PhD Defense - Séverine Toussaint

Toward Primate Origins: Hands and feet in Interdisciplinary Perspective

Composition du jury :

• Marian Dagosto, Professeure, Northwestern University, Chicago (rapporteure)
• Jesse W. Young, Professeur, Northeast Ohio Medical University, Rootstown (rapporteur)
• John Nyakatura, Professeur, Humboldt University, Berlin (examinateur)
• Annemiek Cornelissen, CR CNRS, HDR, Université Paris-Diderot (examinatrice)
• Gaël Clément, Professeur, MNHN (examinateur)
• Marc Godinot, DE EPHE, MNHN (directeur de thèse)
• Denis Youlatos, Professeur, Aristotle University of Thessaloniki (co-directeur de thèse)

Abstract:

Primate origins are subject to important controversies. The initial radiation of first Primates and their precise relationships within Euarchontans (the clade including Primates, Scandentians, Dermopterans, and Plesiadapiformes) are still debated. Moreover, the functional and evolutionary interpretation of some of the morphological characters that define Primates is still uncertain. Among them are the acquisition of manual and pedal prehensile abilities, with a specialized grasping foot bearing an opposable hallux, and nails instead of claws on the distal phalanges. Thus, the ancestral morphotype of Primates is under active investigation, despite the consensus on the arboreality and small size of our early ancestors. This PhD dissertation aimed at revisiting some blurry aspects of primate origins focusing on hand and foot grasping mechanisms, through an interdisciplinary approach blending ethology, biomechanics, comparative morphology and phylogenetics. A reappraisal of the genus Plesiadapis (Plesiadapiformes) led to question a recent hypothesis on early Primates’ phylogeny. In addition, a quantitative analysis of manual and pedal postures relatively to substrate type used during locomotion, followed by a morphological study of hand and foot metapodials and phalanges were also conducted on series of primate and non-primate species. The results were analyzed in an integrative way to relate morphological features to functional attributes, along with assessing their phylogenetic importance. Among many results, this work allowed proposing alternative hypotheses regarding two key characters of primates, the primary function of nails: more linked to sensitivity than to a mechanical advantage; and the environmental scenario that may have driven the evolution of hallucal grasping capabilities: small vertical substrates instead of the fine branch niche. Moreover, in an effort to better understand biomechanical constraints at play during arboreal locomotion, a novel spatially-resolved force sensor was created, which has potential applications in various fields such as robotics.