Pygostyles probably began to
evolve very early in the
Cretaceous period, perhaps 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 as found in species such as
Archaeopteryx is detrimental to its use for flight control. Modern birds still develop longer caudal vertebrae in their
embryonic 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 show traces of 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. The related
Similicaudipteryx, described in 2008, also had a rod-like pygostyle, associated with a fan of tail feathers. '', one of the oldest known species with a pygostyle like that in living birds 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 maneuvering. 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
Hongshanornis longicresta. As evidenced by the
oviraptorosaurian cases, the pygostyle
evolved at least twice, and rod-shaped pygostyles seem to have evolved several times, in association with shortening of the tail but not necessarily with a retractable fan of tail feathers. In other words, the pygostyles of oviraptorosaurs and
Confuciusornis were likely weight-saving measures, and the specialized "true" pygostyles of ornithurans were adapted from these later to improve flight performance. 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. ==Etymology==