The Rise of Modern Avian Flight: Wing Movement and Joint Mobility Within Ornithurae

How and when avian powered flight arose has intrigued scientists for well over a century. Birds have a remarkable fossil record demonstrating their macroevolutionary transition from dinosaurs to birds during the Mesozoic Era. Thus, Mesozoic fossils offer us a unique opportunity to study the evolution of avian flight. The origin of many bird-like characteristics (e.g., feathers, a toothless beak, small body size) among non-avian theropod dinosaurs and Mesozoic bird-like taxa are now well understood. Yet, despite more than a century of research, the origin and evolution of avian flight itself remain poorly understood and the precise timing and mechanisms behind the emergence of active, flapping flight remain elusive. This thesis aims to quantitatively resolve the ancestral flying capabilities of crown birds and elucidate the emergence of avian flight through modelling of the flight performance of *Ichthyornis dispar*, which can provide crucial insights into the evolution of flight in birds due to its phylogenetic position just outside the avian crown. It is essential to assess the ancestral morphologies and flying capabilities of such stem birds to investigate the evolution of the different morphologies and the associated locomotor specialisations in the pectoral girdle of living birds. I developed a novel technique for the retrodeformation of taphonomically deformed fossils to reconstruct the remains of *Ichthyornis*. To decipher a link between 3D wing joint mobility and the flight ecology in living birds, I captured and analysed the 3D wing kinematics of bird cadavers representing nine species through biplanar fluoroscopy (XROMM) and quantified their *ex vivo* movement envelopes. I developed a new quantitative framework for joint mobility analyses in a phylogenetic context: the *EcoPhyloMobilitySpace*. This enabled me to investigate links between flight ecology and joint mobility in living birds and trace the evolution of their locomotor specialisations through time. I projected the simulated range of motion envelopes of *Ichthyornis* into the data of extant birds to infer its locomotory behaviour and retrace the evolution of bird flight ecology. The results highlight that a flight analogue to modern seabirds was already achieved in the Cretaceous period before other bird characteristics, such as a toothless beak, evolved.

European Association of Vertebrate Palaeontologists (EAVP) Research Grant