Xenarthra (; from Ancient Greek
ξένος
''Xenos'' (; ) is a word used in the Greek language from Homer onwards. The most standard definition is "stranger". However, the word itself can be interpreted to mean different things based upon context, author and period of writing/speaking, sig ...
, xénos, "foreign, alien" +
ἄρθρον, árthron, "joint") is a major
clade of
placental mammals
Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
native to the
Americas
The Americas, which are sometimes collectively called America, are a landmass comprising the totality of North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World.
Along with th ...
. There are 31 living species: the
anteaters, tree
sloth
Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their l ...
s, and
armadillos.
Extinct xenarthrans include the
glyptodonts,
pampatheres and
ground sloths. Xenarthrans originated in South America during the late
Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...
about 60 million years ago. They evolved and diversified extensively in
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
during the continent's long period of isolation in the early to mid
Cenozoic Era. They spread to the
Antilles
The Antilles (; gcf, label=Antillean Creole, Antiy; es, Antillas; french: Antilles; nl, Antillen; ht, Antiy; pap, Antias; Jamaican Patois: ''Antiliiz'') is an archipelago bordered by the Caribbean Sea to the south and west, the Gulf of Mex ...
by the early
Miocene
The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
and, starting about 3 Mya, spread to
Central and
North America as part of the
Great American Interchange. Nearly all of the formerly abundant
megafaunal xenarthrans
became extinct at the end of the
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in ...
.
Xenarthrans share several characteristics not present in other placental mammals, which suggest their ancestors were subterranean diggers for insects. The name Xenarthra derives from the two
ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...
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.
Their limb bone structures are unusual, and 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 internal
testicle
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoste ...
s, which are located between the bladder and the rectum. Xenarthrans have the lowest
metabolic rates among
theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes.
...
ns.
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
In zoology, a folivore is a herbivore that specializes in eating leaves. Mature leaves contain a high proportion of hard-to-digest cellulose, less energy than other types of foods, and often toxic compounds.Jones, S., Martin, R., & Pilbeam, D. (1 ...
s specialized for life hanging upside-down in trees
* Ground sloths: Medium to very large ground-living herbivores (and possibly omnivores)
* Aquatic sloths: ''
Thalassocnus
''Thalassocnus'' is an extinct genus of semiaquatic ground sloths from the Miocene and Pliocene of the Pacific South American coast. It is monotypic within the subfamily Thalassocninae. The five species—''T. antiquus'', ''T. natans'', ''T. lit ...
'', a medium-sized herbivore, is the only known aquatic sloth
Evolutionary relationships
Xenarthrans were previously classified alongside the
pangolins and
aardvark
The aardvark ( ; ''Orycteropus afer'') is a medium-sized, burrowing, nocturnal mammal native to Africa. It is the only living species of the order Tubulidentata, although other prehistoric species and genera of Tubulidentata are known. Unlik ...
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
A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
grouping whose New World and Old World taxa are unrelated, and it was split up to reflect their true
phylogeny
A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spe ...
. 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 a
cohort
Cohort or cohortes may refer to:
* Cohort (educational group), a group of students working together through the same academic curriculum
* Cohort (floating point), a set of different encodings of the same numerical value
* Cohort (military unit ...
, while others consider it to be a superorder.
Whatever the rank, Xenarthra is now generally considered to be divided into two orders:
*
Cingulata
Cingulata, part of the superorder Xenarthra, is an order of armored New World placental mammals. Dasypodids and chlamyphorids, the armadillos, are the only surviving families in the order. Two groups of cingulates much larger than extant ar ...
(Latin, "the ones with belts/armor"), the armadillos and the extinct glyptodonts and pampatheres
*
Pilosa
The order Pilosa is a clade of xenarthran placental mammals, native to the Americas. It includes the anteaters and sloths (which includes the extinct ground sloths). The name comes from the Latin word for "hairy".
Origins and taxonomy
The bi ...
(Latin, "the ones with fur"), which is subdivided into:
**
Vermilingua
Anteater is a common name for the four extant mammal species of the suborder Vermilingua (meaning "worm tongue") commonly known for eating ants and termites. The individual species have other names in English and other languages. Together with ...
, the anteaters
**
Folivora
Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their l ...
, 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
Afrotheria ( from Latin ''Afro-'' "of Africa" + ''theria'' "wild beast") is a clade of mammals, the living members of which belong to groups that are either currently living in Africa or of African origin: golden moles, elephant shrews (also k ...
(in the group
Atlantogenata
Atlantogenata is a proposed clade of mammals containing the cohorts or superorders Xenarthra and Afrotheria. These groups originated and radiated in the South American and African continents, respectively, presumably in the Cretaceous. Together ...
), or to
Boreoeutheria
Boreoeutheria (, "northern true beasts") is a magnorder of placental mammals that groups together superorders Euarchontoglires and Laurasiatheria. With a few exceptionsExceptional clades whose males lack the usual boreoeutherian scrotum are mo ...
(in the group
Exafroplacentalia
Exafroplacentalia or Notolegia is a clade of placental mammals proposed in 2001 on the basis of molecular research.
Exafroplacentalia places Xenarthra as a sister group to the Boreoeutheria (comprising Laurasiatheria and Euarchontoglires), thus ...
), or to
Epitheria
Epitherians comprise all the placental mammals except the Xenarthra. They are primarily characterized by having a stirrup-shaped stapes in the middle ear, which allows for passage of a blood vessel. This is in contrast to the column-shaped stape ...
(Afrotheria+Boreoeutheria, i.e. as a sister group to all other placental mammals). A comprehensive phylogeny by Goloboff et al. includes xenarthrans as a sister clade of
Euarchontoglires
Euarchontoglires (synonymous with Supraprimates) is a clade and a superorder of mammals, the living members of which belong to one of the five following groups: rodents, lagomorphs, treeshrews, colugos, and primates.
Evolutionary affinities wi ...
within
Boreoeutheria
Boreoeutheria (, "northern true beasts") is a magnorder of placental mammals that groups together superorders Euarchontoglires and Laurasiatheria. With a few exceptionsExceptional clades whose males lack the usual boreoeutherian scrotum are mo ...
(
Laurasiatheria
Laurasiatheria ("laurasian beasts") is a superorder of placental mammals that groups together true insectivores ( eulipotyphlans), bats ( chiropterans), carnivorans, pangolins ( pholidotes), even-toed ungulates ( artiodactyls), odd-toed ungulat ...
+
Euarchontoglires
Euarchontoglires (synonymous with Supraprimates) is a clade and a superorder of mammals, the living members of which belong to one of the five following groups: rodents, lagomorphs, treeshrews, colugos, and primates.
Evolutionary affinities wi ...
). 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
EA Phenomic was a real-time strategy video game developer, headquartered in Ingelheim, Germany, and founded as Phenomic Game Development in 1997 by Volker Wertich, who had previous worked in Blue Byte and developed '' The Settlers'' and '' The ...
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).
Phylogeny
Below is a recent simplified phylogeny of the xenarthran families based on Slater et al. (2016) and Delsuc et al. (2016). The dagger symbol, "†", denotes extinct groups.
Classification
XENARTHRA
* Order
Cingulata
Cingulata, part of the superorder Xenarthra, is an order of armored New World placental mammals. Dasypodids and chlamyphorids, the armadillos, are the only surviving families in the order. Two groups of cingulates much larger than extant ar ...
** Family
Chlamyphoridae
Chlamyphoridae is a family of cingulate mammals. While glyptodonts have traditionally been considered stem-group cingulates outside the group that contains modern armadillos, there had been speculation that the extant family Dasypodidae could ...
: armadillos and
glyptodonts
Glyptodonts are an extinct subfamily of large, heavily armoured armadillos. They arose in South America around 48 million years ago and spread to southern North America after the continents became connected several million years ago. The best-k ...
***
Greater fairy armadillo, ''Calyptophractus retusus''
***
Pink fairy armadillo, ''Chlamyphorus truncatus''
***
Northern naked-tailed armadillo, ''Cabassous centralis''
***
Chacoan naked-tailed armadillo
The Chacoan naked-tailed armadillo (''Cabassous chacoensis'') is a species of South American armadillo.
It is the smallest of the naked-tailed armadillos, having an average head-body length of , while the other species range from . They also ha ...
, ''Cabassous chacoensis''
***
Southern naked-tailed armadillo, ''Cabassous unicinctus''
***
Greater naked-tailed armadillo, ''Cabassous tatouay''
***
Screaming hairy armadillo
The screaming hairy armadillo (''Chaetophractus vellerosus'') is a species of armadillo also known as the small screaming armadillo, crying armadillo or the small hairy armadillo. It is a burrowing armadillo found in the central and southern part ...
, ''Chaetophractus vellerosus''
***
Big hairy armadillo, ''Chaetophractus villosus''
***
Andean hairy armadillo
The Andean hairy armadillo (''Chaetophractus nationi'') is an armadillo found mainly in Bolivia, in the region of the Puna; the departments of Oruro, La Paz, and Cochabamba (Gardner, 1993). Nowark (1991) describes it as distributed in Bolivia a ...
, ''Chaetophractus nationi''
***
Six-banded armadillo
The six-banded armadillo (''Euphractus sexcinctus''), also known as the yellow armadillo, is an armadillo found in South America. The sole extant member of its genus, it was first described by Swedish zoologist Carl Linnaeus in 1758. The six-b ...
or yellow armadillo, ''Euphractus sexcinctus''
***
Giant armadillo, ''Priodontes maximus''
***
Southern three-banded armadillo
The southern three-banded armadillo (''Tolypeutes matacus''), also known as La Plata three-banded armadillo or Azara's domed armadillo, is an armadillo species from South America. It is found in parts of southwestern Brazil, northern Argentina, ...
, ''Tolypeutes matacus''
***
Brazilian three-banded armadillo, ''Tolypeutes tricinctus''
***
Pichi or dwarf armadillo, ''Zaedyus pichiy''
*** Subfamily †
Glyptodontinae: glyptodonts
** Family
Dasypodidae
Dasypodidae is a family of mostly extinct genera of armadillos. One genus, ''Dasypus'', is extant, with at least seven living species.
__TOC__
Classification
Below is a taxonomy of armadillos in this family.
Family Dasypodidae
*† Genus ...
: long-nosed armadillos
***
Nine-banded armadillo
The nine-banded armadillo (''Dasypus novemcinctus''), also known as the nine-banded long-nosed armadillo or common long-nosed armadillo, is a mammal found in North, Central, and South America, making it the most widespread of the armadillos. ...
or long-nosed armadillo, ''Dasypus novemcinctus''
***
Seven-banded armadillo
The seven-banded armadillo (''Dasypus septemcinctus''), also known as the Brazilian lesser long-nosed armadillo, is a species of armadillo from South America found in Paraguay, Argentina, Bolivia and Brazil.
It is a solitary nocturnal, terrest ...
, ''Dasypus septemcinctus''
***
Southern long-nosed armadillo, ''Dasypus hybridus''
***
Llanos long-nosed armadillo, ''Dasypus sabanicola''
***
Great long-nosed armadillo
The greater long-nosed armadillo (''Dasypus kappleri'') is a South American species of armadillo found in Colombia, Venezuela, Ecuador, Guyana, Suriname, French Guiana, Peru, Bolivia and Brazil. It is a solitary, nocturnal, terrestrial animal tha ...
, ''Dasypus kappleri''
***
Hairy long-nosed armadillo, ''Dasypus pilosus''
***
Yepes's mulita, ''Dasypus yepesi''
** Family †
Pampatheriidae
Pampatheriidae ("Pampas beasts") is an extinct family of large plantigrade armored armadillos related to extant armadillos in the order Cingulata. However, pampatheriids have existed as a separate lineage since at least the middle Eocene Muste ...
: pampatheres
* Order
Pilosa
The order Pilosa is a clade of xenarthran placental mammals, native to the Americas. It includes the anteaters and sloths (which includes the extinct ground sloths). The name comes from the Latin word for "hairy".
Origins and taxonomy
The bi ...
** Suborder
Folivora
Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their l ...
: sloths
*** Family
Bradypodidae: three-toed sloths
****
Pygmy three-toed sloth
The pygmy three-toed sloth (''Bradypus pygmaeus''), also known as the monk sloth or dwarf sloth, is a sloth endemic to Isla Escudo de Veraguas, a small island off the Caribbean coast of Panama. The species was first described by Robert P. Ander ...
, ''Bradypus pygmaeus''
****
Brown-throated three-toed sloth
The brown-throated sloth (''Bradypus variegatus'') is a species of three-toed sloth found in the Neotropical realm of Central and South America.
It is the most common of the four species of three-toed sloth, and is found in the forests of South ...
, ''Bradypus variegatus''
****
Pale-throated three-toed sloth
The pale-throated sloth (''Bradypus tridactylus''), occasionally known as the ai (), is a species of three-toed sloth that inhabits tropical rainforests in northern South America.
It is similar in appearance to, and often confused with, the brow ...
, ''Bradypus tridactylus''
****
Maned three-toed sloth, ''Bradypus torquatus''
*** Family †
Megalonychidae
Megalonychidae is an extinct family of sloths including the extinct ''Megalonyx''. Megalonychids first appeared in the early Oligocene, about 35 million years (Ma) ago, in southern Argentina (Patagonia). There is actually one possible find datin ...
: megalonychid ground sloths
*** Family †
Megatheriidae
Megatheriidae is a family of extinct ground sloths that lived from approximately 23 mya—11,000 years ago.
Megatheriids appeared during the Late Oligocene (Deseadan in the SALMA classification), some 29 million years ago, in South America ...
: megatheriid ground sloths
*** Family †
Nothrotheriidae
Nothrotheriidae is a family of extinct ground sloths that lived from approximately 17.5 mya—10,000 years ago, existing for approximately . Previously placed within the tribe Nothrotheriini or subfamily Nothrotheriinae within Megatheriidae, th ...
: nothrotheriid ground sloths and aquatic sloths
*** Family
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
Mylodontidae is a family of extinct South American and North American ground sloths within the suborder Folivora of order Pilosa, living from around 23 million years ago (Mya) to 11,000 years ago. This family is most closely related to another f ...
: mylodontid ground sloths
** Suborder
Vermilingua
Anteater is a common name for the four extant mammal species of the suborder Vermilingua (meaning "worm tongue") commonly known for eating ants and termites. The individual species have other names in English and other languages. Together with ...
: anteaters
*** Family
Cyclopedidae
The Cyclopedidae is a family of anteaters that includes the silky anteater
The silky anteater, also known as the pygmy anteater, has traditionally been considered a single species of anteater, ''Cyclopes didactylus'', in the genus ''Cyclopes ...
: silky anteaters
****
Silky anteater
The silky anteater, also known as the pygmy anteater, has traditionally been considered a single species of anteater, ''Cyclopes didactylus'', in the genus ''Cyclopes'', the only living genus in the family Cyclopedidae. Found in southern Mexico ...
, ''Cyclopes didactylus''
*** Family
Myrmecophagidae
The Myrmecophagidae are a family of anteaters, the name being derived from the Ancient Greek words for 'ant' and 'eat' (''myrmeco-'' and '). Two genera and three species are in the family, consisting of the giant anteater, and the tamanduas. T ...
: anteaters
****
Giant anteater
The giant anteater (''Myrmecophaga tridactyla'') is an insectivorous mammal native to Central and South America. It is one of four living species of anteaters, of which it is the largest member. The only extant member of the genus ''Myrmecophag ...
, ''Myrmecophaga tridactyla''
****
Northern tamandua
The northern tamandua (''Tamandua mexicana'') is a species of tamandua, an anteater in the family Myrmecophagidae. They live in tropical and subtropical forests from southern Mexico, through Central America, and to the edge of the northern Andes ...
, ''Tamandua mexicana''
****
Southern tamandua
The southern tamandua (''Tamandua tetradactyla''), also called the collared anteater or lesser anteater, is a species of anteater from South America and the island of Trinidad in the Caribbean. It is a solitary animal found in many habitats, fro ...
, ''Tamandua tetradactyla''
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
George Gaylord Simpson (June 16, 1902 – October 6, 1984) was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern synthesis, contributing '' Tempo ...
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 (
aardvark
The aardvark ( ; ''Orycteropus afer'') is a medium-sized, burrowing, nocturnal mammal native to Africa. It is the only living species of the order Tubulidentata, although other prehistoric species and genera of Tubulidentata are known. Unlik ...
s 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. With the single exception of ''Dasypus'' armadillos and their ancestral genus ''Propraopus'', xenarthrans do not have a
milk dentition. They have a single set of teeth through their lives; these teeth have no functional
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
dentine 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
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiolog ...
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
thoracic
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
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
Monochromacy (from Greek ''mono'', meaning "one" and ''chromo'', meaning "color") is the ability of organisms or machines to perceive only light intensity, without respect to spectral composition (color). Organisms with monochromacy are called ...
(single color vision), common in nocturnal, aquatic and subterranean mammals.
Further losses led to rod monochromacy in a stem
cingulate and a stem
pilosa
The order Pilosa is a clade of xenarthran placental mammals, native to the Americas. It includes the anteaters and sloths (which includes the extinct ground sloths). The name comes from the Latin word for "hairy".
Origins and taxonomy
The bi ...
n, 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
theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes.
...
ns.
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.
References
External links
*
*
{{Taxonbar, from=Q173612
Mammal superorders
Extant Paleocene first appearances
Taxa named by Edward Drinker Cope