Ornithurae

Ornithurae (meaning 'bird tails' in Greek) is the name of a natural group which includes all modern birds as well as their extinct relatives with plough-shaped pygostyles (†Ichthyornis, †Hesperornithes, †Limenavis). This is a bone at the end of the tail which allows the tail feathers to fan and retract. "Gansus", from the Lower Cretaceous of China is the earliest known at 110 million years ago.
Simple classification: