Webbed foot of an Early Cretaceous ornithurine bird Gansus from China

Gansus yumenensis is the first Mesozoic bird found in China. It was collected from the Xiagou Formation in northwestern Gansu, which is considered to be equivalent to the Early Cretaceous strata yielding the Jehol biota in western Liaonlng. Based on some characters such as elongated phalanges and short claws with pointed flexor tubercles, Gansus yurnenensis is regarded as the bird adapted to life in the aquatic environment. Gansus yurnenensis was originally erected only based on a unique specimen of the left pes, and some specimens have been found for a further study recendy. In this paper, a newly discovered complete hindlimb of Gansus yurnenensis is described, which shows the clear webbed structures among the distal portions of pedal phalanges. It is the first direct evidence to show that Gansus yurnenensis with well-developed footwebs is really the oldest known bird specifically well-adapted to life in the aquatic environment in the world.