Pygostylia

Pygostylia is a group of birds which includes Confuciusornis and all of the more derived birds; the Ornithothoraces. Chiappe (1997) defined the Pygostylia as "the common ancestor of the Confuciusornithidae and Neornithes (Ornithurae) 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. Next is the absence of a hyposphene - hypantrum. Next is a retroverted pubis separated from the main axis of the sacrum by an angle of 45 to 65 degrees. Last is a bulbous medial condyle of the tibiotarsus.

The pygosylians fall into two distinct groups with regard to the pygostyle. The Ornithothoraces have a ploughshare-shaped end, while the more primitive members have a longer, rod-shaped pygostyle.

The earliest known member of the group is the enantiornithine species Protopteryx fengningensis, from the Sichakou Member of the Huajiying Formation of China, which dates to around 131 Ma ago, though at least one other enantiornithine, Noguerornis, may be even older, at up to 145.5 million years ago, though its exact age is uncertain.