Ornithurae

Ornithurans Temporal range: Late Cretaceous - Present, 99.6–0 Ma PreЄ Є O S D C P T J K Pg N
Cast skeleton of Ichthyornis dispar, Rocky Mountain Dinosaur Resource Center
Australian brushturkey (Alectura lathami)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Ornithuromorpha
Clade:	Ornithurae Haeckel, 1866
Subgroups
†Apatornis †Guildavis †Iaceornis †Ichthyornis †Limenavis †Qinornis? †Hesperornithes Neornithes

Ornithurae (meaning "bird tails" in Greek) is a natural group which includes the common ancestor of Ichthyornis, Hesperornis, and all modern birds as well as all other descendants of that common ancestor.

Ernst Haeckel coined the name in 1866 and included in the group all "true birds" with the "characteristic tail morphology of all extant birds" (translation by Jacques Gauthier). This distinguishes the group from Archaeopteryx, which Haeckel placed in another new group called Sauriurae. Said simply, modern birds have short, fused pygostyle tails, while Archaeopteryx retained the long tail characteristic of non-avian theropod dinosaurs.

Gauthier converted Ornithurae into a clade, giving it a branch-based definition: "extant birds and all other taxa, such as Ichthyornis and Hesperornithes, that are closer to extant birds than is Archaeopteryx". Later he and de Queiroz redefined it as an apomorphy-based clade more in keeping with Haeckel's original usage, including the first pan-avian with a "bird tail" homologous with that of Vultur gryphus, and all of its descendants. They defined "bird tail" as a tail that is shorter than the femur, with a pygostyle that is a ploughshare-shaped, compressed element, with the bones fused in the adult, composed of less than six caudal vertebrae, and shorter than the free part of the tail, which itself is composed of less than eight caudal vertebrae. They included Aves (which they defined as the "crown group" of modern birds), Ichthyornis, Hesperornithes, and Apsaravis in Ornithurae.