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. Neornithes was originally proposed as a replacement for Ornithurae by
Gadow in 1892 and 1893. Gauthier and de Queiroz, therefore, consider Neornithes a junior synonym of Ornithurae,
Relationships The
cladogram below is the result of a 2017 analysis by McLachlan and colleagues. }} ==References==