A New Enantiornitine Bird with Four Long Rectrices from the Early Cretaceous of Northern Hebei,China

Paraprotopteryx gracilis, a new enantiornithine bird from the Early Cretaceous Yixian Formation in Fengning, northern Hebei Province is erected, based on the following characters： Yshaped furcula with a long hypocleidum and a much narrow interclavicular angle, and the morphology of the sternum are different from other enantiornithines. Additionally, alular digit bearing the biggest manual claw extends distally to the distal end of the major metacarpal; the minor metacarpal is slender than the major metacarpal. Carpometacarpus only fused proximally; astragalus and calcaneum partially fused to one another but unfused to the tibia. This is the first record of Mesozoic birds in having four long rectrices, which may represent morphologically a secondary sexual character, an intermediate stage from elongated scale to branched feather, and possess functional advantage in supplementing the lifting surface to compensate the unskilled flight.