All living birds are Neornithes, a subclass in traditional taxonomy. The Ornithurae is the natural group made up of modern birds and their extinct relatives. Their relatives include "Ichthyornis" and "Hesperornis", and some of the Chinese Cretaceous birds. They all have a plough-shaped pygostyle, a bone at the end of the tail which allows the tail feathers to fan and retract. Other groups of birds became extinct in the K/T extinction event.