Red: anteater, yellow: armadillo, blue: sloth, orange: both anteater and armadillo, green: both armadillo and sloth, purple: anteater, armadillo and sloth
Xenarthrans share several characteristics that are not present in other placental mammals. The name Xenarthra derives from the twoancient Greek wordsξένος (xénos), meaning "strange, unusual", andἄρθρον (árthron), meaning "joint",[4][5] and refers to their vertebral joints, which have extra articulations that are unlike other mammals. Theischium of the pelvis is also fused to thesacrum of the spine.[6] Xenarthran limb bones are typically robust, with large processes for muscle attachment. Relative to their body size, living xenarthrans are extremely strong.[7] 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 the uterus and vagina, and males havetesticles inside the body, which are located between the bladder and the rectum.[8] Xenarthrans have the lowestmetabolic rates amongtherians.[9][10]
The teeth of xenarthrans differ from all other mammals. The dentition of most species is either significantly reduced and highly modified, or absent.[11] With the single exception ofDasypus armadillos and their ancestral genusPropraopus, xenarthrans do not have amilk dentition. They have a single set of teeth through their lives; these teeth have no functionalenamel, 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 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 evolvecheek teeth to chew plants, but since they lacked enamel, patterns of harder and softerdentine created grinding surfaces. Dentine is less resistant to wear than the enamel-cusped teeth of other mammals, and xenarthrans developed open-rooted teeth that grow continuously.[12] Currently, no living or extinct xenarthrans have been found to have the standard mammaliandental formula or crown morphology derived from the ancienttribosphenic pattern.[13]
The name Xenarthra, which means "strange joints", was chosen because the vertebral joints of members of 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 vertebraestrengthen 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 a reduced number of lumbar vertebrae with either more or fewercervical vertebrae than most mammals, while cingulates have neck vertebrae fused into a cervical tube, with glyptodonts fusingthoracic andlumbar vertebrae as well.[1]
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.[14] Further losses led to rod monochromacy in a stemcingulate and a stempilosan, pointing to a subterranean ancestry; the ancestors of Xenarthra had the reduced eyesight characteristic of vertebrates that live underground.[14] Some authorities state that xenarthrans lack a functionalpineal gland; pineal activity is related to the perception of light.[15]
Living xenarthrans have the lowest metabolic rates amongtherians.[9][16] Paleoburrows have been discovered which are up to 1.5 m (5 ft) wide and 40 m (130 ft) long, with claw marks from excavation referred to the ground slothsGlossotherium orScelidotherium. 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 AmericanLujan 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 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 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.[12]
Xenarthrans were previously classified alongside thepangolins andaardvarks in the orderEdentata (meaning toothless, because the members do not have incisors and lack, or have poorly developed, molars). Subsequently, Edentata was found to be apolyphyletic grouping whose New World and Old World taxa are unrelated, and it was split up to reflect their truephylogeny. 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 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 acohort, while others consider it to be a superorder.
Whatever the rank, Xenarthra is now generally considered to be divided into two orders:
Cingulata (Latin, "the ones with belts/armor"), the armadillos and the extinct glyptodonts and pampatheres
Pilosa (Latin, "the ones with fur"), which is subdivided into:
Folivora ("leaf-eaters"), the sloths (both tree sloths and the extinct ground sloths). Folivora is also called Tardigrada or Phyllophaga.[17]
Their relationship to other placental mammals is obscure. Xenarthrans have been defined as most closely related toAfrotheria[18] (in the groupAtlantogenata), or toBoreoeutheria (in the groupExafroplacentalia), or toEpitheria[19] (Afrotheria+Boreoeutheria, i.e. as a sister group to all other placental mammals). A comprehensive phylogeny by Goloboff et al.[20] includes xenarthrans as a sister clade ofEuarchontoglires withinBoreoeutheria (Laurasiatheria+Euarchontoglires). Overall, studies using mitochondrial DNA have tended to group them as a sister clade toFerungulata (carnivorans+ungulates+pholidotans), while studies using nuclear DNA have identified them as 1) a sister clade to Afrotheria, 2) a sister clade to all placentalsexcept 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, a largephenomic 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).[21]
Phylogenetic position of xenarthrans (in orange) among placentals in a genus-level molecular phylogeny of 116 extant mammals inferred from the gene tree information of 14,509coding DNA sequences.[22] The other major clades are colored: marsupials (magenta), afrotherians (red), laurasiatherians (green), and Euarchontoglires (blue).
Below is a recent simplified phylogeny of the xenarthran families based on Slater et al. (2016)[23] and Delsuc et al. (2016).[24] The dagger symbol, "†", denotes extinct groups.
It has been suggested that the last common ancestor of xenarthrans wasmyrmecophagous (feeding on ants and termites), with digging and possibly climbing capabilities. The oldest fossils of xenarthrans are isolated remains known from theItaboraí Formation of Brazil, dating to the earlyEocene or possibly latestPaleocene, which already includes remains recognisable as armadillos. Most Eocene remains of xenarthrans are attributed to armadillos. The oldest known sloth,Pseudoglyptodon is known from the late Eocene, with remains known from across South America. The oldest fossils of anteaters date to theMiocene epoch.[25]
The name Pan-Xenarthra is used for thetotal group, with the alternative Americatheria having been abandoned.[26] However no unambiguousstem group taxa have been identified.
^abVizcaíno, Sergio F.; Loughry, W. J., eds. (2008).The biology of the Xenarthra. Gainesville: University Press of Florida.ISBN978-0-8130-3718-9.OCLC741613153.
^Webb, S. David (2001). "Chapter 10: Mammalia 2: Xenarthrans". In Hulbert, Richard C. (ed.).The Fossil Vertebrates of Florida. University Press of Florida. p. 176.ISBN0-8130-1822-6.
^Kleisner, K; Ivell, R; Flegr, J (2010). "The evolutionary history of testicular externalization and the origin of the scrotum".Journal of Biosciences.35 (1):27–37.doi:10.1007/s12038-010-0005-7.PMID20413907.S2CID11962872.
^abFarina, Richard A; Sergio F. Vizcaino; Gerry de Iuliis (2013).Megafauna; Giant Beasts of Pleistocene South America. Bloomington: Indiana University Press.ISBN978-0-253-00230-3.
^McKenna, M.C.; Bell, S.K. (1997).Classification of Mammals Above the Species Level. New York: Columbia University Press. p. 93.ISBN978-0-231-11013-6.OCLC37345734.
^Slater, G., Cui, P., Forasiepi, A. M., Lenz, D., Tsangaras, K., Voirin, B., ... & Greenwood, A. D. (2016). Evolutionary relationships among extinct and extant sloths: the evidence of mitogenomes and retroviruses. Genome Biology and Evolution, evw023.
^Delsuc, F., Gibb, G. C., Kuch, M., Billet, G., Hautier, L., Southon, J., ... & Poinar, H. N. (2016). The phylogenetic affinities of the extinct glyptodonts. Current Biology, 26(4), R155-R156.
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