometric repatterning. However, pygidia of these species share the same allometric pattern with a distinct developmental rate suggesting that heterochrony could be a partial explanation for the body shape evolution. Pygidial ontogeny of D. calxensis corresponds to an allometric repatterning with respect to both other species. This work illustrates the complexity of evolutionary modifications of ontogeny constituting an important process in morphological novelties. [ABSTRACT FROM AUTHOR]