In phylogenetics , a sister group or sister taxon , also called an adelphotaxon , comprises the closest relative(s) of another given unit in an evolutionary tree .
26-814: Proterochampsia is a clade of early archosauriform reptiles from the Triassic period. It includes the Proterochampsidae (e.g. Proterochampsa , Chanaresuchus and Tropidosuchus ) and probably also the Doswelliidae . Nesbitt (2011) defines Proterochampsia as a stem-based taxon that includes Proterochampsa barrionuevoi and all forms more closely related to it than Euparkeria capensis , Erythrosuchus africanus , Passer domesticus (the House Sparrow), or Crocodylus niloticus (the Nile crocodile). Therefore,
52-525: 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 a phylogenetic tree . In the taxonomical literature, sometimes the Latin form cladus (plural cladi ) is used rather than the English form. Clades are
78-407: A monophyletic group, the clade AB. Clade AB and taxon C are also sister groups. Taxa A, B, and C, together with all other descendants of their MRCA form the clade ABC. The whole clade ABC is itself a subtree of a larger tree which offers yet more sister group relationships, both among the leaves and among larger, more deeply rooted clades. The tree structure shown connects through its root to
104-479: 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
130-623: 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
156-403: A clade formed by Proterochampsidae and Doswelliidae , defined Proterochampsia by up-facing nostrils, maxilla -to- prefrontal contact, the tooth bearing part of the upper jaw being curved downwards, neck and front back vertebrae lacking a postzygodiapophyseal lamina, tibia with straight cnemial crest, fifth metatarsal that is not hook-shaped in its inner end, well developed foot phalanges on
182-1228: A highly diverse Proterochampsia, which includes a deep split between doswelliids, including forms with aquatic adaptations, and proterochampsids. Proterochampsia was found to be the sister taxon of Archosauria , whose living representatives consist of birds and crocodilians . Multiple updated versions of the analysis performed by Ezcurra have been published, including the analysis of Paes-Neto and colleagues in their 2023 description of two new genera of proterochampsids, Kuruxuchampsa and Pinheirochampsa . Proterosuchidae Sarmatosuchus Cuyosuchus Erythrosuchidae Asperoris Dorosuchus Euparkeria Halazhaisuchus Avemetatarsalia Pseudosuchia Litorosuchus Vancleavea Polymorphodon Jaxtasuchus Doswellia Rugarhynchos ? Stenoscelida Sphodrosaurus Proterochampsa barrionuevoi Proterochampsa nodosa ? Stenoscelida Cerritosaurus ? Stenoscelida Tropidosuchus MCP-4195-V ? Stenoscelida Gualosuchus Pseudochampsa Chanaresuchus Kuruxuchampsa Pinheirochampsa Rhadinosuchus Clade In biological phylogenetics ,
208-422: 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
234-429: 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 ,
260-414: Is birds , whose commonly cited living sister group is the crocodiles , but that is true only when discussing extant organisms ; when other, extinct groups are considered, the relationship between birds and crocodiles appears distant. Although the bird family tree is rooted in the dinosaurs , there were a number of other, earlier groups, such as the pterosaurs , that branched off of the line leading to
286-499: 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,
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#1733085833319312-472: Is an internal clade of the family. Dilkes and Arcucci (2012) combined data from several phylogenetic analyses of the Archosauriformes, such as Dilkes and Sues (2009), Ezcurra et al. (2010) and Nesbitt (2011), and added ten new characters to their matrix. The monophyly of Proterochampsia, which was restricted to proterochampsids, was supported by 12 unambiguous synapomorphies in their analysis, including
338-471: 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
364-515: 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
390-489: 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
416-455: The quadratojugal , a bone positioned toward the back of the skull behind the jugal. There is also a depression on the squamosal bone of the skull roof . The second metatarsal of the foot is wider than the other metatarsals. Proterochampsians lack a fifth digit on the foot; the fifth metatarsal is reduced to a small pointed bone. However, Nesbitt (2011) only considered Proterochampsia to include Proterochampsidae . Ezcurra (2016), who recovered
442-494: The fifth digit but with a poorly developed first phalanx, among other traits. One of the earliest phylogenies of proterochampsians was proposed by Kischlat and Schultz (1999). They considered Proterochampsia as a substitute name for Proterochampsidae , and recovered a monophyletic Rhadinosuchidae within this clade. However, more recent studies regard Proterochampsidae as the appropriate name for this clade (since no doswelliids were included), and use Rhadinosuchinae instead, as it
468-451: 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
494-546: 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"
520-407: The inclusion of Doswelliidae in it is dependent upon whether Doswellia and Proterochampsa form a monophyletic group to the exclusion of Archosauria and other related groups. Nesbitt (2011) found that Proterochampsians share several distinguishing characteristics, or synapomorphies . A prominent ridge runs along the length of the jugal , a bone below the eye. Another ridge is present on
546-590: 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"
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#1733085833319572-518: 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
598-832: The node Tropidosuchus + Chanaresuchus . Trotteyn and Haro (2012) conducted a phylogenetic analysis of proterochampsians and other basal archosauriforms using only braincase characters, and found Doswellia , an unusual long-necked, heavily armored archosauriform from Virginia, to nest within Proterochampsia. A close relationship between Doswellia and proterochampsids was also found by Benton and Clark (1988) and Dilkes and Sues (2009). Trotteyn and Haro (2012) considered Proterochampsia to include proterochampsids and Doswellia , and proterochampsids to include all proterochampsians more closely related to Proterochampsa than to Doswellia . Ezcurra (2016) combined data several sets, including Nesbitt (2011) and Trotteyn and Haro (2012), and recovered
624-432: The presence of dermal sculpturing on skull that consists of prominent ridges or tubercles on frontals, parietals and nasals; a contact between the maxilla and the prefrontal, separating lacrimal and nasal; a strongly convex dorsal margin of surangular and palatal teeth that are inserted into alveoli. Some of the synapomorphies recovered by Nesbitt (2011) were found to support either the node Cerritosaurus + Chanaresuchus or
650-430: The rest of the universal tree of life . In cladistic standards, taxa A, B, and C may represent specimens, species , genera , or any other taxonomic units. If A and B are at the same taxonomic level, terminology such as sister species or sister genera can be used. The term sister group is used in phylogenetic analysis , however, only groups identified in the analysis are labeled as "sister groups". An example
676-788: The spread of viral infections . HIV , for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. Sister taxon The expression is most easily illustrated by a cladogram : Taxon A Taxon B Taxon C More tree branches Taxon A and taxon B are sister groups to each other. Taxa A and B, together with any other extant or extinct descendants of their most recent common ancestor (MRCA), form
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