Editing Xenarthra (section)

==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" />