Pygostyle

Pygostyle describes a skeletal condition in which the final few caudal vertebrae are fused into a single ossification, supporting the tail feathers and musculature.

[citation needed] 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.

There are two main types of pygostyle:[citation needed] 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.

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 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.

[4] 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.

Because the cut has a very high fat content, it was banned from 2007 to 2013 to combat obesity, only allowed back when Samoa joined the World Trade Organization.