The peripheral vestibular membranous ducts that detect angular motion are contained within bony semicircular canals of mammalian petrosals. I investigated morphology and function in the three membranous semicircular ducts through measurements on the bony semicircular canals of 31 skeletonized skulls from different genera. While the prevailing theory of semicircular canal researchers is that the locomotor agility of extant and extinct mammals can be understood by measuring the size of the three bony semicircular canal arcs, I propose that there are important and quantifiable features other than the adult size of radius of curvature of the semicircular canal arc (R) that influence angular movement detection in mammals and perhaps in their ancestors. Initially, I sought to verify that there was no significant asymmetry of R across the study specimens. However, there was significant asymmetry in canal pair angles between contralateral sets: ipsilateral canal pair angles differed by up to 14◦, and contralateral synergistic angle pairs differed by up to 18◦. Canal pair contralateral differences were lower for specimens of more agile taxa. In addition, the angle between the left and right lateral canals varied by up to 27◦ from parallelism, so the use of the lateral bony canal in one petrosal to represent the horizontal animal resting position could result in significant skull orientation errors. I utilized a program to quantify the effects of canal plane non-orthogonality and to calculate a maximum rotational sensitivity axis for a given taxon. My results concur with earlier research indicating that canal orientation significantly affects the location of maximum rotational sensitivity axes in the head, and should be considered in future quantitative research. Finally, I determined the volumes of the subarcuate fossa and the petrosal lobule in three Monodelphis domestica animals (76 days postnatal) by utilizing both cranial and tissue volumes in fresh specimens. The petrosal lobule fills 93-97% of the Monodelphis domestica fossa, a greater volume than the 50% estimated by previous researchers. These results highlight the difficulties of using histologic or preserved specimens to make quantitative determinations of brain tissue volumes, and reopen the question of whether the subarcuate fossa volume provides a record of the agility for an extinct taxon. / text

The morphological diversity of the external and internal surfaces of the petrosal bone, which contains the structures of the inner ear, across a broad range of therian mammals is documented, and patterns of variation across taxa are identified. One pattern of variation is the result of ontogenetic changes in the ear region, as described for the external surface morphology of a sample of isolated petrosal bones referred to Proboscidea from Pleistocene deposits in central Texas. The morphology of the aquaeductus Fallopii for passage of the greater petrosal branch of the facial nerve supports an ontogenetic explanation for some variation within the proboscidean sample, and a sequence of ossification surrounding the aquaeductus Fallopii is hypothesized. Further ontogenetic patterns are investigated using digital endocasts of the bony labyrinth (preserved on the internal surfaces of the petrosal) constructed from CT data across a growth series of the opossum Monodelphis domestica. Strong correlation between skull length and age is found, but from 27 days after birth onward, there is no correlation with age among most dimensions of the inner ear. Adult dimensions of several of the inner ear structures are achieved before the inner ear is functional in M. domestica. Morphological variation within the inner ear of several eutherian mammals from the Cretaceous of Asia, including zhelestids from the Bissekty Formation of Uzbekistan, is described. The variation within the fossil sample is compared to that observed within extant species of placental mammals, and it is determined that the amount of variation within the Bissekty zhelestid population is within the range of that measured for extant species. Additional evolutionary and physiological patterns preserved within the walls of the bony labyrinth are identified through a high level anatomical comparison of the inner ear cavities across Placentalia as a whole. In particular, features of the inner ear support monophyly of Cetacea, Carnivora, Primatomorpha, and caviomorph Rodentia. The volumetric percentage of the vestibular apparatus (vestibule plus semicircular canals) of aquatic mammals is smaller than that calculated for terrestrial relatives of comparable body size. Thus, aspects of the bony labyrinth are both phylogenetically and physiologically informative. / text

L'étude des caractères basicrâniens et endocraniens chez les vertébrés fossiles et actuels connait un regain d'intérêt depuis quelques années. Ces caractères bénéficient en effet d'une bonne réputation en tant que marqueurs phylogénétiques, ainsi que d'un biais taphonomique favorable à leur fossilisation. C'est pourquoi dans ce volume, la région auditive et l'oreille interne (Partie I) et l'endocrâne (Partie II) de mammifères fossiles appartenant au clade des Afrotheria ont été scannés par (micro)tomographie à rayon-X, décrits, étudiés et comparés à ceux des espèces actuelles. Il s'agit plus précisément des Afrotheria fossiles que sont les Bibymalagasia (Chapitre 1), le Macroscelididae Chambius (Chapitre 2), de nombreux Hyracoidea du Paléogène (Chapitre 3) ainsi que des Tethytheria basaux tels que le proboscidien Numidotherium, l'embrithopode Arsinoitherium et le sirénien Prorastomus (Chapitre 4). Cette étude met en lumière l'intéressant signal phylogénétique fournit par les caractères de la région auditive et de l'endocrâne, capable de soutenir certains clades moléculaires (e.g. Afroinsectivora, ‘Panelephantulus'). Elle montre aussi comment, sous certaines pressions de sélection, ces caractères peuvent converger de façon remarquable (e.g. chez les téthythères). Cette étude apporte aussi des éléments qui appuient l'hypothèse d'une origine Africaine des Afrotheria, ainsi que celle faisant de l'ancêtre commun des afrothères un mammifère plutôt ‘ongulé' que ‘insectivore'. Ces inférences fondées sur l'observation des représentants fossiles les plus anciens de ce groupe sont importants pour notre compréhension de l'origine et de la diversification des afrothères et des mammifères placentaires en général. / The study of basicranial and endocranial characters in vertebrate paleontology has recently known a regain of interest. Indeed, these characters are renowed for their phylogenetic stability and are usually easily preserved in the fossil record. Here, the auditory region (Part I) and endocranium (Part II) of some fossil representatives of the mammalian order Afrotheria have been scanned using X-ray (micro)tomography, described and compared to their modern day's counterparts. These fossil afrotheres are the Bibymalagasia (Chapter 1), the stem sengi Chambius (Chapter 2), numerous Hyracoidea (Chapter 3) and stem Tethytheria such as the proboscideans Numidotherium, the embrithopod Arsinoitherium and the sirenian Prorastomus (Chapter 4). This study highlights the strong phylogenetic signal carried by the characters of the auditory region and endocast, which could even support some molecular clades (e.g. Afroinsectivora, ‘Panelephantulus'). It also shows that convergences can occur under certain selective pressure (e.g. in Tethytheria). Finally this study brings new clues that support the hypothesis of an African origin of Afrotheria as well as that of an ‘ungulate-like' instead of ‘insectivore-like' ancestor. These inferences based on the observation of the oldest and basal-most fossil representatives of this group are crucial to understand the origin, evolution and diversification of afrotherian and placental mammals across the Cenozoic.

Les éléphants font partie des rares mammifères capables d’entendre les infrasons et leur comportement locomoteur est unique. Ces singularités correspondent à des spécialisations de leurs organes sensoriels contenus dans l’oreille interne : la cochlée (audition) et le système vestibulaire (locomotion). Alors que la diversité actuelle des proboscidiens est très faible (trois espèces), ce groupe a été bien plus diversifié pendant les 60 millions d’années qui composent son histoire. Cette thèse étudie pour la première fois de façon complète la morphologie et la fonction de la région de l’oreille (périotique, labyrinthes osseux et membraneux) d’éléphants actuels grâce aux techniques CT scan 3D. De plus, les périotiques de quatorze genres de proboscidiens fossiles sont décrits afin de documenter l’évolution de ce complexe anatomique au sein de cet ordre de mammifères. Les résultats montrent que certains caractères de l’oreille sont très variables au niveau spécifique. Notre analyse inclut les plus anciens proboscidiens connus et suggère que la morphologie de la région otique a évolué graduellement pendant la première moitié de l’histoire évolutive du groupe et que le morphotype moderne éléphantin est déjà acquis chez les Deinotheriidae et généralisé chez les éléphantimorphes. Les inférences sur la locomotion et l’audition des taxons fossiles confirment ces observations. Ce travail permet ainsi de lever le voile sur l’évolution d’une région anatomique majeure, jusque-là méconnue chez un groupe emblématique de mammifères. / Elephants are among the few mammals able to hear infra-sounds, and they display a unique locomotor behavior. It corresponds to specializations of their sensory organs contained in the inner ear: the cochlea (audition) and the vestibular system (spatial orientation). While only three species are living today, they were a much more diverse group found in five continents and with a 60 Ma-long history. We provide here the first comprehensive morphological and functional study of the ear region (petrosal, bony and membranous labyrinths) of extant elephants using 3D CT scan techniques. Additionally, we describe and compare the petrosals of fourteen extinct proboscidean genera in order to shed light on the evolution of this anatomical complex in the Proboscidea. The results show that some features of the petrosal and bony labyrinth of extant elephants - such as the number of turns of the cochlea – display a noticeable level of intra-specific variability. Our analysis includes the earliest-known proboscideans and suggests that the petrosal and bony labyrinth morphology evolved gradually during the first half of the proboscidean evolutionary history, but also that the modern morphotype exhibited in elephants was probably already acquired in deinotheriids and generalized in elephantimorphs. Functional inferences on the locomotor behavior and the audition of extinct proboscideans confirm these observations. This work hence provides new insights on the evolution of a major anatomical region hitherto poorly known in an emblematic group of mammals.