A new Lower Cretaceous bird from China and tooth reduction in early avian evolution.

A new avian genus and species, Zhongjianornis yangi gen. et sp. nov., is reported from the Lower Cretaceous lacustrine deposits of the Jiufotang Formation in Liaoning, northeast China. The new taxon is characterized by possessing the following combination of features: upper and lower jaws toothless, snout pointed, humerus with large and robust deltopectoral crest, second phalanx of the major manual digit longer than the first phalanx, unguals of the alular and major digits of similar length and significantly shorter than the corresponding penultimate phalanges, tibiotarsus slender and more than twice the length of the tarsometatarsus, and metatarsal IV longer than the other metatarsals. Phylogenetic analysis indicates that Zhongjianornis is phylogenetically basal to Confuciusornis and the dominant Mesozoic avian groups, Enantiornithes and Ornithurae, and therefore provides significant new information regarding the diversification of birds in the Early Cretaceous. It also represents the most basal bird that completely lacks teeth, suggesting that tooth loss was more common than expected in early avian evolution and that the avian beak appeared independently in several avian lineages, most probably as a response to selective pressure for weight reduction. Finally, the presence of a significantly enlarged humeral deltopectoral crest suggests that Zhongjianornis shares with other basal birds such as Jeholornis, Sapeornis and Confuciusornis a distinctive mode of adaptation for flight contrasting with that seen in more advanced birds, which instead possess an elongated sternum and a prominent keel.