Avialae

Avialae is the bird-line clade of theropod dinosaurs, encompassing the only living dinosaurs, the birds, together with their closest extinct relatives. It is usually defined as all theropods more closely related to modern birds than to the deinonychosaurs, and in most recent analyses it forms the sister group of Deinonychosauria (Xu and colleagues, 2011). Its members range from early forms such as Archaeopteryx and the anchiornithid-grade taxa of the late Jurassic, known from Germany and from the roughly 160-million-year-old deposits of China, through the pygostylian and ornithothoracine birds. Avialans diversified into a wide variety of forms during the Cretaceous, when many still retained primitive traits such as clawed wings and toothed jaws. Over the course of their evolution the long bony tail of their ancestors was progressively shortened, and the group as a whole shows a trend toward feathered wings used in flapping flight.