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http://animaldiversity.ummz.umich.edu/chordata/mammalia/

Order Xenarthra

(armadillos, anteaters, and sloths)

Xenarthrans radiated in South America during the Tertiary, when that continent was isolated by sea from other continents. The group currently includes armadillos, 2-toed sloths, 3-toed sloths, and anteaters, placed in four families containing 29 species. These animals are mostly insectivores and herbivores of small to medium body size (up to around 60 kg). In the past, however, xenarthrans were much more diverse and numerous. They radiated into around a dozen families, including not only the groups known today, but also such animals as giant ground sloths, some of which were larger than elephants; glyptodonts, reaching 3 m in length and the most heavily armored vertebrates that ever existed; and a large number of smaller grazing and browsing forms. Several groups of xenarthrans successfully crossed the Central American land bridge to North America when it formed in the Pliocene; these included a number of kinds of ground sloths and armadillos. Only one species, however, an armadillo(Dasypus novemcinctus), is still alive today.

Xenarthrans lack incisors or canines, and if present, their molars and premolars are simply cylinders without the covering of enamel that is found on the teeth of most other mammals. These teeth have a single root. Xenarthrans have small brains, and some have an unusually long and cylindrical braincase. Their tympanic bone is ring-shaped. The most notable feature of their postcranial skeleton are the special articulations (xenarthrous processes) on the lumbar vertebrae. These are found in no other mammals. The number of cervical vertebrae varies from five to nine, depending on the species; this degree of variation is extremely unusual in mammals (almost all other mammals have seven). The forefeet have five toes (with a few exceptions), but two or three predominate and have long, sharp, curved claws. Xenarthrans also have a clavicle and an unusually well-developed coracoid process.

Xenarthrans can be found throughout Central and South America, ranging northwards to the central United States. They are sometimes referred to as edentates (order Edentata). Their fossil record extends to the Paleocene.

  
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Click on a term below to find out more about that family:



Family Dasypodidae (armadillos)
Family Myrmecophagidae (anteaters)
Family Bradypodidae (sloths)
Family Megalonychidae (sloths)
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Literature and references cited

Barlow, J. C. 1984. Xenarthrans and pholidotes. Pp. 219-239 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.

Paradiso, J. L. 1975. Walker's Mammals of the World, Third Edition. Johns Hopkins University Press, Baltimore.

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.

Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.


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Dasypodidae

armadillos

Armadillos range from the central United States south through Central and South America. They are by far the most diverse group of xenarthrans, with 20 species in 8 genera.

To most of us, the defining feature of armadillos is their "shell." This structure consists of bony scutes covered with thin keratinous (horny) plates. The scutes cover most of the dorsal surface of the body. They are interrupted by bands of flexible skin at least behind the head, and in most species, at intervals across the back as well. The belly is soft and unprotected by bone except insofar as some species are able to curl into a ball. Hairs project from the areas between scutes, and in some species the ventral surface is densely hairy as well. The limbs have irregular horny plates covering at least parts of their surfaces; they also may be hairy. The top of the head is always covered by a shield of keratin-covered scutes, and the tail is covered by bony rings.

Armadillos vary in size from the tiny fairy armadillo (120 gms) to the giant armadillo (60 kg). Body length ranges from about 125 mm to around 1 m. The snout is short and triangular in some species, long and tubular in others. Some species have large external ears, others do not. The eyes generally seem small. All armadillos have powerful forelimbs, with 3-5 digits (depending on the species) tipped with heavy, curved claws. Body colors are mostly gray or brown; pink fairy armadillos have a pinkish shell and pure white, dense fur on their sides and venters.

The postcranial skeleton of armadillos is much modified for digging and to accomodate the shell. The axial skeleton is rigid and may or may not contact the carapace. The pelvis of some species is especially strongly built and enlarged. The ribs may be broadened, and parts of ribs that in most mammals are cartilage are ossified in some species of armadillos. The limb bones are stout and include expanded crests and processes for the attachment of muscles.

The skulls of these peculiar animals are flattened with a long lower jaw. The zygomatic arch is complete and a jugal is present. The premaxillae are small, as are the lacrimals. The cheek teeth vary from 7-8/7-8 to 18/19. They are homodont and simple. Armadillos lack canines, and most have no incisors.

Most armadillos are not gregarious, living solitarily or in sometimes in pairs. A few sometimes travel in small bands. All armadillos are strictly terrestrial, but some are strong swimmers. All are strong diggers that live in burrows and find their food by scratching or digging. Most feed on insects or other invertebrates, although they may also consume carrion, small vertebrates, and in some cases, plant material.

Armadillos are an ancient group; fossilized scutes are known from the late Paleocene of South America. Armadillos evolved and diversified in that continent during the Tertiary, probably entering North America when a land bridge connected the continents in the Pliocene. The range of the species that occurs in North American (Dasypus novemcinctus, which is also found throughout most of Central and South America) is expanding rapidly northwards. Originally known only from central and southern Texas, 9-banded armadillos have recently been found as far north as Nebraska.

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Species included in the Animal Diversity Web:


  Subfamily Chlamyphorinae 
  Subfamily Dasypodinae 

Accounts marked with a p contain pictures, t contain narrative text (student authored), a contain anatomical still/QTVR images, and s contain digitized sound clips. 
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Literature and references cited

Barlow, J. C. 1984. Xenarthrans and pholidotes. Pp. 219-239 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.

Paradiso, J. L. 1975. Walker's Mammals of the World, Third Edition. Johns Hopkins University Press, Baltimore.

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.

Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.


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Myrmecophagidae

anteaters

Four species in three genera make up this family, whose members are found in Central and South America.

Anteaters range from the very small Cyclopes, which weighs around 250 gms, to the large Myrmecophaga, which weighs over 30 kg. All anteaters have long, tapered snouts; that of Myrmecophaga is extraordinarily elongated. The tongue is also long. Anteaters secrete a sticky substance from their salivary glands that coats the tongue when they feed. The ears are small and rounded, and the eyes are small. The tails are long and prehensile in 2 of the 3 genera. The forelimbs are remarkable. They have 5 digits, each with long and sharp claws, the third claw being especially well developed. The hind feet are less specialized, with 4 or 5 toes and strong but not remarkable claws. Myrmecophaga walks with a peculiar, shuffling gait; Tamandua species walk on the sides of their hands; and Cyclopes are almost exclusively arboreal. The body fur varies from coarse and long (Myrmecophaga) to short, soft, and silky(Cyclopes). All species have some sort of contrasting color pattern.

Besides its extreme elongation, the skull of anteaters is remarkable because it completely lacks teeth. The jugal is small or absent, and the zygomatic arch is incomplete. The premaxillae are small, but the the lacrimals are large and well developed. The palate is very long; its posterior margin is extended by the fusion of the pterygoids in some species. The mouth opening is tubular and very small. The mandibles are long, thin, and weak.

Anteaters feed almost exclusively on ants and termites, whose nests they rip open with their powerful forelimbs. They also take some beetle larvae and bees, and in captivity giant anteaters accept some fruit. The prey of anteaters adheres to their long, sticky tongues. It is swallowed, and part of the digestive process involves grinding by the unusually muscular pyloric region of the stomach. Cyclopes forages arboreally; Tamandua on the ground or in trees, and Myrmecophaga strictly on the ground.

These animals are not gregarious; they come together primarily for the purpose of breeding. The foreclaws of the larger species are highly effective defensive weapons against predators. Giant anteaters forage during day and night; the other species are most often nocturnal or crepuscular. All anteaters have an excellent sense of smell; sight and hearing are not as well developed.

Geologically, the family is known only back to the Early Miocene in South America. The record is poor, however, and the group is likely to be older.

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Species included in the Animal Diversity Web:


  Cyclopes didactylus (Silky Anteater) -t- 
  Myrmecophaga tridactyla (Giant Anteater) -pt- 
  Tamandua mexicana (Northern Tamandua) -t- 
  Tamandua tetradactyla (Collared Anteater, Southern Tamandua) -pta- 

Accounts marked with a p contain pictures, t contain narrative text (student authored), a contain anatomical still/QTVR images, and s contain digitized sound clips. 
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Literature and references cited

Barlow, J. C. 1984. Xenarthrans and pholidotes. Pp. 219-239 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.

Paradiso, J. L. 1975. Walker's Mammals of the World, Third Edition. Johns Hopkins University Press, Baltimore.

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.

Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.


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Bradypodidae

three toed sloths

This family includes 3 Recent species in one genus, Bradypus. It is distributed through Central and South America, south to southern Brazil.

Three-toed sloths weigh 3 - 5 kg; their bodies run around 0.5 m in length. They are covered with dense, long, shaggy fur made up of thick hairs with longitudinal grooves. Beneath the overfur is short underfur of finer texture. Some have unusually long neck hairs, which form a mane. Individual hairs are directed so that they point towards the ground when the animal hangs beneath a branch, perhaps helping it shed rain. Three-toed sloths are mostly tan or yellow-brown in color (with some contrasting markings on the face and mane), and the grooves in the individual hairs contain algal cells that give the coat a greenish cast. Bradypodids have a short and very stout tail.

The forearms of three-toed sloths are longer than the hind limbs. Fore and hind feet have three enlarged, hook-like claws. The claws are separate, but the digits to which they attach are syndactylous.

Three-toed sloths have a short, rounded skull. The profile is somewhat flattened dorsoventrally. The zygomatic arch is incomplete. A large jugal is present, and at its termination it forks broadly into dorsal and ventral processes. A tympanic bulla is present. The mandible has a well developed coronoid process, and the condyloid process is elevated above the level of the toothrow.

The teeth of three-toed sloths are highly distinctive. Incisors and canines are lacking and the number of cheek teeth is reduced to 5/4-5 = 18-20. Each tooth is a simple, ever-growing peg surrounded by a thin layer of dentine (but no enamel) and coated with cementum. The anteriormost teeth are smaller than the more posterior ones, and the anterior upper teeth do not occlude with the anterior lower teeth.

Members of this family have 9 cervical vertebrae, 2 more than the number found in almost all other mammals.

Three-toed sloths are solitary and arboreal. They rarely descend from the trees, coming to ground once every 8 days to defecate, and occasionally to cross open ground. They are strong swimmers. Their movements in the trees and on the ground are very slow and deliberate, but they are capable of slashing rapidly with their claws in defense against predators. It has been suggested that their slow and deliberate style of moving is a defense against visually-orienting hawks and eagles. They are also preyed upon by mammalian predators such as cats, and by snakes such as boas and anacondas.

The diet of these species consists almost entirely of leaves. Their senses of sight and hearing are poorly developed, and they probably forage almost entirely using their senses of smell and touch. Their body temperature and metabolic rate are unusually low, and they thermoregulate partly by sunning.

No fossils of bradypodids are known.

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Species included in the Animal Diversity Web:


  Bradypus torquatus (Maned Sloth) -pt- 
  Bradypus tridactylus (Three-Toed Sloth) -t- 
  Bradypus variegatus (Three-Toed Sloth) -ta- 

Accounts marked with a p contain pictures, t contain narrative text (student authored), a contain anatomical still/QTVR images, and s contain digitized sound clips. 
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Literature and references cited

Barlow, J. C. 1984. Xenarthrans and pholidotes. Pp. 219-239 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.

Paradiso, J. L. 1975. Walker's Mammals of the World, Third Edition. Johns Hopkins University Press, Baltimore.

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.

Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.


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Megalonychidae

two-toed sloths

This family contains 2 species placed in a single genus, Choloepus. Two-toed sloths are found in Central America, throughout northern South America and in the Amazon basin.

Two toed sloths are medium-sized animals, with a body slightly more than half a meter in length and weight up to around 9 kg. They are covered with long, usually pale gray-brown fur (paler on the head) that takes on a greenish hue due to symbiotic algae living on the hairs. Under the coarse outer fur, the pelage grades into a layer of finer, shorter underfur. External ears are much reduced in size. The forelimbs and hindlimbs are long, with the forelimbs somewhat longer than the hindlimbs (but the difference is not as extreme as it is in three-toed sloths, Bradypodidae). The forelimbs end in two large, curved claws; these are attached to digits that are enclosed in a web of skin (syndactylous). The hindlimbs have three claws and are also syndactylous. The tail is small or absent.

The skulls of these sloths are relatively short. The rostrum is distinctly convex in profile. True tympanic bullae are lacking. There is, however, an ectotympanic, and the pterygoids and alisphenoids are swollen, forming a sort of inflated bullae. The mandible has a large coronoid process, and the condyloid process lies in the same plane as the toothrow. The zygomatic arch is incomplete, the jugal ending in flared dorsal and ventral processes. The premaxilla is small.

Megalonychids have 5/4-5 cheek teeth. Incisors and canines are absent, but the anterior cheek teeth are enlarged, triangular in cross section, and canine-like. They are separated from the rest of the cheek teeth by a diastema. The anterior surface of the lower canine-like tooth meets the posterior surface of the upper, wearing against each other and continually sharpening their edges.

These sloths feed mainly on leaves, but also include some fruits, buds, and even small vertebrates in their diets. Like three-toed sloths, they move in a very slow and deliberate fashion (but megalonychids are faster than bradypodids). Most of their lives are spent high up in trees, although they, like bradypodids, come to the ground to defecate. They swim well. Two-toed sloths are capable of vigorous self defense, using their foreclaws to slash a predator and biting with their enlarged, canine-like cheek teeth.

Two toed sloths are perhaps more heterothermic than any other mammal. Their body temperatures fluctuate from as low as 24 degrees C to as high as 33 degrees C. Unlike three-toed sloths, however, they do not thermoregulate by basking.

Megalonychids first appear in the fossil record in the Early Miocene. They were found in North America in the Pleistocene, becoming extinct as recently as 11,000 years BP (giant ground sloths).

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Species included in the Animal Diversity Web:


  Subfamily Choloepinae 

Accounts marked with a p contain pictures, t contain narrative text (student authored), a contain anatomical still/QTVR images, and s contain digitized sound clips. 
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Literature and references cited

Barlow, J. C. 1984. Xenarthrans and pholidotes. Pp. 219-239 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.

Paradiso, J. L. 1975. Walker's Mammals of the World, Third Edition. Johns Hopkins University Press, Baltimore.

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.

Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.