Abstract
The discovery by both molecular biology and palaeontological data that cetaceans are artiodactyls constitutes one of the major breakthroughs in mammal’s evolutionary history of the past 30 years. However, no consensus has yet been reached regarding the basal relationship within the enlarged Artiodactyla clade and major questions of its evolutionary history remain to be solved. This thesis explores a promising source of phylogenetic characters: the auditory region (petrosal bone, tympanic bulla, middle ear ossicles, inner ear) from the new perspectives offered by µCT Scan imaging.The main objectives of this thesis are (1) to determine the phylogenetic signal carried by the auditory region in artiodactyls in order to provide a new source of characters to the analyses and (2) to explore the ecological signal carried by the different elements of this sensory region dedicated to hearing (outer ear, middle ear and cochlear canal of the bonny labyrinth) and to equilibrioception (vestibule and semicircular canals of the bony labyrinth).The first part of this thesis (I) brings us to Togo, where many fossil remains of the auditory region of ancient “legged whales” (Protocetida Stromer 1908) have been collected. From an anatomical viewpoint, these fossil remains document a nearly complete petrotympanic complex and allowed us to describe for the first time, the stapes, incus and bony labyrinth of a protocetid whale, which are crucial elements to understand their hearing. Morphofunctional analysis indicates that optimal hearing was probably possible both in air and underwater for these semi-aquatic whales. In addition, the morphology of their cochlea indicates that their hearing ability was close to that of their terrestrial kin and that the specializations related to the remarkable hearing abilities of modern cetaceans (i.e. sensitivity to infra- or ultrasound) occurred after the historical separation between mysticetes and odontocetes.The second part of this work (II) focuses on the origins of amphibiosis in Cetancodonta, through the study of several fossil families, known for their potamophilous tendencies. The study of the auditory region of hippopotamoids (Anthracotheriidae + Hippopotamidae) reveals that adaptation to a semi-aquatic lifestyle has emerged several times (i.e. in a convergent way) in its evolutionary history and seems to indicate a terrestrial origin for this group. As for the raoellid Indohyus, its petro-tympanic complex presents a combination of features suggesting some degree of adaptation to the aquatic environment, but the functional study of its cochlea indicates that this taxon probably could not hear efficiently underwater.The last point of this thesis explores the phylogenetic potential of the auditory region through an analysis built upon morphological characters of the petrosal and bony labyrinth at Artiodactyla scale. For the first time, the results of our analysis are consistent with that of molecular analyses. Among the most notable points, the Cetancodonta clade is well supported by the morphology of the petrosal and Indohyus’ position strongly suggests that raoellids are cetaceans.Thus, the auditory region turns out to be an essential element from a phylogenetic and morphofunctional viewpoint. Indeed, as we have seen throughout this thesis, when the complex and multifaceted nature of the auditory region is apprehended as a whole, it allows to infer the ecology of a given taxon and to clarify its phylogenetic relationships. Thus, the auditory region is still far from having said its last words… and we are not done hearing about it yet.