Pygostylia is a group of

avialan Avialae ("bird wings") is a clade containing the only living dinosaurs, the birds. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to deinonychosaurs, though alternative definitions are occasionally used ...

s which includes the

Confuciusornithidae Confuciusornithidae is an extinct family of pygostylian avialans known from the Early Cretaceous, found in northern China. They are commonly placed as a sister group to Ornithothoraces, a group that contains all extant birds along with their clo ...

and all of the more advanced species, the Ornithothoraces.

Definition

The group Pygostylia was intended to encompass all avialans with a short, stubby tail, as opposed to the longer, unfused tails of more primitive species like '' Archaeopteryx lithographica''. It was named by

Sankar Chatterjee Sankar Chatterjee (born May 28, 1943) is a paleontologist, and is the Paul W. Horn Professor of Geosciences at Texas Tech University and Curator of Paleontology at the Museum of Texas Tech University. He earned his Ph. D. from the University o ...

in 1997.Chatterjee, S. 1997. ''The Rise of Birds.'' Baltimore: Johns Hopkins University Press. 312 pp. Luis Chiappe later defined Pygostylia as a node-based

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

, "the common ancestor of the

and

Neornithes Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

plus all its descendants". In 2001,

Jacques Gauthier Jacques Armand Gauthier (born June 7, 1948 in New York City) is an American vertebrate paleontologist, comparative morphologist, and systematist, and one of the founders of the use of cladistics in biology. Life and career Gauthier is the so ...

and

Kevin de Queiroz Kevin de Queiroz is a vertebrate, evolutionary, and systematic biologist. He has worked in the phylogenetics and evolutionary biology of squamate reptiles, the development of a unified species concept and of a phylogenetic approach to biologica ...

recommended that Chatterjee's original apomorphy-based clade concept be used instead of Chiappe's node-based definition,Gauthier, J., & De Queiroz, K. (2001). Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name" Aves. ''New perspectives on the origin and early evolution of birds'', 7-41. but this recommendation has been inconsistently followed. Luis Chiappe and co-authors continue to use Chiappe's definition, often attributing authorship of the name to Chiappe 2001 or Chiappe 2002 rather than to Chatterjee. Cladogram following the results of a phylogenetic study by Jingmai O'Connor and colleagues in 2016:O’Connor, J.K., Wang X., Zheng X., Hu H., Zhang X., & Zhou Z. (2016). An Enantiornithine with a Fan-Shaped Tail, and the Evolution of the Rectricial Complex in Early Birds. ''Current Biology'', 26(1): 114-119.

Description

Chiappe noted that under his definition, all members of the Pygostylia share four unique characteristics. The trait that gives the group its name is the presence of a

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. In modern birds, the rectrices attach to these. The pygostyle is the main compone ...

, or set of fused vertebrae at the end of the tail. Next is the absence of a hyposphene - hypantrum. Next is a reversed

pubic bone In vertebrates, the pubic region ( la, pubis) is the most forward-facing ( ventral and anterior) of the three main regions making up the coxal bone. The left and right pubic regions are each made up of three sections, a superior ramus, inferior ...

separated from the main axis of the

sacrum The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part o ...

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

condyle A condyle (;Entry "condyle"
in
tibiotarsus (lower leg bone). The pygostylians fall into two distinct groups with regard to the pygostyle. The Ornithothoraces have a

ploughshare In agriculture, a plowshare ( US) or ploughshare ( UK; ) is a component of a plow (or plough). It is the cutting or leading edge of a moldboard which closely follows the coulter (one or more ground-breaking spikes) when plowing. The plowshar ...

-shaped pygostyle, while the more primitive members had longer, rod-shaped pygostyles. The earliest known member of the group is the

enantiornithine The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth and cla ...

species ''

Protopteryx fengningensis ''Protopteryx'' is an extinct bird and the most basal enantiornithean, from the Cretaceous period.Zhou, Zhonghe. (2002). "A new and primitive enantiornithine bird from the Early Cretaceous of China." ''Journal of Vertebrate Paleontology'', 22(1): ...

'', from the Sichakou Member of the

Huajiying Formation The Huajiying Formation is a geological formation in Hebei, People's Republic of China. Known for its fossils including feathered dinosaurs, the age of the formation is uncertain. It may represent an early portion of the Jehol Biota, dating to som ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

, which dates to around 131 Ma ago,O'Connor, J.K., Zhou Z. and Zhang F., 2011. A reappraisal of ''Boluochia zhengi'' (Aves: Enantiornithes) and a discussion of intraclade diversity in the Jehol avifauna, China. ''Journal of Systematic Palaeontology'', (published online before print 16 December 2010). though at least one other enantiornithine, ''

Noguerornis ''Noguerornis'' is a genus of enantiornithine bird possibly related to '' Iberomesornis''. It lived during the Early Cretaceous (early Barremian age) about 130 mya and is known from fossils found in the La Pedrera de Rúbies Formation at El Mon ...

'', may be even older, at up to 145.5 million years ago, though its exact age is uncertain.Holtz, Thomas R. Jr. (2012) ''Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages,'
Winter 2011 Appendix.
/ref>

References

{{Taxonbar, from=Q135800 Avialans Extant Early Cretaceous first appearances Tetrapod unranked clades Taxa named by Sankar Chatterjee