Résumé
The Late Devonian is punctuated by a series of environmental perturbations documented by the development of anoxic facies in many depositional settings. We focus on two of these events: the upper Kellwasser (UKW) marking the Frasnian-Famennian boundary that severely affected the marine biota, and the lower Kellwasser (LKW) that predates the UKW by about 0.8 Ma. Pelagic organisms were less affected than benthic faunas and thus can be used to trace fine scale paleobiological reactions to the Kellwasser events. Conodonts are one of the most common tropical planktonic fossils of the Upper Devonian and are interpreted as elements of the buccal apparatus of an animal related to early vertebrates. The genus Palmatolepis is the major component of the conodont fauna in Late Devonian open ocean waters and has been chosen for morphometrical analyses. Conodonts were extracted from conodont-rich limestone levels of the Coumiac and the La Serre sections in the Montagne Noire (France). Fine morphological variations of the genus Palmatolepis were quantified using an outline analysis of its platform. Oxygen isotope analyses were performed on conodont apatite and compared to conodont shape changes through the late Frasnian and early Famennian. Paleotemperatures derived from oxygen isotopes of conodont apatite indicate two cooling phases associated with both Kellwasser events, the larger shift being associated with the LKW. In parallel, the tooth-like Palmatolepis elements show two morphological shifts, the larger one being associated with the UKW. The discrepancy between the two signals points to a paradox: the UKW caused a stronger biotic response despite a minor environmental change. Despite this paradox, the changes in the shape of Palmatolepis are overall correlated with changes in the oxygen isotope ratios, with different relationships observed in the Frasnian and the Famennian. We speculate that climatic changes affected nutrient levels in surface waters and thus the diet of the conodont animals, selecting for a different functioning of the feeding apparatus and hence a different shape of the conodont elements.