Xenarthra

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Xenarthrans
Temporal range:
Ma
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Placentalia
Superorder: Xenarthra
Cope, 1889
Orders and suborders
  • Suborder
    Folivora
  • Suborder
    Vermilingua


See text for more details

Red: anteater, yellow: armadillo, blue: sloth, orange: both anteater and armadillo, green: both armadillo and sloth, purple: anteater, armadillo and sloth

Xenarthra (

became extinct at the end of the Pleistocene
.

Xenarthrans share several characteristics that are not present in other placental mammals, and that suggest that Xenarthrans descend from subterranean diggers for insects. The name Xenarthra derives from the two ancient Greek words ξένος (xénos), meaning "strange, unusual", and ἄρθρον (árthron), meaning "joint",[4][5] 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.[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 have testicles inside the body, which are located between the bladder and the rectum.[8] Xenarthrans have the lowest metabolic rates among therians.[9][10]

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 folivores 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

Pink fairy armadillo (Chlamyphorus truncatus)

Xenarthrans were previously classified alongside the

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.[11]

Their relationship to other placental mammals is obscure. Xenarthrans have been defined as most closely related to Afrotheria[12] (in the group Atlantogenata), or to Boreoeutheria (in the group Exafroplacentalia), or to Epitheria[13] (Afrotheria+Boreoeutheria, i.e. as a sister group to all other placental mammals). A comprehensive phylogeny by Goloboff et al.[14] 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 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, 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).[15]

Phylogeny

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 DNA sequences.[16] 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)[17] and Delsuc et al. (2016).[18] The dagger symbol, "†", denotes extinct groups.

Xenarthra
Cingulata

Dasypodidae

Pampatheriidae

Chlamyphoridae

Pilosa
Vermilingua

Cyclopedidae

Myrmecophagidae

Folivora

Mylodontidae

Choloepodidae
(two-toed sloths)

Megalonychidae

Bradypodidae
(three-toed sloths)

Nothrotheriidae

Megatheriidae

Classification

Screaming hairy armadillo
Southern three-banded armadillo
Brown-throated three-toed sloth
Giant anteater
Southern tamandua juvenile

XENARTHRA

Characteristics

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.[19] These extreme characteristics led to their confusion with unrelated groups that had similar specializations (aardvarks and pangolins), 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.

tribosphenic pattern.[22]

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 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 thoracic and lumbar vertebrae as well.[1]

Vision

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.[23] Further losses led to rod monochromacy in 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.[23] Some authorities state that xenarthrans lack a functional pineal gland; pineal activity is related to the perception of light.[24]

Metabolism

Living xenarthrans have the lowest metabolic rates among therians.[9][25] Paleoburrows have been discovered which are up to 1.5m wide and 40m 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 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-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.[21]

References

  1. ^
    OCLC 741613153.{{cite book}}: CS1 maint: others (link
    )
  2. .
  3. .
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  5. ^ Bailly, Anatole. "Greek-french dictionary online". www.tabularium.be. Retrieved May 2, 2020.
  6. PMID 11487408. Archived from the original
    (PDF) on 2016-03-04. Retrieved 2013-01-04.
  7. .
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  9. ^ .
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  11. .
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  16. .
  17. ^ 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.
  18. ^ 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.
  19. S2CID 12547340.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  20. .
  21. ^
    ISBN 9780253002303.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  22. .
  23. ^ .
  24. ISBN 9781475714517 – via Google Books.[full citation needed
    ]
  25. .

External links