Xenarthra (; from Ancient Greek , xénos, "foreign, alien" + , árthron, "joint") is a superorder and major clade of Placentalia native to the Americas. There are 31 living species: the , tree , and .
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 (), meaning "strange, unusual", and (), meaning "joint",
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.
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 the uterus and vagina, and males have
inside the body, which are located between the bladder and the rectum.
Xenarthrans have the lowest metabolic rates among
.
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 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
Xenarthrans were previously classified alongside the
and
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). 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, 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.
Their relationship to other placental mammals is obscure. Xenarthrans have been defined as most closely related to Afrotheria
Phylogeny
Below is a recent simplified phylogeny of the xenarthran families based on Slater et al. (2016)
[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.] and Delsuc et al. (2016).
[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.] The dagger symbol, "†", denotes extinct groups.
Classification
XENARTHRA
-
Order Cingulata
-
Family Chlamyphoridae: armadillos and Glyptodontinae
-
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: 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
Characteristics
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.
These extreme characteristics led to their confusion with unrelated groups that had similar specializations (
and
), 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.
With the single exception of
Dasypus armadillos and their ancestral genus
Propraopus, xenarthrans do not have a
Baby teeth. They have a single set of teeth through their lives; these teeth have no functional
Tooth 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
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.
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.
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
Thorax and
lumbar vertebrae as well.
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.
Further losses led to rod monochromacy in a stem
Cingulata and a stem
, 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.
Metabolism
Living xenarthrans have the lowest metabolic rates among
.
Paleoburrows have been discovered which are up to wide and 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.
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