前肢仍有三个发育的指爪,胸骨无龙骨突,肱骨有一大气囊孔等等(The most primitive bird except Arcbaeopteryx.Skull bones unfused with postorbital retained.Teeth lost,horny beak present.Forelimb with three well-developed claws.Sternum keel-less.Humerus with alarge pneumatic fenestra.)。