Editing Xenarthra

{{Short description|Superorder of mammals including anteaters, sloths, and armadillos}}
{{Automatic taxobox
| name = Xenarthrans
| fossil_range = [[Late Paleocene]]&nbsp;–Recent, {{fossil range|60|0}}
| image = Xenarthra.jpg
| image_upright = 1.3
| image_caption = Clockwise from upper left: ''[[Megatherium]]'', [[giant anteater]], two-toed [[sloth]], [[nine-banded armadillo]].
| taxon = Xenarthra
| authority = [[Edward Drinker Cope|Cope]], 1889
| subdivision_ranks = Orders and suborders
| subdivision = * Order [[Cingulata]]
* Order [[Pilosa]]
:* Suborder [[Folivora]]
:* Suborder [[Vermilingua]]
<br/>''See text for more details''
| range_map = Xenarthra range.jpg
| range_map_caption = Red: anteater, yellow: armadillo, blue: sloth, orange: both anteater and armadillo, green: both armadillo and sloth, purple: anteater, armadillo and sloth
}}

'''Xenarthra''' ({{IPAc-en|z|E|'|n|ɑr|T|r|@}}; from Ancient Greek [[wikt:ξένος|ξένος]], xénos, "foreign, alien" + [[wikt:ἄρθρον|ἄρθρον]], árthron, "joint") is a [[Order (biology)|superorder]] and major [[clade]] of [[Placentalia|placental mammals]] native to the [[Americas]]. There are 31 living species: the [[anteater]]s, tree [[sloth]]s, and [[armadillo]]s.<ref name=":0">{{Cite book|title=The biology of the Xenarthra|date=2008|publisher=University Press of Florida |editor=Vizcaíno, Sergio F. |editor2=Loughry, W. J.|isbn=978-0-8130-3718-9|location=Gainesville|oclc=741613153}}</ref> Extinct xenarthrans include the [[glyptodont]]s, [[Pampatheriidae|pampatheres]] and [[ground sloth]]s. Xenarthrans originated in South America during the late [[Paleocene]] about 60 million years ago.<ref>{{cite journal | last1 = O'Leary | first1 = M. A. | last2 = Bloch | first2 = J. I. | last3 = Flynn | first3 = J. J. | last4 = Gaudin | first4 = T. J. | last5 = Giallombardo | first5 = A. | last6 = Giannini | first6 = N. P. | last7 = Cirranello | first7 = A. L. | year = 2013 | title = The placental mammal ancestor and the post–K-Pg radiation of placentals | journal = Science | volume = 339 | issue = 6120| pages = 662–667 | doi=10.1126/science.1229237 | pmid=23393258| hdl = 11336/7302 | bibcode = 2013Sci...339..662O | s2cid = 206544776 | hdl-access = free }}</ref> 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 America|Central]] and [[North America]] as part of the [[Great American Interchange]].<ref>{{cite journal |doi=10.1007/s10914-010-9144-8 |title=The Great American Biotic Interchange: Dispersals, Tectonics, Climate, Sea Level and Holding Pens |year=2010 |last1=Woodburne |first1=Michael O. |journal=Journal of Mammalian Evolution |volume=17 |issue=4 |pages=245–264 |pmid=21125025 |pmc=2987556}}</ref> Nearly all of the formerly abundant [[megafauna]]l xenarthrans [[Quaternary extinction event|became extinct]] at the end of the [[Pleistocene]].

==Characteristics==
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 {{wikt-lang|grc|ξένος}} ({{grc-transl|ξένος}}), meaning "strange, unusual", and {{wikt-lang|grc|ἄρθρον}} ({{grc-transl|ἄρθρον}}), meaning "joint",<ref>{{Cite book |title=Abrégé du dictionnaire grec français |last=Bailly |first=Anatole |date=1981-01-01 |publisher=Hachette |isbn=978-2010035289 |location=Paris |oclc=461974285 }}</ref><ref>{{Cite web |url=http://www.tabularium.be/bailly/ |title=Greek-french dictionary online |last=Bailly |first=Anatole |website=www.tabularium.be |access-date=May 2, 2020 }}</ref> and refers to their vertebral joints, which have extra articulations that are unlike other mammals. The [[ischium]] of the pelvis is also fused to the [[sacrum]] of the spine.<ref name="Delsuc2001">{{Cite journal | last1 = Delsuc | first1 = Frédéric | last2 = Catzteflis | first2 = François M. | last3 = Stanhope | first3 = Michael J. | last4 = Douzery | first4 = Emmanuel J. P. | title = The evolution of armadillos, anteaters and sloths depicted by nuclear and mitochondrial phylogenies: implications for the status of the enigmatic fossil ''Eurotamandua'' | journal = Proc. R. Soc. Lond. B | date = August 2001 | volume = 268 | issue = 1476 | pages = 1605–15 | url = http://fdelsuc.perso.neuf.fr/fd_files/Delsuc-ProcRSocB01.pdf | doi = 10.1098/rspb.2001.1702 | pmc = 1088784 | pmid = 11487408 | access-date = 2013-01-04 | archive-date = 2016-03-04 | archive-url = https://web.archive.org/web/20160304042457/http://fdelsuc.perso.neuf.fr/fd_files/Delsuc-ProcRSocB01.pdf | url-status = dead }}</ref> Xenarthran limb bones are typically robust, with large processes for muscle attachment. Relative to their body size, living xenarthrans are extremely strong.<ref>{{cite book |last=Webb |first=S. David |date=2001 |editor-last=Hulbert |editor-first=Richard C. |title=The Fossil Vertebrates of Florida |publisher=University Press of Florida |pages=176 |chapter=Chapter 10: Mammalia 2: Xenarthrans |isbn=0-8130-1822-6}}</ref> 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 have [[testicle]]s inside the body, which are located between the bladder and the rectum.<ref>{{cite journal |pmid=20413907 |year=2010 |last1=Kleisner |first1=K |last2=Ivell |first2=R |last3=Flegr |first3=J |title=The evolutionary history of testicular externalization and the origin of the scrotum |volume=35 |issue=1 |pages=27–37 |journal=Journal of Biosciences |doi=10.1007/s12038-010-0005-7|s2cid=11962872 }}</ref> Xenarthrans have the lowest [[Basal metabolic rate|metabolic rates]] among [[theria]]ns.<ref name="Basal Metabolic Rates in Mammals: A">{{cite journal |first1=M. A. |last1=Elgar |first2=P. H. |last2=Harvey |year=1987 |title=Basal Metabolic Rates in Mammals: Allometry, Phylogeny and Ecology |journal=Functional Ecology |volume=1 |issue=1 |pages=25–36 |jstor=2389354 |doi=10.2307/2389354|bibcode=1987FuEco...1...25E }}</ref><ref>{{cite journal |first1=Barry G. |last1=Lovegrove |year=2000 |title=The Zoogeography of Mammalian Basal Metabolic Rate |journal=The American Naturalist |volume=156 |issue=2 |pages=201–19 |doi=10.1086/303383 |pmid=10856202 |jstor=3079219|s2cid=4436119 }}</ref>

Xenarthran forms and lifestyles include:
* Armadillos: Mostly small and some larger omnivores and insectivores with flexible banded body armor
* Glyptodonts: Large herbivores with a rigid semi-spherical carapace
* Pampatheres: Large herbivores (and possibly omnivores) with banded body armor
* Anteaters: Small to large specialized feeders on social insects 
* Tree sloths: Medium-sized [[folivore]]s specialized for life hanging upside-down in trees
* Ground sloths: Medium to very large ground-living herbivores (and possibly omnivores)
* Aquatic sloths: ''[[Thalassocnus]]'', a medium-sized herbivore, is the only known aquatic sloth

==Evolutionary relationships==
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[[File:Pink Fairy Armadillo (Chlamyphorus truncatus) (cropped).jpg|thumb|right|[[Pink fairy armadillo]] (''Chlamyphorus truncatus'')]]
Xenarthrans were previously classified alongside the [[pangolin]]s and [[aardvark]]s in the order '''Edentata''' (meaning toothless, because the members do not have incisors and lack, or have poorly developed, molars). Subsequently, Edentata 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 ([[Monophyly|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 a [[cohort (taxonomy)|cohort]], 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: 
** [[Vermilingua]] ("worm-tongues"), the anteaters
** [[Folivora]] ("leaf-eaters"), the sloths (both tree sloths and the extinct ground sloths). Folivora is also called Tardigrada or Phyllophaga.<ref name="McKenna1997">{{cite book | last1 = McKenna | first1 = M.C. | last2 = Bell | first2 = S.K. | title = Classification of Mammals Above the Species Level | publisher = Columbia University Press | location = New York | isbn = 978-0-231-11013-6 | oclc = 37345734| year = 1997 | pages = 93}}</ref>

Their relationship to other placental mammals is obscure. Xenarthrans have been defined as most closely related to [[Afrotheria]]<ref>{{cite journal |doi=10.1101/gr.5918807 |title=Using genomic data to unravel the root of the placental mammal phylogeny |year=2007 |last1=Murphy |first1=W. J. |last2=Pringle |first2=T. H. |last3=Crider |first3=T. A. |last4=Springer |first4=M. S. |last5=Miller |first5=W. |journal=Genome Research |volume=17 |issue=4 |pages=413–21 |pmid=17322288 |pmc=1832088}}</ref> (in the group [[Atlantogenata]]), or to [[Boreoeutheria]] (in the group [[Exafroplacentalia]]), or to [[Epitheria]]<ref>{{cite journal |doi=10.1371/journal.pbio.0040091 |title=Retroposed Elements as Archives for the Evolutionary History of Placental Mammals |year=2006 |last1=Kriegs |first1=Jan Ole |last2=Churakov |first2=Gennady |last3=Kiefmann |first3=Martin |last4=Jordan |first4=Ursula |last5=Brosius |first5=Jürgen |last6=Schmitz |first6=Jürgen |journal=PLOS Biology |volume=4 |issue=4 |pages=e91 |pmid=16515367 |pmc=1395351 |doi-access=free }}</ref> (Afrotheria+Boreoeutheria, i.e. as a sister group to all other placental mammals). A comprehensive phylogeny by Goloboff et al.<ref>{{cite journal |doi=10.1111/j.1096-0031.2009.00255.x |title=Phylogenetic analysis of 73 060 taxa corroborates major eukaryotic groups |year=2009 |last1=Goloboff |first1=Pablo A. |last2=Catalano |first2=Santiago A. |last3=Marcos Mirande |first3=J. |last4=Szumik |first4=Claudia A. |last5=Salvador Arias |first5=J. |last6=Källersjö |first6=Mari |last7=Farris |first7=James S. |journal=Cladistics |volume=25 |issue=3 |pages=211–30|pmid=34879616 |s2cid=84401375 |doi-access=free |hdl=11336/78055 |hdl-access=free }}</ref> includes xenarthrans as a sister clade of [[Euarchontoglires]] within [[Boreoeutheria]] ([[Laurasiatheria]]+[[Euarchontoglires]]). Overall, studies using mitochondrial DNA have tended to group them as a sister clade to [[Ferungulata]] (carnivorans+ungulates+pholidotans), 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, a large [[Phenomics|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).<ref>{{Cite journal|last1=O'Leary|first1=Maureen A.|last2=Bloch|first2=Jonathan I.|last3=Flynn|first3=John J.|last4=Gaudin|first4=Timothy J.|last5=Giallombardo|first5=Andres|last6=Giannini|first6=Norberto P.|last7=Goldberg|first7=Suzann L.|last8=Kraatz|first8=Brian P.|last9=Luo|first9=Zhe-Xi|last10=Meng|first10=Jin|last11=Ni|first11=Xijun|date=2013-02-08|title=The placental mammal ancestor and the post-K-Pg radiation of placentals|journal=Science|volume=339|issue=6120|pages=662–667|doi=10.1126/science.1229237|issn=1095-9203|pmid=23393258|bibcode=2013Sci...339..662O|s2cid=206544776|hdl=11336/7302|hdl-access=free}}</ref>

===Phylogeny===
[[File:OrthoMaM v10b 2019 116genera circular tree.svg|thumb|upright=1.35|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 14,509 [[Coding region|coding DNA sequences]].<ref name=Scornavacca2019>{{cite journal | vauthors = Scornavacca C, Belkhir K, Lopez J, Dernat R, Delsuc F, Douzery EJ, Ranwez V | title = OrthoMaM v10: Scaling-up orthologous coding sequence and exon alignments with more than one hundred mammalian genomes | journal = Molecular Biology and Evolution | volume = 36 | issue = 4 | pages = 861–862 | date = April 2019 | pmid = 30698751 | pmc = 6445298 | doi = 10.1093/molbev/msz015 }}</ref> 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)<ref>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.</ref> and Delsuc et al. (2016).<ref>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.</ref> The dagger symbol, "†", denotes extinct groups.
{{clade
|style=font-size:100%
|label1='''Xenarthra'''
|1={{clade
   |label1=[[Cingulata]]
   |1={{clade
      |label1=
      |1=[[Dasypodidae]]
      |label2=
      |2={{Clade
         |1=†[[Pampatheriidae]]
         |2=[[Chlamyphoridae]]
         }}
      }}
   |label2=[[Pilosa]]
   |2={{clade
      |label1=[[Anteater|Vermilingua]]
      |1={{clade
         |1=[[Cyclopedidae]]
         |2=[[Myrmecophagidae]]
         }}
      |label2=[[Sloth|Folivora]]
      |2={{clade
         |1={{Clade
             |1=†''[[Mylodontidae]]''
             |2={{Clade
               |2=†[[Megalonychidae]]
               |state2=double
               |1=[[Choloepodidae]] (two-toed sloths)
               }}
            }}
         |2={{Clade
            |1=[[Bradypodidae]] (three-toed sloths)
            |2=†[[Nothrotheriidae]]
            |3=†''[[Megatheriidae]]''
            }}
         }}
      }}
   }}
}}

==Classification==
[[File:2 toed sloth.jpg|thumb|right|[[Hoffmann's two-toed sloth]]]]
[[File:Bicho-preguiça 3.jpg|thumb|right|[[Brown-throated sloth]]]]
[[File:Florida-015.jpg|thumb|right|[[Nine-banded armadillo]]]]
[[File:Myresluger2.jpg|thumb|right|[[Giant anteater]]]]
[[File:Giant armadillo.jpg|thumb|right|[[Giant armadillo]]]]
'''XENARTHRA'''
* Order [[Cingulata]]
** Family [[Chlamyphoridae]]: armadillos and [[Glyptodontinae|glyptodonts]]
*** [[Greater fairy armadillo]], ''Calyptophractus retusus''
*** [[Pink fairy armadillo]], ''Chlamyphorus truncatus''
*** [[Northern naked-tailed armadillo]], ''Cabassous centralis''
*** [[Chacoan naked-tailed armadillo]], ''Cabassous chacoensis''
*** [[Southern naked-tailed armadillo]], ''Cabassous unicinctus''
*** [[Greater naked-tailed armadillo]], ''Cabassous tatouay''
*** [[Screaming hairy armadillo]], ''Chaetophractus vellerosus''
*** [[Big hairy armadillo]], ''Chaetophractus villosus''
*** [[Andean hairy armadillo]], ''Chaetophractus nationi''
*** [[Six-banded armadillo]] or yellow armadillo, ''Euphractus sexcinctus''
*** [[Giant armadillo]], ''Priodontes maximus''
*** [[Southern three-banded armadillo]], ''Tolypeutes matacus''
*** [[Brazilian three-banded armadillo]], ''Tolypeutes tricinctus''
*** [[Pichi]] or dwarf armadillo, ''Zaedyus pichiy''
*** Subfamily †[[Glyptodontinae]]: glyptodonts
** Family [[Dasypodidae]]: long-nosed armadillos
*** [[Nine-banded armadillo]] or long-nosed armadillo, ''Dasypus novemcinctus''
*** [[Seven-banded armadillo]], ''Dasypus septemcinctus''
*** [[Southern long-nosed armadillo]], ''Dasypus hybridus''
*** [[Llanos long-nosed armadillo]], ''Dasypus sabanicola''
*** [[Great long-nosed armadillo]], ''Dasypus kappleri''
*** [[Hairy long-nosed armadillo]], ''Dasypus pilosus''
*** [[Yepes's mulita]], ''Dasypus yepesi''
** Family †[[Pampatheriidae]]: pampatheres
* Order [[Pilosa]]
** Suborder [[Folivora]]: sloths
*** Family [[Bradypodidae]]: three-toed sloths
**** [[Pygmy three-toed sloth]], ''Bradypus pygmaeus''
**** [[Brown-throated three-toed sloth]], ''Bradypus variegatus''
**** [[Pale-throated three-toed sloth]], ''Bradypus tridactylus''
**** [[Maned three-toed sloth]], ''Bradypus torquatus''
*** Family †[[Megalonychidae]]: megalonychid ground sloths
*** Family †[[Megatheriidae]]: megatheriid ground sloths
*** Family †[[Nothrotheriidae]]: nothrotheriid ground sloths and aquatic sloths
*** Family [[Two-toed sloth|Choloepodidae]]: two-toed sloths
**** [[Hoffman's two-toed sloth]], ''Choloepus hoffmanni''
**** [[Linnaeus's two-toed sloth]] or southern two-toed sloth, ''Choloepus didactylus''
*** Family †[[Mylodontidae]]: mylodontid ground sloths
** Suborder [[Vermilingua]]: anteaters
*** Family [[Cyclopedidae]]: silky anteaters
**** [[Silky anteater]], ''Cyclopes didactylus''
*** Family [[Myrmecophagidae]]: anteaters
**** [[Giant anteater]], ''Myrmecophaga tridactyla''
**** [[Northern tamandua]], ''Tamandua mexicana''
**** [[Southern tamandua]], ''Tamandua tetradactyla''

==Characteristics==
[[File:Glyptodon-1.jpg|thumb|Skeleton of ''[[Glyptodon]],'' an extinct [[glyptodont]] related to living armadillos]]
Xenarthrans share several characteristics not present in other mammals. Authorities have tended to agree they are 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.<ref>{{Cite journal|url=https://jeb.biologists.org/content/jexbio/219/19/2991.full.pdf|title=Vertebral bending mechanics and xenarthrous morphology in the nine-banded armadillo (Dasypus novemcinctus)|last=Oliver|first=Jillian D. |author2=Katrina E. Jones |author3=Lionel Hautier |author4=W. J. Loughry |author5=Stephanie E. Pierce|date=2016|journal=Journal of Experimental Biology|volume=219|issue=Pt 19|pages=2991–3002|doi=10.1242/jeb.142331|pmid=27473436|s2cid=12547340|doi-access=free}}</ref> These extreme characteristics led to their confusion with unrelated groups that had similar specializations ([[aardvark]]s and [[pangolin]]s), and obscures their relationships with other mammals.

=== Dentition ===
The teeth of xenarthrans differ from all other mammals. The dentition of most species is either significantly reduced and highly modified, or absent.<ref>{{Cite journal|last=Vizcaíno|first=Sergio F.|date=2009|title=The teeth of the "toothless": novelties and key innovations in the evolution of xenarthrans (Mammalia, Xenarthra)|journal=Paleobiology|volume=35|issue=3|pages=343–366|doi=10.1666/0094-8373-35.3.343|bibcode=2009Pbio...35..343V |s2cid=86798959|issn=0094-8373}}</ref> With the single exception of ''Dasypus'' armadillos and their ancestral genus ''Propraopus'', xenarthrans do not have a [[Baby teeth|milk dentition]]. They have a single set of teeth through their lives; these teeth have no functional [[Tooth enamel|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 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 [[dentin]]e 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.<ref name=":1">{{Cite book|last=Farina|first=Richard A |author2=Sergio F. Vizcaino |author3=Gerry de Iuliis|title=Megafauna; Giant Beasts of Pleistocene South America|publisher=Indiana University Press|year=2013|isbn=9780253002303|location=Bloomington}}</ref> Currently, no living or extinct xenarthrans have been found to have the standard mammalian [[dental formula]] or crown morphology derived from the ancient [[Tribosphenic molar|tribosphenic]] pattern.<ref>{{Cite journal|last1=Gaudin|first1=Timothy J.|last2=Croft|first2=Darin A.|date=2015-06-24|title=Paleogene Xenarthra and the evolution of South American mammals|journal=Journal of Mammalogy|volume=96|issue=4|pages=622–634|doi=10.1093/jmammal/gyv073|issn=0022-2372|doi-access=free}}</ref>

===Spine===
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 vertebrae [[Thor's hero shrew|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 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 [[Thorax|thoracic]] and [[lumbar]] vertebrae as well.<ref name=":0" />

===Vision===
Xenarthrans have been determined to have single-color vision. [[Polymerase chain reaction|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.<ref name="Emerling2015">{{Cite journal |last1=Emerling |first1=Christopher A. |last2=Springer |first2=Mark S. |date=2015-02-07 |title=Genomic evidence for rod monochromacy in sloths and armadillos suggests early subterranean history for Xenarthra |journal=Proceedings of the Royal Society B: Biological Sciences |volume=282 |issue=1800 |pages=20142192 |doi=10.1098/rspb.2014.2192 |issn=0962-8452 |pmc=4298209 |pmid=25540280}}</ref> Further losses led to rod monochromacy in a stem [[Cingulata|cingulate]] and a stem [[pilosa]]n, pointing to a subterranean ancestry; the ancestors of Xenarthra had the reduced eyesight characteristic of vertebrates that live underground.<ref name="Emerling2015" /> Some authorities state that xenarthrans lack a functional [[pineal gland]]; pineal activity is related to the perception of light.<ref>{{cite book|last1=Axelrod|first1=J.|url=https://books.google.com/books?id=VH7SBwAAQBAJ&q=pineal+regularly+present+Xenarthra&pg=PA62|title=The Pineal Gland and its Endocrine Role|date=December 2013|publisher=Springer |isbn=9781475714517|via=Google Books}}{{full citation needed|date=June 2019}}</ref>

===Metabolism===
Living xenarthrans have the lowest metabolic rates among [[theria]]ns.<ref name="Basal Metabolic Rates in Mammals: A"/><ref>{{cite journal |first1=Barry G. |last1=Lovegrove |year=2000 |title=The Zoogeography of Mammalian Basal Metabolic Rate |journal=The American Naturalist |volume=156 |issue=2 |pages=201–219 |doi=10.1086/303383 |pmid=10856202 |jstor=3079219|s2cid=4436119 }}</ref> Paleoburrows have been discovered which are up to {{Convert|1.5|m|ft|0|abbr=on}} wide and {{Convert|40|m|abbr=on}} long, with claw marks from excavation referred to 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 [[Luján River|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 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-[[Great American Interchange|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.<ref name=":1" />

==References==<!-- Syst. Biol. 56(5):753 -->
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* {{cite journal |first1=Derek E. |last1=Wildman |first2=Caoyi |last2=Chen |first3=Offer |last3=Erez |first4=Lawrence I. |last4=Grossman |first5=Morris |last5=Goodman |first6=Roberto |last6=Romero |doi=10.1073/pnas.0511344103 |title=Evolution of the mammalian placenta revealed by phylogenetic analysis |year=2006 |journal=Proceedings of the National Academy of Sciences |volume=103 |issue=9 |pages=3203–3208 |bibcode=2006PNAS..103.3203W |jstor=30048561 |pmid=16492730 |pmc=1413940|doi-access=free }}
* {{Cite web | title = Armadillos: Biology, Ecology and Images | publisher = Armadillo Online | url = https://armadillo-online.org/species.html | date = 1995 | access-date = 19 December 2017}}

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[[Category:Xenarthra| ]]
[[Category:Mammal superorders]]
[[Category:Extant Paleocene first appearances]]
[[Category:Taxa named by Edward Drinker Cope]]