TL;DR: A new, exquisitely preserved, bird from the Lower Cretaceous Konservat-Lagerstätte of Las Hoyas (Cuenca, Spain) which provides evidence for the oldest known alula (bastard wing) and the recognition of a new enantiornithine taxon, Eoalulavishoyasi.
Abstract: AVIAN flight is one of the most remarkable achievements of vertebrate evolution, yet there is little evidence of its early phases. Specimens of Archaeopteryx shed important (albeit controversial) light on this evolutionary phenomenon, but the large morphological (and almost certainly functional) gap between Archaeopteryx and modern avians remained virtually empty until recently. Here we report a new, exquisitely preserved, bird from the Lower Cretaceous Konservat-Lagerstatte of Las Hoyas (Cuenca, Spain) which provides evidence for the oldest known alula (bastard wing). Crustacean remains found inside its belly also provide the oldest direct evidence of feeding habits in birds. The new specimen has numerous synapomorphies with the Enantiornithes, but its unique sternal morphology, along with other autopomorphies in the furcula and vertebral centra, support the recognition of a new enantiornithine taxon, Eoalulavishoyasi. The combination in Eoalulavis of a decisive aerodynamic feature, such as the alula, with the basic structures of the modern flight apparatus indicates that as early as 115 million years ago, birds had evolved a sophisticated structural system that enabled them to fly at low speeds and to attain high manoeuvrability.
TL;DR: The fossil record provides a general view of the morphological changes of the avian flight apparatus from non-volant ancestors to the first derived fliers of the Early Cretaceous.
Abstract: Information on the first steps of the avian evolutionary history has dramatically increased during the last few years. The fossil record provides a general view of the morphological changes of the avian flight apparatus from nonvolant ancestors (non-avian theropod dinosaurs) to the first derived fliers of the Early Cretaceous. The Las Hoyas bird record includes three genera: Iberomesornis, Concornis and Eoalulavis. This fossil material has yielded information about the early avian evolutionary history. These Early Cretaceous birds (some 120 Myr old) had a wingbeat cycle and breathing devices similar to those of extant birds. The function of the rectricial fan was also similar. In the evolutionary transition from cursorial ancestors to derived fliers it is possible to verify a trend to increase lift. Primitive wing aspect ratio morphotypes were elliptical ones, other derived morphotypes appeared, for example, in the Neornithes (extant birds). Some primitive fliers, like the Las Hoyas genus Eoalulavis, had an alula (feathers attached to the first digit of the hand) similar to that of present day birds, indicating braking and manoeuvring skills similar to those of their extant relatives. Primitive avian life habits are poorly understood. Some evidence from the Las Hoyas bird record indicates that Early Cretaceous birds were present in the trophic chains.