GIRDLE ANATOMY OF AQUATIC CROCODYLOMORPHS

Nowadays, Crocodylomorpha is represented by twenty-eight species of semi-aquatic ambush predators with some of the more famous members including the saltwater crocodile, the gharial, and the American alligator. However, Crocodylomorpha was significantly more diverse in the past, as re ported by its extensive fossil record. Unfortunately, craniodental remains have received much more attention, due to either historical reasons (i.e. lack of collected materials other than craniodental parts) or general contempt. As a result, the rich and complex postcranial anatomy of crocody lomorphs is little-known, further shrouding the locomotive adaptations that possibly underpinned
their radiations and longevity. Recently, several works on different extinct clades of Crocody lomorpha have pointed out the necessity to refocus attention on postcranial anatomy in order to better understand both their lifestyle and their relationships. As such, this work tackles these issues and aspires to shed new light on the evolution of postcranial morphology of two extinct groups of aquatic crocodylomorphs: Thalattosuchia and Dyrosauridae. Thalattosuchia is a clade of secondarily aquatic crocodylomorph of uncertain phylogenetic origin. Thalattosuchia namely comprises the two very unique and distinct subclades: the mainly semi-aquatic Teleosauroidea superficially resembling modern gharials and Metriorhynchoidea, which became highly aquatic. Dyrosaurids
are nested within Neosuchia; they have relatively long ventral processes on their last cervical and anterior thoracic vertebra, and also possess markedly elongated neural spines in their thoracic region and all along their tail. Dyrosaurids also show a ventral and dorsal osteodermal shield and stout limb bones which may appear superficially similar to extant crocodylians at first glance.In this thesis, I thoroughly describe the postcranial anatomy (with a particular focus on girdles) of these clades in a comparative framework with extant crocodylians. I also use high-resolution 3D scanning and modelling to reconstruct the girdles of about 30 species. The main result of this thesis is that the postcranial anatomy of crocodylomorphs is distinctive and discriminating both
interclade and intraclade. In addition, it is shown that extinct crocodylomorphs have developed distinct strategies in order to invade the aquatic realm, which are also dissimilar from other iconic marine reptiles like plesiosaurians and aquatic cryptodirans. Those claims are supported by sev eral osteological observations and quantitative analyses (linear and 3D geometric morphometrics, phylogenetic inferences). Anatomical evidence, in particular proportions, suggest that the basal dyrosaurid Cerrejonisuchus improcerus was mostly terrestrial and had a unique lifestyle which
contrasts with that of other members of the clade. Thorough reinvestigation of the scapular pelvic girdle anatomy of Thalattosuchia and Dyrosauridae highlights the presence of major dissimilarities between and within those clades. These differences have functional implications, allowing me to postulate terrestrial walking and swimming capabilities. Geometric morphometrics analyses of the 3 dimensional models of the thoracic and pelvic girdles of Thalattosuchia and Dyrosauridae also reveals markedly distinct architectures between clades and clearly indicate that extant crocodylians are unsatisfactory functional analogues for ancient clades of Crocodylomorpha. The re-assessment of both thoracic and pelvic girdle anatomy also lead to the creation of 42 new postcranial phylogenetic characters along with the revision of several other postcranial characters taken in the literature. A new repurposed matrix centred on postcranial anatomy was built employing those characters. Results indicate that a clear phylogenetic signal resides in the postcranium of Crocodylomorpha. This signal partially differs from those recovered from cranial dominated datasets, further emphasizing the importance of postcranial anatomy in understanding the relations of extinct crocodylomorphs, in addition to their development, ecology, and biomechanics.