Article: A new bird from the Early Cretaceous of Las Hoyas, Spain, and the early radiation of birds

Concornis lacustris gen. et sp. nov. is a new fossil bird from the Early Cretaceous (Barremian) of Las Hoyas, Cuenca, Spain. Concornis is roughly twice the size of Iberomesornis, the first bird described from Las Hoyas, but it is not the adult of the latter. Concornis is more derived than Iberomesornis, with a true tibiotarsus and a fan-like distal metatarsal zone with distinct trochlear structures. Only the proximal metatarsal region is fused. The sternum has a slight but conspicuous posterior median keel. The phalangeal structure of the manus is like that of extant birds. The metacarpus is not distally fused. Concornis is one of the closer known sister taxa of the Ornithurae. This sister group plus Iberomesornis form the clade Euornithes, diagnosed by a strut-like coracoid, a derived avian furcula and a pygostyle. The more probable topological structure within the Euornithes is (Iberomesornis + (Concornis+ (Ambiortns+(Enantiornathes + Ornithurae)))). There is increasing evidence for an adaptative radiation of birds during the Early Cretaceous. After feathers appeared, the first stage of the radiation was the rapid evolution of active flight structures. Later transformations probably involved refinement of the flight apparatus (tail reduction, development of the sternum) that altered the centres of gravity and balance. The synsacrum evolved as the maximum tail reduction was reached, and can be envisaged as a new flight functional unit. Based on a ' legginess index' in extant birds, we suggest an aquatic habitat for Las Hoyas birds.