PhD Defense - Nicolas Séon

Oxygen isotope determination of the thermoregulatory strategies of modern and fossil marine vertebrates: paleoenvironmental implications

Composition du jury :

• M. Christophe GUINET, Directeur de recherche CNRS, Université de La Rochelle - Rapporteur
• M. Thomas TÜTKEN, Associate Professor, Johannes Gutenberg Universität Mainz - Rapporteur
• Mme Nathalie BARDET, Directrice de recherche CNRS, Muséum national d’Histoire naturelle de Paris - Examinatrice
• Mme Klervia JAOUEN, Chargée de recherche CNRS, Observatoire Midi-Pyrénées de Toulouse - Examinatrice
• Mme Paula MÉNDEZ-FERNANDEZ, Ingénieure de recherche CNRS, Université de La Rochelle - Examinatrice
• Mme Peggy VINCENT, Chargée de recherche CNRS, Muséum national d’Histoire naturelle de Paris - Directrice de thèse
• M. Romain AMIOT, Chargé de recherche CNRS, Université Claude Bernard Lyon 1- Co-directeur de thèse
• M. Sylvain CHARBONNIER, Professeur, Muséum national d’Histoire naturelle de Paris - Co-directeur de thèse

Abstract

Recent paleobiogeographic, osteohistological and geochemical studies indicate that Mesozoic Ichthyosauria and Plesiosauria were endothermic and probably homeothermic organisms such as Cetacea. By this thermoregulatory strategy, the oxygen isotope composition of the phosphate group of the bioapatite of their bones and teeth (δ18Op) could constitute a good proxy to trace the variations of the oxygen isotopic composition of the oceans (δ18Osw) of the Mesozoic and thereby improve the estimates of paleotemperatures of the oceans of the epoch based on the thermo-dependence of the isotopic fractionation of the oxygen. However, to use these marine vertebrates as biotracers of the variations of δ18Osw, it is necessary to determine what is the relationship between the oxygen isotope composition of the body water (δ18Obody water), the water from which mineralize skeletal elements and teeth, and that of the oceanic water (δ18Osw); but also to characterize the intra-skeletal variability of the δ18Op in order to define the skeletal elements that it is relevant to use for future paleoceanographic studies.

To resolve these issues, the oxygen isotope composition of the body fluids (blood plasma and urine) of three species of extant marine vertebrates (the loggerhead turtle Caretta caretta, the common bottlenose dolphin Tursiops truncatus and the killer whale Orcinus orca) hosted under controlled conditions was compared to that of their basin and their diet. The results indicate that the oxygen isotope composition of the body fluids and that of the water in which they live is largely dependent on the presence of salt glands and the type of diet. In parallel, six intra-skeletal isotopic maps of the δ18Op were carried out on two species of Cetacea (two specimens of short-beaked common dolphin Delphinus delphis delphis and one specimen of Commerson’s dolphin Cephalorhynchus commersonii kerguelensis), two species of Osteichthyes (one specimen of Atlantic bluefin tuna Thunnus thynnus and one specimen of swordfish Xiphias gladius) and one species of Pinnipedia (one specimen of harbour seal Phoca vitulina vitulina). The production of these δ18Op maps demonstrate that the observed spatial heterogeneities of the δ18Op at the skeletal scale reflects the known regional heterothermies, thus offering the opportunity to characterize the regional heterothermies in fossil marine vertebrates from the intra-skeletal variability of the δ18Op.

In a second step, seven maps were carried out on sub-complete fossil specimens of Ichthyosauria, Plesiosauria and Metriorhynchidae, an extinct lineage of fully marine crocodylomorph, from the Middle and Upper Jurassic (175.6 million years ago to 145.5 million years ago) of the western Tethys in order to clarify their thermoregulatory strategy by trying to locate the possible presence of regional heterothermies. The interpretation of the data from a thermophysiological point of view still remains quite difficult given the diagenetic processes and the lack of knowledge concerning migrations in these organisms. Thus, the presence of regional heterothermies in Ichthyosauria, Plesiosauria and Metriorhynchidae still remains an open question. However, the compilation and the acquisition of new δ18Op data from isolated skeletal elements of Ichthyosauria, Plesiosauria and Metriorhynchidae from different deposits and ages has shed light on the potential of these organisms as biotracers of δ18Osw of the Mesozoic oceans.

Keywords: thermophysiology, isotopes, oxygen, cetaceans, Ichthyosauria, Plesiosauria.