39-553: See text for more details Xenarthra ( / z ɛ ˈ n ɑːr θ r ə / ; from Ancient Greek ξένος , xénos, "foreign, alien" + ἄρθρον , árthron, "joint") is a major clade of placental mammals native to the Americas . There are 31 living species: the anteaters , tree sloths , and armadillos . Extinct xenarthrans include the glyptodonts , pampatheres and ground sloths . Xenarthrans originated in South America during
78-590: 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 the fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual,
117-588: 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 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
156-506: A taxonomic rank but has been called a cohort, magnorder, or superorder. Relations among the four cohorts (Euarchontoglires, Xenarthra , Laurasiatheria , Afrotheria ) and the identity of the placental root remain controversial. So far, few, if any, distinctive anatomical features have been recognized that support Euarchontoglires; nor does any strong evidence from anatomy support alternative hypotheses. Although both Euarchontoglires and diprotodont marsupials are documented to possess
195-650: A vermiform appendix , this feature evolved as a result of convergent evolution . Euarchontoglires probably split from the Boreoeutheria magnorder about 85 to 95 million years ago, during the Cretaceous , and developed in the Laurasian island group that would later become Europe . This hypothesis is supported by molecular evidence; so far, the earliest known fossils date to the early Paleocene . The combined clade of Euarchontoglires and Laurasiatheria
234-479: 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
273-446: 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"
312-623: 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
351-414: A feature of the entire clade, allowing relatively low-resource scrublands to support large numbers of huge animals. Faunal analysis also shows far fewer large predators in pre- GABI South American faunas than would be expected based on current faunas in similar environments. This suggests other factors than predation controlled the numbers of xenarthrans. South America had no placental predatory mammals until
390-809: A large phenomic analysis of living and fossil mammals suggests placental mammals evolved shortly after the end of the Cretaceous, and first split into Xenarthra and Epitheria (all other placentals). Below is a recent simplified phylogeny of the xenarthran families based on Slater et al. (2016) and Delsuc et al. (2016). The dagger symbol, "†", denotes extinct groups. Dasypodidae † Pampatheriidae Chlamyphoridae Cyclopedidae Myrmecophagidae † Mylodontidae Choloepodidae (two-toed sloths) † Megalonychidae Bradypodidae (three-toed sloths) † Nothrotheriidae † Megatheriidae XENARTHRA Xenarthrans share several characteristics not present in other mammals. Authorities have tended to agree they are
429-724: A primitive group of placental mammals not very closely related to other orders, without agreeing on how to classify them. George Gaylord Simpson first suggested in 1931 that their combination of unique characteristics shows the group evolved from highly specialized early ancestors that lived underground or were nocturnal and dug with their forelimbs to feed on social insects like ants or termites. Most researchers since then have agreed. These extreme characteristics led to their confusion with unrelated groups that had similar specializations ( aardvarks and pangolins ), and obscures their relationships with other mammals. The teeth of xenarthrans differ from all other mammals. The dentition of most species
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#1732773158188468-540: A reduced number of lumbar vertebrae with either more or fewer cervical vertebrae than most mammals, while cingulates have neck vertebrae fused into a cervical tube, with glyptodonts fusing thoracic and lumbar vertebrae as well. Xenarthrans have been determined to have single-color vision. PCR analysis determined that a mutation in a stem xenarthran led to long-wavelength sensitive-cone (LWS) monochromacy (single color vision), common in nocturnal, aquatic and subterranean mammals. Further losses led to rod monochromacy in
507-422: 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
546-409: A sister clade to Ferrungulata (carnivores+ungulates and cetaceans), while studies using nuclear DNA have identified them as 1) a sister clade to Afrotheria, 2) a sister clade to all placentals except Afrotheria, or 3) a trichotomy (three-way split): Afrotheria, Xenarthra, and everything else (i.e. Boreoeutheria). Among studies that use physical characteristics rather than DNA to look at relationships,
585-500: A stem cingulate and a stem pilosan , pointing to a subterranean ancestry; the ancestors of Xenarthra had the reduced eyesight characteristic of vertebrates that live underground. Some authorities state that xenarthrans lack a functional pineal gland ; pineal activity is related to the perception of light. Living xenarthrans have the lowest metabolic rates among therians . Paleoburrows have been discovered which are up to 1.5m wide and 40m long, with claw marks from excavation referred to
624-429: 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 ,
663-423: Is a clade and a superorder of mammals , the living members of which belong to one of the five following groups: rodents , lagomorphs , treeshrews , primates , and colugos . The Euarchontoglires clade is based on DNA sequence analyses and retrotransposon markers that combine the clades Glires (Rodentia + Lagomorpha) and Euarchonta (Scandentia + Primates + Dermoptera). It is usually discussed without
702-406: Is either significantly reduced and highly modified, or absent. With the single exception of Dasypus armadillos and their ancestral genus Propraopus , xenarthrans do not have a milk dentition . They have a single set of teeth through their lives; these teeth have no functional enamel , and usually there are few or no teeth in the front of the mouth and the rear teeth all look alike. As a result, it
741-625: Is impossible to define Xenarthra as having incisors, canines, premolars, or molars. Since most mammals are classified by their teeth, it has been difficult to determine their relationships to other mammals. Xenarthrans may have evolved from ancestors that had already lost basic mammalian dental features like tooth enamel and a crown with cusps; reduced, highly simplified teeth are usually found in mammals that feed by licking up social insects. Several groups of xenarthrans did evolve cheek teeth to chew plants, but since they lacked enamel, patterns of harder and softer dentine created grinding surfaces. Dentine
780-476: 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
819-423: Is less resistant to wear than the enamel-cusped teeth of other mammals, and xenarthrans developed open-rooted teeth that grow continuously. Currently, no living or extinct xenarthrans have been found to have the standard mammalian dental formula or crown morphology derived from the ancient tribosphenic pattern. The name Xenarthra, which means "strange joints", was chosen because the vertebral joints of members of
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#1732773158188858-510: Is obscure. Xenarthrans have been defined as most closely related to Afrotheria (in the group Atlantogenata ), or to Boreoeutheria (in the group Exafroplacentalia ), or to Epitheria (Afrotheria+Boreoeutheria, i.e. as a sister group to all other placental mammals). A comprehensive phylogeny by Goloboff et al. includes xenarthrans as a sister clade of Euarchontoglires within Boreoeutheria ( Laurasiatheria + Euarchontoglires ). Overall, studies using mitochondrial DNA have tended to group them as
897-546: Is recognized as Boreoeutheria . The hypothesized relationship among the Euarchontoglires is as follows: Laurasiatheria Lagomorpha ( rabbits , hares , pikas ) [REDACTED] Rodentia (rodents) [REDACTED] Scandentia ( treeshrews or banxrings) [REDACTED] Dermoptera (colugos) [REDACTED] Primates [REDACTED] One study based on DNA analysis suggests that Scandentia and Primates are sister clades, but does not discuss
936-515: 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
975-489: 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
1014-431: Is to the armadillos, glyptodonts, and pampatheres; this idea is upheld by molecular studies. Since its conception, Xenarthra has increasingly come to be considered to be of a higher rank than 'order'; some authorities consider it to be a cohort , while others consider it to be a superorder. Whatever the rank, Xenarthra is now generally considered to be divided into two orders: Their relationship to other placental mammals
1053-506: The Pleistocene, and xenarthran large-mammal faunas may have been vulnerable to many factors including a rise in numbers of mammalian predators, resource use by spreading North American herbivores with faster metabolisms and higher food requirements, and climate change. [REDACTED] [REDACTED] Clade In biological phylogenetics , a clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as
1092-524: The end of the Pleistocene . Xenarthrans share several characteristics that are not present in other placental mammals, which suggest that xenarthrans descend from subterranean diggers. The name Xenarthra derives from the two ancient Greek words ξένος ( xénos ), meaning "strange, unusual", and ἄρθρον ( árthron ), meaning "joint", and refers to their vertebral joints, which have extra articulations that are unlike other mammals. The ischium of
1131-501: The ground sloths Glossotherium or Scelidotherium . Remains of ground sloths ( Mylodon and others) in caves are particularly common in colder parts of their range, suggesting ground sloths may have used burrows and caves to help regulate their body temperature. Analysis of the fossil South American Lujan fauna suggests far more large herbivorous mammals were present than similar contemporary environments can support. As most large Lujan herbivores were xenarthrans, low metabolic rate may be
1170-518: The group have extra articulations of a type unlike any other mammals. This trait is referred to as "xenarthry". (Tree sloths lost these articulations to increase the flexibility of their spines, but their fossil ancestors had xenarthrous joints.) Additional points of articulation between vertebrae strengthen and stiffen the spine , an adaptation developed in different ways in various groups of mammals that dig for food. Xenarthrans also tend to have different numbers of vertebrae than other mammals; sloths have
1209-648: The late Paleocene about 60 million years ago. They evolved and diversified extensively in South America during the continent's long period of isolation in the early to mid Cenozoic Era. They spread to the Antilles by the early Miocene and, starting about 3 million years ago, spread to Central and North America as part of the Great American Interchange . Nearly all of the formerly abundant megafaunal xenarthrans became extinct at
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1248-518: 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
1287-453: The pelvis is also fused to the sacrum of the spine. Xenarthran limb bones are typically robust, with large processes for muscle attachment. Relative to their body size, living xenarthrans are extremely strong. Their limb bone structures are unusual. They have single-color vision. The teeth of xenarthrans are unique. Xenarthrans are also often considered to be among the most primitive of placental mammals. Females show no clear distinction between
1326-470: The position of Dermoptera. Although it is known that Scandentia is one of the most basal Euarchontoglires clades, the exact phylogenetic position is not yet considered resolved, and it may be a sister of Glires, Primatomorpha or Dermoptera or to all other Euarchontoglires. Some old studies place Scandentia as sister of the Glires, invalidating Euarchonta. Whole-genome duplication may have taken place in
1365-482: 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" 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"
1404-509: 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. Euarchontoglires Euarchontoglires (from: Euarchonta ("true rulers") + Glires ("dormice")), synonymous with Supraprimates ,
1443-463: The uterus and vagina, and males have testicles inside the body, which are located between the bladder and the rectum. Xenarthrans have the lowest metabolic rates among therians . Xenarthran forms and lifestyles include: Xenarthrans were previously classified alongside the pangolins and aardvarks in the order Edentata (meaning toothless, because the members do not have incisors and lack, or have poorly developed, molars). Subsequently, Edentata
1482-423: 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, the group consists of a common ancestor with all its descendant branches. Rodents, for example, are
1521-436: Was found to be a polyphyletic grouping whose New World and Old World taxa are unrelated, and it was split up to reflect their true phylogeny . Aardvarks and pangolins are now placed in individual orders, and the new order Xenarthra was erected to group the remaining families ( which are all related ). The morphology of xenarthrans generally suggests that the anteaters and sloths are more closely related to each other than either
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