Xenarthra

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Xenarthrans
Temporal range: Late Paleocene  –Recent, 60–0  Ma
Xenarthra.jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Placentalia
Superorder: Xenarthra
Cope, 1889
Orders and suborders


See text for more details

Xenarthra range.jpg
Red: anteater, yellow: armadillo, blue: sloth, orange: both anteater and armadillo, green: both armadillo and sloth, purple: anteater, armadillo and sloth

Xenarthra ( /zɛˈnɑːrθrə/ ; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. [1] Extinct xenarthrans include the glyptodonts, pampatheres and ground sloths. Xenarthrans originated in South America during the late Paleocene about 60 million years ago. [2] 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 and North America as part of the Great American Interchange. [3] Nearly all of the formerly abundant megafaunal xenarthrans 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:

Evolutionary relationships

Pink fairy armadillo (Chlamyphorus truncatus) Pink Fairy Armadillo (Chlamyphorus truncatus) (cropped).jpg
Pink fairy armadillo (Chlamyphorus truncatus)

Xenarthrans were previously classified alongside the pangolins and aardvarks 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 (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, while others consider it to be a superorder.

Whatever the rank, Xenarthra is now generally considered to be divided into two orders:

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. The other major clades are colored: Marsupials (magenta), afrotherians (red), laurasiatherians (green), and euarchontoglires (blue). OrthoMaM v10b 2019 116genera circular tree.svg
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. 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

Classification

Screaming hairy armadillo Chaetophractus vellerosus3.jpg
Screaming hairy armadillo
Southern three-banded armadillo SouthernThreeBandedArmadillo065b.jpg
Southern three-banded armadillo
Brown-throated three-toed sloth Bradypus variegatus.jpg
Brown-throated three-toed sloth
Giant anteater Myresluger2.jpg
Giant anteater
Southern tamandua juvenile Tamandua tetradactyla qtl1-2.jpg
Southern tamandua juvenile

XENARTHRA

Characteristics

Skeleton of Glyptodon, an extinct glyptodont related to living armadillos Glyptodon-1.jpg
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. [20] 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. [21] 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. [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]

Related Research Articles

<span class="mw-page-title-main">Armadillo</span> New World placental mammals in the order Cingulata

Armadillos are New World placental mammals in the order Cingulata. They form part of the superorder Xenarthra, along with the anteaters and sloths. 21 extant species of armadillo have been described, some of which are distinguished by the number of bands on their armor. All species are native to the Americas, where they inhabit a variety of different environments.

<span class="mw-page-title-main">Placentalia</span> Infraclass of mammals in the clade Eutheria

Placental mammals 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 distinguished from monotremes and marsupials in that the fetus is carried in the uterus of its mother to a relatively late stage of development. The name is something of a misnomer considering that marsupials also nourish their fetuses via a placenta, though for a relatively briefer period, giving birth to less developed young which are then nurtured for a period inside the mother's pouch. Placentalia represents the only living group within Eutheria, which contains all mammals more closely related to placentals than to marsupials.

<i>Megatherium</i> Genus of ground sloth (extinct)

Megatherium is an extinct genus of ground sloths endemic to South America that lived from the Early Pliocene through the end of the Pleistocene. It is best known for the elephant-sized type species M. americanum, sometimes known as the giant ground sloth, or the megathere, native to the Pampas through southern Bolivia during the Pleistocene. Various other smaller species belonging to the subgenus Pseudomegatherium are known from the Andes.

<span class="mw-page-title-main">Glyptodont</span> Subfamily of extinct mammals belonging to the armadillo order of xenarthrans

Glyptodonts are an extinct clade of large, heavily armoured armadillos, reaching up to 1.5 metres (4.9 ft) in height, and maximum body masses of around 2 tonnes. They had short, deep skulls, a fused vertebral column, and a large bony carapace made up of hundreds of individual scutes. Some glyptodonts had clubbed tails, similar to ankylosaurid dinosaurs.

<span class="mw-page-title-main">Cingulata</span> Order of armored mammals from the Americas

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 armadillos existed until recently: pampatheriids, which reached weights of up to 200 kg (440 lb) and chlamyphorid glyptodonts, which attained masses of 2,000 kg (4,400 lb) or more.

<span class="mw-page-title-main">Pilosa</span> Order of mammals

The order Pilosa is a clade of xenarthran placental mammals, native to the Americas. It includes anteaters and sloths. The name comes from the Latin word for "hairy".

<i>Doedicurus</i> An extinct genus of mammals belonging to the armadillo order, Cingulata

Doedicurus is an extinct genus of glyptodont from North and South America containing one species, D. clavicaudatus. Glyptodonts are a member of the family Chlamyphoridae, which also includes some modern armadillo species, and they are classified in the superorder Xenarthra alongside sloths and anteaters. Being a glyptodont, it was a rotund animal with heavy armor and a carapace. Averaging at an approximate 1,400 kg (3,100 lb), it was one of the largest glyptodonts to have ever lived. Though glyptodonts were quadrupeds, large ones like Doedicurus may have been able to stand on two legs like other xenarthrans. It notably sported a spiked tail club, which may have weighed 40 or 65 kg in life, and it may have swung this in defense against predators or in fights with other Doedicurus at speeds of perhaps 11 m/s.

<span class="mw-page-title-main">Sloth</span> Group of tree dwelling mammals noted for slowness

Sloths are a Neotropical group of 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 lives hanging upside down in the trees of the tropical rainforests of South America and Central America. Sloths are considered to be most closely related to anteaters, together making up the xenarthran order Pilosa.

<i>Eurotamandua</i> Extinct genus of pangolins

Eurotamandua is an extinct genus of mammal from extinct family Eurotamanduidae that lived during the middle Eocene.

<span class="mw-page-title-main">Tolypeutinae</span> Subfamily of mammals

Tolypeutinae is a subfamily of armadillos in the family Chlamyphoridae, consisting of the giant, three-banded and naked-tailed armadillos.

<i>Glyptodon</i> Genus of large, heavily armored mammals

Glyptodon is a genus of glyptodont, an extinct group of large, herbivorous armadillos, that lived from the Pliocene, around 3.2 million years ago, to the early Holocene, around 11,000 years ago, in Brazil, Uruguay, Paraguay, Bolivia, Peru, Argentina, and Colombia. It is one of, if not the, best known genus of glyptodont. Glyptodon has a long and storied past, being the first named extinct cingulate and the type genus of the subfamily Glyptodontinae. Fossils of Glyptodon have been recorded as early as 1814 from Pleistocene aged deposits from Uruguay, though many were incorrectly referred to the ground sloth Megatherium by early paleontologists.

<span class="mw-page-title-main">Euphractinae</span> Subfamily of mammals

Euphractinae is an armadillo subfamily in the family Chlamyphoridae.

<i>Holmesina</i> An extinct genus of mammals belonging to the armadillo order of xenarthrans

Holmesina is a genus of pampathere, an extinct group of armadillo-like xenarthrans that were distantly related to extant armadillos. Like armadillos, and unlike the other extinct branch of megafaunal cingulates, the glyptodonts, the shell was made up of flexible plates which allowed the animal to move more easily. Holmesina species were herbivores that grazed on coarse vegetation; armadillos are mostly insectivorous or omnivorous.

<span class="mw-page-title-main">Paratheria (mammals)</span> Former taxonomic group including xenarthran and similar mammals

Paratheria is an obsolete term for a taxonomic group including the xenarthran mammals and various groups thought to be related to them. It was proposed by Oldfield Thomas in 1887 to set apart the sloths, anteaters, armadillos, and pangolins, usually classified as placentals, from both marsupial and placental mammals, an arrangement that received little support from other workers. When teeth of the extinct gondwanathere mammals were first discovered in Argentina in the 1980s, they were thought to be related to xenarthrans, leading to renewed attention for the hypothesis that xenarthrans are not placentals. However, by the early 1990s, gondwanatheres were shown to be unrelated to xenarthrans, and xenarthrans are still considered to be placentals.

<span class="mw-page-title-main">Chlamyphoridae</span> Family of armadillos

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 be paraphyletic based on morphological evidence. In 2016, an analysis of Doedicurus mtDNA found it was, in fact, nested within the modern armadillos as the sister group of a clade consisting of Chlamyphorinae and Tolypeutinae. For this reason, all extant armadillos but Dasypus were relocated to a new family.

<span class="mw-page-title-main">Chlamyphorinae</span> Subfamily of mammals

Chlamyphorinae is a subfamily of South American armadillos in the family Chlamyphoridae. Members of this subfamily, the fairy armadillos, are largely fossorial and have reduced eyes and robust forearms with large claws for digging.

<i>Macroeuphractus</i> An extinct genus of mammals belonging to the armadillo order of xenarthrans

Macroeuphractus is a genus of extinct armadillos from the Late Miocene to Late Pliocene of South America. The genus is noted for its large size, with Macroeuphractus outesi being the largest non-pampathere or glyptodont armadillo discovered, as well as its specializations for carnivory, unique among all xenarthrans.

<i>Pampatherium</i> Extinct genus of mammal

Pampatherium is an extinct genus of xenarthran that lived in the Americas during the Pleistocene. Some species went extinct right at the Pleistocene-Holocene border.

References

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