Pygostylia is a group of birds which includes Confuciusornis

Confuciusornis

Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago...

and all of the more derived birds; the Ornithothoraces

Ornithothoraces

Ornithothoraces is a clade of birds which includes all enantiornithines and modern birds .The name Ornithothoraces means "bird thoraxes". This refers to a modern, highly derived, anatomy of the thorax which gave the ornithothoracines superior flight capability compared to more primitive birds...

. Chiappe (1997) defined the Pygostylia as "the common ancestor of the Confuciusornithidae

Confuciusornithidae

Confuciusornithidae is a family of primitive birds from the early Cretaceous Period of China. It consists of four genera. About half of all the confuciusornithid specimens, including reprersentatives of all species, that have preserved feathers possess a pair of distinctive ribbon-like tail...

 and Neornithes (Ornithurae

Ornithurae

Ornithurae is the name of a natural group which includes all modern birds as well as their extinct relatives with plough-shaped pygostyles, a bone at the end of the tail which allows the tail feathers to fan and retract....

) plus all its descendants". This is a node-based definition.

Chiappe (2001) united the Pygostylia in possessing four unambiguous synapomorphies. The trait that gives the group its name is the presence of a pygostyle

Pygostyle

Pygostyle refers to a number of the final few caudal vertebrae fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these....

. Next is the absence of a hyposphene - hypantrum. Next is a retroverted pubis

Pubis (bone)

In vertebrates, the pubic bone is the ventral and anterior of the three principal bones composing either half of the pelvis.It is covered by a layer of fat, which is covered by the mons pubis....

 separated from the main axis of the sacrum

Sacrum

In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...

 by an angle of 45 to 65 degrees. Last is a bulbous medial

Medial

- Medial magmas :In abstract algebra, a medial magma is a set with a binary operation which satisfies the identity \cdot = \cdot , or more simply, xy\cdot uv = xu\cdot yv...

 condyle of the tibiotarsus

Tibiotarsus

The tibiotarsus is the large bone between the femur and the tarsometatarsus in the leg of a bird. It is the fusion of the proximal part of the tarsus with the tibia.A similar structure also occurred in the Mesozoic Heterodontosauridae...

.

The pygosylians fall into two distinct groups with regard to the pygostyle. The Ornithothoraces have a ploughshare

Ploughshare

A ploughshare or plowshare is a component of a plough or plow.Ploughshare or Plowshare may also refer to:* Operation Plowshare, United States project to use nuclear explosives for civilian purposes* vomer, facial bone in the human skull...

-shaped end, while the more primitive members have a longer, rod-shaped pygostyle.