Thesis defense

Axelle GARDIN

Amphi PBS, Bât. B36, University of Poitiers or from a distance: Soutenance d’Axelle Gardin

Skeletal tissues of vertebrates: paleoenvironmental and paleoecological archives

Contributions of the study of skeletochronology, isotopic geochemistry and functional anatomy to the understanding of ancient continental ecosystems

Material of the vertebrist paleontologist, the fossilized skeletal tissues represent an original archive, rich in ecological and environmental information in ancient times. They also record information on the history and living conditions and provide information on the functional adaptations of organisms, essential elements for understanding biological evolution in relation to environmental changes. These aspects are addressed in the thesis, through the exploration and development of different methodological approaches.

The skeletal growth marks of freshwater actinopterygians and chelonians from intertropical Africa are considered a marker of paleo-seasonality. The development of this tool involves the construction of a reference database on extant species close to fossil ones and is broken down into two complementary studies. In a first part, I demonstrate from the analysis of the results of an experiment on 3 intertropical species (Polypterus senegalus, Auchenoglanis occidentalis and Pelusios castaneus), that the seasonal growth of these species is mainly controlled by the abundance of food, and sometimes by water temperature. In a second part, I demonstrate that the exploration of bone growth in wild individuals of Lates niloticus is indeed correlated with seasonal cycles. These first results also make it possible to specify the future developments to be carried out to calibrate this seasonality marker. In a third part is presented the sampling carried out for an application in the Shungura Formation (Plio-Pleistocene, Ethiopia) with the first observations.

Based on the synthesis of the ecology and physiology of crocodilians, I was able to show that the oxygen isotopic composition of their dental tissues mainly reflects that of the water in which they lived during the formation of the tooth. In a first study, the oxygen isotopic composition of the bulk enamel of fossils from the Shungura Formation is interpreted, and provides information on the evolution of freshwater environments and their local diversity over two million years. In a second study sampling dentin serially, preliminary results show fluctuation consistent with the seasonality of climate for some samples. These first data allow proposing a protocol in order to interpret the recorded signal and to discuss the evaluation of the impact of diagenesis.

Illustration Axelle Gardin

Finally, the study of the detailed functional anatomy of the external bony structures (morphology, articular facets, muscular insertions, proportions) on the appendicular skeleton allows inferences on the locomotor behaviour and more generally on the paleoecology of Amphicynodon leptorhynchus (Carnivora, Arctoidea) from the Phosphorites of Quercy (Lower Oligocene, France). In a first part, the functional analysis of the hind limb reveals adaptations to climbing and allows to conclude on probable arborealism. In the second part, a paleoartistic restoration by Amphicynodon leptorhynchus is elaborated on the basis of anatomical arguments and in coherence with the environmental context.

A perspective of the methodological contributions addressed to scientific questions on evolution in its environmental context is proposed, with a particular focus on questions of seasonality and paleoecological inferences.

Jury

• Naomi Levin (Rapporteure) – Professeure Associée, Université du Michigan, Ann Arbor
• Loïc Ségalen (Rapporteur) – Professeur, Sorbonne Université, Paris
• Pascal Godefroit (examinateur) – Professeur, Royal Belgian Institute of Natural Sciences, Bruxelles
• Lionel Hautier (examinateur) – Directeur de recherche, CNRS, Université de Montpellier
• Alexandra Houssaye (examinatrice) – Directrice de recherche, CNRS, MNHN, Paris
• Antoine Souron (examinateur) – Maître de conférences, Université de Bordeaux
• Géraldine Garcia (co-Directrice de thèse) – Maîtresse de conférences, Université de Poitiers
• Olga Otero (Directrice de thèse) – Professeure, Université de Poitiers