Pygostyle

Pygostyle refers to a number of the final few caudal vertebrae fused into a single ossification, supporting the tail feathers and musculature.

Evolution
Pygostyles probably started to evolve very early in the Cretaceous, maybe 140-130 million years ago. The earliest known species to have evolved a pygostyle were members of the Confuciusornithidae. The structure provided an evolutionary advantage, as a completely mobile tail like in Archaeopteryx is detrimental to its use for flight control. Modern birds still develop longer caudal vertebrae in their embryonal state, which later fuse to form a pygostyle.

There are two main types of pygostyle: one, found in Confuciusornithidae, Enantiornithes, and some other Mesozoic birds, as well as in some oviraptorosaurs like Nomingia, is long and rod- or dagger blade-like. None of the known fossils with such pygostyles that also retain feather traces had well-developed rectrices. The tail feathers in these animals consisted of downy fuzz and sometimes 2-4 central "streamers" such as those found in some specimens of Confuciusornis or in Paraprotopteryx.

By contrast, the function of the pygostyle in the terrestrial Nomingia is not known. It is notable however that its older relative Caudipteryx had no pygostyle but a "fan" of symmetrical feathers which were probably used in social display. Perhaps such ornaments were widespread in Caenagnathoidea and their relatives, and ultimately the oviraptorosaurian pygostyle evolved to help support them. However, few oviraptorosaurs were found in sediment fine-grained enough to permit fossilization of feathers, and Nomingia is not among these. The related Similicaudipteryx, described in 2008, also had a rod-like pygostyle, though it does not retain feather traces either.

The other pygostyle type is plowshare-shaped. It is found in Ornithurae (living birds and their closest relatives), and in almost all flying species is associated with an array of well-developed rectrices used in maneuvring. The central pair of these attach directly to the pygostyle, just as in Confuciusornis. The other rectrices of Ornithurae are held in place and moved by structures called bulbi rectricium (rectricial bulbs), a complex feature of fat and muscles located on either side of the pygostyle. The oldest known species with such a pygostyle is Yixianornis grabaui. This bird lived at the same time as Confuciusornis (around 125-120 million years ago), supporting the theory that the two pygostyle types of birds evolved independently.

As evidenced by the oviraptorosaurian cases, the pygostyle evolved at least twice; as indicated by the differences between Enantiornithes and modern birds, it may well have evolved thrice. In other words, the pygostyle of Nomingia and that of a chicken is a homoplasy, whereas that of Enantiornis and a chicken is either a homoplasy too or a synapomorphy.

The bird clade Pygostylia was named in 1996, by Luis Chiappe, for the presence of this feature and roughly corresponds to its appearance in the bird family tree, though the feature itself is not included in its definition. In 2001, Jacques Gauthier and Kevin de Queiroz (2001) re-defined Pygostylia to refer specifically to the apomorphy of a short tail bearing an avian pygostyle.