Liaoxiornis delicatus gen. et sp nov., the smallest Mesozoic bird

Recently, a very small and complete bird specimen was collected from the Confuciusornis-bearing Yixian Formation. This finding represents a new kind of Mesozoic bird, named Liaoxiornis delicatus. It is characterized by a short skull with teeth, a long neck and a very small sternum with only posterior process; the humeral head is not internally curved, and the hindlimbs are longer than the forelimbs. Liaoxiornis delicatus is not only much smaller than Confuciusornis, but also smaller than Cathayornis and other early Cretaceous birds. It is the smallest bird known from Mesozoic deposit. Liaoxiornis provides new evidence indicating that the early adaptive radiation of birds is complicated.