Ornithurae

Ornithurae (meaning "bird tails" in Greek) is the name of a natural group of birds coined by Ernst Haeckel in 1866.

Haeckel 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 tails, while Archaeopteryx has a long tail like that of theropod dinosaurs.

Gauthier converted Ornithurae into a clade, defining it as a stem-based taxon: "extant birds and all other taxa, such as Ichthyornis and Hesperornithes, that are closer to extant birds than is Archaeopteryx". This clade includes the first panavian with a "bird tail", and all of its descendants. He 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.

Gauthier included Aves (which he defines as the "crown group" of modern birds, alternately Neornithes), Ichthyornis, Hesperornithes, and Apsaravis in Ornithurae.

Neornithes was originally proposed as a replacement for Ornithurae by Gadow in 1892 and 1893. Gauthier, therefore, considers Neornithes a junior synonym for Ornithurae, though many other scientists use Neornithes to refer to the much more restrictive crown group consisting only of modern birds (a group for which Gauthier uses the name Aves). Alternately, some researchers have used Ornithurae to refer to a much more restrictive, node-based clade, anchored on Hesperornis, Ichthyornis, and modern birds.

Clarke at al. (2006) found that Yixianornis, Songlingornis, and Yanornis form an unnamed clade which is the most basal group in the Ornithurae (following the restrictive definition; alternately, Ornithuromorpha). They found that this group has a mosaic of advanced and primitive features. These three taxa retain primitive features like gastralia and a pubic symphysis. They also show the first fully modern pygostyles, and the type specimen of Yixianornis (IVPP 13631) preserves eight elongated rectrices in a modern arrangement. No lower pygostylians are known which preserve a fan of rectrices of this sort; instead they show only paired plumes or a tuft of short feathers.