Abstract
Within modern marine vertebrate communities, the elasmobranchs (sharks and rays) present an incredible taxonomic and ecological diversity placing them as key elements of the structure of their food webs. However, despite their success in surviving most of the major biological crises of the Phanerozoic, many species belonging to this group are now in danger of extinction. The reconstruction of their trophic ecology in the fossil record (mainly composed of teeth) is a major asset in understanding the resilience of these groups to mass extinction events. The parallel analysis of conodonts, another group of marine vertebrates also known by their dental remains and which had a flourishing evolutionary history through the Paleozoic crises before becoming extinct during the Triassic/Jurassic boundary, allows us to consider the understanding of the factors that led to the decline and total extinction of this group.The aim of this thesis is to characterize trophism and the impact of biological crises on elasmobranch and conodont fossil communities using two proxies: the calcium isotopic composition of tooth enamel and the dental topography at the edge of two major crises: The Devonian/Carboniferous crisis and the Cretaceous/Paleogene crisis. The calibration of these proxies on modern elasmobranch faunas has shown the strength of the isotopic tool in the reconstruction of trophic ecology in these groups. On the other hand, the topographic tool has allowed to underline the opportunism in present and fossil elasmobranchs and a certain decoupling between dental type and trophism. The analysis of the isotopic composition within the elasmobranch fossil record has allowed to highlight 1- The resilience of taxa of low trophic level on both sides of the Cretaceous/Tertiary crisis as well as the diversification of selachimorphs within apical predators during the extinction of the large reptiles. 2- To trace the trophic ecology of the giant shark lineage of the genus Otodus. 3- To highlight singular behaviors such as scavenging in Cretaceous large sharks. The analysis of dental topography in the fossil record has shown a morphological segregation of platform elements within conodonts, implying their ability to process different foods depending on the species. The results of this thesis confirm the interest of isotopic and morphological tools for the understanding of trophic ecology in modern and fossil marine vertebrates, including in the long term, thus allowing a better understanding of the resilience of these taxa in times of crisis.