25-543: Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals . First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives. It is one of two groups placed in the clade Theria alongside Eutheria , which contains the placentals. Remains of metatherians have been found on all of Earths continents. Distinctive characteristics ( synapomorphies ) of Metatheria include:
50-534: 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
75-399: A prehensile tail , the development of a capitular tail on the humerus, the loss of tooth replacement on the 2nd and 5th premolars and the retention of decidious teeth on the lower fifth premolars, the lower canines outwardly diverge from each other, the angular process on the dentary is equal to or less than half the length of the ramus , the dentary has a posterior masseteric shelf, and
100-479: 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
125-623: 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
150-422: 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
175-429: 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 ,
200-635: Is Sinodelphys szalayi , which lived in China during the Early Cretaceous around 125 million years ago (mya). This makes it a contemporary to some early eutherian species that have been found in the same area. However, Bi et al. (2018) reinterpreted Sinodelphys as an early member of Eutheria. The oldest uncontested metatherians are now 110 million year old fossils from western North America. Metatherians were widespread in Asia and North America during
225-499: 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,
250-476: 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
275-515: 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
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#1732783612683300-489: 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
325-777: The Pliocene , as well as the Polydolopimorphia , which likely had a wide range of diets. Metatherians then declined in diversity in South America during the Late Eocene as well as the later Oligocene epoch. The oldest known Australian marsupials are from the early Eocene, and are thought to have arrived in the region after having dispersed via Antarctica from South America. During the Oligocene epoch, Australian metatherians radiated rapidly, which contributed most to
350-734: The Pliocene - Pleistocene as part of the Great American interchange ). Metatherians first arrived in Afro-Arabia during the Paleogene , probably from Europe, including the possible peradectoid Kasserinotherium from the Early Eocene of Tunisia and the herpetotheriid Peratherium africanum from the Early Oligocene of Egypt and Oman. The youngest African metatherian is the possible herpetotheriid Morotodon from
375-691: The Cenozoic. The two major groups of Cenozoic Laurasian metatherians, the opossum-like herpetotheriids and peradectids persisted into the Miocene before becoming extinct, with the North American herpetotheriid Herpetotherium , the European herpetotheriid Amphiperatherium and the peradectids Siamoperadectes and Sinoperadectes from Asia being the youngest Laurasian non-marsupial metatherians (with marsupials invading North America during
400-798: The Late Cretaceous, including both Deltatheroida and Marsupialiformes, with fossils also known from Europe during this time. During the Late Cretaceous, metatherians were more diverse than eutherians in North America. Metatherians underwent a severe decline during the Cretaceous–Paleogene extinction event , more severe than that suffered by contemporary eutherians and multituberculates , and were slower to recover diversity. Morphological and species diversity of metatherians in Laurasia remained low in comparison to eutherians throughout
425-451: 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
450-1445: The global peak in metatherian diversity during the Early Miocene. The only known Antarctic metatherians are from the Early Eocene La Meseta Formation of the Antarctic Peninsula , where they are the most diverse group of mammals, and include marsupials as well as polydolopimorphians. Below is a metatherian cladogram from Wilson et al. (2016): Holoclemensia Pappotherium Sulestes Oklatheridium Tsagandelta Lotheridium Deltatheroides Deltatheridium Nanocuris Atokatheridium Gurlin Tsav skull Borhyaenidae Mayulestes Jaskhadelphys Andinodelphys Pucadelphys Asiatherium Iugomortiferum Kokopellia Aenigmadelphys Anchistodelphys Glasbius Pediomys Pariadens Eodelphis Didelphodon Turgidodon Alphadon Albertatherium Marsupialia Cladogram after: Deltatheriidae Kokopellia Asiatherium Peradectidae Stagodontidae Pucadelphyidae Sparassodonta Amphiperatherium Peratherium Herpetotherium Marsupialia Clade In biological phylogenetics ,
475-546: 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"
500-590: 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"
525-538: The late Early Miocene of Uganda . Metatherians arrived in South America from North America during the latest Cretaceous or Paleocene and underwent a major diversificiation, with South American metatherians including both the ancestors of extant marsupials as well as the extinct Sparassodonta , which were major predators in South American ecosystems during most of the Cenozoic , up until their extinction in
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#1732783612683550-518: 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
575-402: The lower 5th premolar has a "very trenchant" cristid obliqua/ectolophid. The permanent deciduous lower 5th premolars are molar like and were historically identified as 1st molars, with the third premolar found in basal therians being lost, leaving 4 premolars in the halves of each jaw. The relationships between the three extant divisions of mammals ( monotremes , marsupials, and placentals )
600-521: 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. Peratherium africanum Peratherium is a genus of metatherian mammals in the family Herpetotheriidae that lived in Europe and Africa from
625-422: Was long a matter of debate among taxonomists . Most morphological evidence comparing traits, such as the number and arrangement of teeth and the structure of the reproductive and waste elimination systems , favors a closer evolutionary relationship between marsupials and placental mammals than either has with the monotremes, as does most genetic and molecular evidence. The earliest possible known metatherian
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