PhD Defense - Chloé Olivier

Phylogeny, paleobiogeography, and paleophysiology of the Triassic dicynodonts (Therapsida, Anomodontia): contributions of the Laotian and Moroccan forms

Thèse de doctorat sous la direction de Jorge Cubo et Nour-Eddine Jalil

Composition du jury :

• Jennifer Botha-Brink, Professor, National Museum of Bloemfontein (South of Africa), Rapporteure
• Christian F. Kammerer, Research Curator of Paleontology, North Carolina Museum of Natural Sciences (U.S.A.), Rapporteur
• Régine Vignes-Lebbe, Professeure, Sorbonne Université (France), Examinatrice
• Jörg Fröbisch, Professor, Museum für Naturkunde Leibniz (Germany), Examinateur
• Michel Laurin, Directeur de recherche, Centre national de la recherche scientifique (France), Examinateur
• Jorge Cubo, Professeur, Sorbonne Université (France), Directeur de thèse
• M. Nour-Eddine Jalil, Professeur, Muséum national d’Histoire naturelle, Co-directeur de thèse

Abstract:

Dicynodonts represent emblematic Permian-Triassic (P-Tr) taxa that survived the most severe biotic crisis in Earth’s history, at the end of the Permian. While they were strongly impacted, they highly diversified during the Middle Triassic. They form a major component of the herbivorous fauna in their ecosystems. However, the major Late Triassic (Carnian) environmental changes deeply affected the diversity and abundance of dicynodonts. They finally became extinct at the end of Triassic. The evolution of the Triassic dicynodonts and the patterns of their Triassic diversification and extinction still unclear. This work focused on Laotian and Moroccan dicynodonts and aims to enhance our knowledge on the phylogeny, paleobiogeography, and paleophysiology of the Triassic dicynodonts.

The discovery of two new species (Counillonia superoculis and Repelinosaurus robustus), in the Early Triassic of Laos supported a rapid and earlier post-crisis recovery of dicynodonts. Repelinosaurus is recovered within Kannemeyeriiformes and Counillonia within Dicynodontidae. The latter assignment involves an additional group of dicynodonts surviving the P-Tr crisis. However, the phylogenetic relationships of the Laotian forms appear non-consensual. A high termometabolism has been proposed as the main cause to explain their survivorship beyond the crisis. Paleophysiological models built on the basis of histological features inferred a high metabolism in both Permian and Triassic dicynodonts, which thus refuted the previous hypothesis. They moreover support a unique acquisition of the mammalian endothermy within Synapsida occurring at least during middle Permian, at the Neotherapsida node.

The presence of dicynodonts in Laos, in the Indochina Block, provides new data on the current debate on the paleogeography of South East Asia. The stratigraphic age of the Laotian dicynodonts and the terrestrial lifestyle of dicynodonts consequently suggest a terrestrial connection between the Indochina Block and Pangea at least in late Permian-Early Triassic, most likely via the South and North China blocks.

An abundant and unpublished post-cranial material of dicynodonts from Morocco (Argana Basin), was described and the taxonomy of the Moroccan dicynodonts (Moghreberia nmachouensis, Azarifeneria robustus and A. barrati) was revised. The species Moghreberia nmachouensis, the mostly represented form in the Argana Basin, is proved to be a valid taxon clearly distinct from the North American Placerias to which it was previously assigned as a junior synonym. Moreover, contrary to all previous studies, Moghreberia is recovered sister-taxa of the Polish Lisowicia rather than to Placerias. The revision of the available material of Azarifeneria did not emphasize any significant diagnostic characters distinguishing it from other Triassic dicynodonts. However, a second morphotype in the postcranial material, distinct from Moghreberia, has been highlighted on the basis of its robustness. The large size of Moghreberia and the hypothetical second new taxon confirmed the increase of the body size in dicynodonts during the Triassic.