32-537: Pygostylia is a group of avialans which includes the Confuciusornithidae and all of the more advanced species, the Ornithothoraces . 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 in 1997. Luis Chiappe later defined Pygostylia as
64-708: A phylogenetic tree . In the taxonomical literature, sometimes the Latin form cladus (plural cladi ) is used rather than the English form. Clades are the fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual, a population , or a species ( extinct or extant ). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups. Over
96-479: A "ladder", with supposedly more "advanced" organisms at the top. Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming , this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a rank can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, phylogenetic nomenclature has been developed; it
128-839: A better sense of smell. The following cladogram is based on the analysis by Hartman et al . (2019), which found flight likely evolved five separate times among paravian dinosaurs, two of those among Avialae (in Scansoriopterygids and other avialans). Archaeopteryx and "anchiornithids" were placed in Deinonychosauria, Avialae's sister group. † Archaeopterygidae † Unenlagiidae † Dromaeosauridae † Troodontidae † Scansoriopterygidae † Yandangornis † Bauxitornis † Balaur † Shenzhouraptor † Jixiangornis † Zhongornis † Sapeornis † Confuciusornithidae † Changchengornis † Chongmingia † Jinguofortis † Zhongjianornis Ornithothoraces In
160-623: A clade can be described based on two different reference points, crown age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils , or from molecular clock estimates. Viruses , and particularly RNA viruses form clades. These are useful in tracking
192-426: A definition similar to "all theropods closer to birds than to Deinonychus ." A nearly identical definition, "the theropod group that includes all taxa closer to Passer than to Dromaeosaurus ", was used by Agnolín and Novas (2013) for their clade Averaptora , operating under the assumption that troodontids and birds were more closely related to each other than to dromaeosaurs. They also redefine Avialae as
224-443: A node-based clade , "the common ancestor of the Confuciusornithidae and Neornithes plus all its descendants". In 2001, Jacques Gauthier and Kevin de Queiroz recommended that Chatterjee's original apomorphy-based clade concept be used instead of Chiappe's node-based definition, but this recommendation has been inconsistently followed. Luis Chiappe and co-authors continue to use Chiappe's definition, often attributing authorship of
256-422: A revised taxonomy based on a concept strongly resembling clades, although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley . German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics . He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on
288-428: A study conducted in 2020, Archaeopteryx was recovered as an avialan. [REDACTED] [REDACTED] Clade In biological phylogenetics , a clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group , is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on
320-429: A suffix added should be e.g. "dracohortian". A clade is by definition monophyletic , meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics ,
352-410: Is also occasionally defined as an apomorphy-based clade (that is, one based on derived characteristics that were not present among lineage predecessors). Jacques Gauthier , who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight , and the birds that descended from them. The clade Avialae was given a formal phylogenetic definition in
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#1733106816820384-499: Is also used with a similar meaning in other fields besides biology, such as historical linguistics ; see Cladistics § In disciplines other than biology . The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis , the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch . Many commonly named groups – rodents and insects , for example – are clades because, in each case,
416-471: Is in turn included in the mammal, vertebrate and animal clades. The idea of a clade did not exist in pre- Darwinian Linnaean taxonomy , which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus's original Systema Naturae (mostly vertebrate groups) do represent clades. The phenomenon of convergent evolution
448-515: Is responsible for many cases of misleading similarities in the morphology of groups that evolved from different lineages. With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary tree of life . The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley , an early advocate of evolutionary theory, proposed
480-489: Is still controversial. As an example, see the full current classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand". The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade Dracohors , which was made by haplology from Latin "draco" and "cohors", i.e. "the dragon cohort "; its form with
512-412: Is uncertain. [REDACTED] [REDACTED] Avialan Avialae ("bird wings") is a clade containing the only living dinosaurs , the birds , and their closest relatives. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to deinonychosaurs , though alternative definitions are occasionally used (see below). Archaeopteryx lithographica , from
544-459: The PhyloCode by Juan Benito and colleagues in 2022 as "the largest clade containing Vultur gryphus , but not Dromaeosaurus albertensis and Saurornithoides mongoliensis ". This definition ensures that both dromaeosaurids and troodontids are excluded from Avialae . Gauthier and de Queiroz (page 34) identified four conflicting ways of defining the term "Aves", which is a problem since
576-717: The Tiaojishan Formation of China , which has been dated to the late Jurassic period ( Oxfordian stage), about 160 million years ago. The avialan species from this time period include Anchiornis huxleyi and Aurornis xui . Xiaotingia zhengi used to be considered a member, but was later classified within the clade Dromaeosauridae . The well-known Archaeopteryx dates from slightly later Jurassic rocks (about 155 million years old) from Germany . Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds, but were later lost during bird evolution. These features include enlarged claws on
608-436: The cladogram below: † Archaeopteryx † Jeholornis † Sapeornis Jinguofortis Cratonavis Chongmingia † Confuciusornithidae Ornithuromorpha † Enantiornithes Pygostylia has been recovered as being within the clade Avebrevicauda . Avebrevicauda (meaning "birds with short tails") is a group of birds which includes all avialan species with ten or fewer free vertebrae in
640-531: The Pygostylia share four unique characteristics . The trait that gives the group its name is the presence of a pygostyle , 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 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 (lower leg bone). The pygostylians fall into two distinct groups with regard to
672-534: The field of paleontology and bird evolution, though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is sometimes used synonymously with the vernacular term "bird" by these researchers. † Anchiornis † Archaeopteryx † Rahonavis † Jixiangornis † Jeholornis [REDACTED] † Sapeornis † Confuciusornis [REDACTED] † Chongmingia Ornithothoraces [REDACTED] The earliest known avialans come from
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#1733106816820704-546: The group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant , each of which consists of even smaller clades. The clade "rodent"
736-590: The last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic . Some of the relationships between organisms that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria , and multicellular organisms may have evolved from archaea. The term "clade"
768-526: The late Jurassic Period Solnhofen Formation of Germany , is usually considered the earliest known avialan which may have had the capability of powered flight; a minority of studies have suggested that it might have been a deinonychosaur instead. Several older (but non flight-capable) possible avialans are known from the late Jurassic Tiaojishan Formation of China , dated to about 160 million years ago. Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used
800-518: The latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms ; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature : node-, stem-, and apomorphy-based (see Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions). The relationship between clades can be described in several ways: The age of
832-437: The latter were lost independently in a number of avialan groups, including modern birds (Aves). While the earliest forms, such as Archaeopteryx and Shenzhouraptor , retained the long bony tails of their ancestors, the tails of more advanced avialans were shortened with the advent of the pygostyle bone in the group Pygostylia . In the late Cretaceous, around 95 million years ago, the ancestor of all modern birds also evolved
864-434: 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: † Confuciusornithiformes † Didactylornis † Sapeornis Ornithothoraces In 2023, Li et al recovered their new taxon, Cratonavis , as the new jinguofortisid that belongs to Pygostylia. The results of their phylogenetic analyses are shown in
896-599: The pygostyle. The Ornithothoraces have a ploughshare -shaped pygostyle, while the more primitive members had longer, rod-shaped pygostyles. 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
928-471: The same biological name is being used four different ways. They proposed a solution, number 4 below, which is to reserve the term Aves only for the crown group , the last common ancestor of all living birds and all of its descendants. Other definitions of Aves found in literature were reassigned to other clade names. Under the fourth definition Archaeopteryx is an avialan, and not a member of Aves. Gauthier's proposals have been adopted by many researchers in
960-534: The second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering. It is also thought that early avialans were either cranially akinetic or had otherwise limited cranial kinesis . Avialans diversified into a wide variety of forms during the Cretaceous Period . Many groups retained primitive characteristics , such as clawed wings and teeth, though
992-552: The smallest clade containing Archaeopteryx and modern birds. Additionally, beginning in the late 2000s and early 2010s, several groups of researchers began adding the genus Troodon as an additional specifier in the definition of Avialae. Troodon had long been considered a close relative of the dromaeosaurids in the larger group Deinonychosauria, though some contemporary studies found it and other troodontids more closely related to modern birds, and so it has been specifically excluded from Avialae in more recent studies. Avialae
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1024-525: The tail. The group was named in 2002 by Gregory S. Paul to distinguish short-tailed avialans from their ancestors, such as Archaeopteryx , which had long, reptilian tails. Depending on the analysis, Sapeornis may or may not be a member of Pygostylia, but is always within Avebrevicauda. Depending on the true phylogenetic position of Sapeornis , Avebrevicauda may be a junior synonym of Pygostylia. Chiappe noted that under his definition, all members of
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