Xenarthrans Edentates

Living and extinct xenarthrans are endemic to the New World and distinguished from all other living and extinct mammals by their extra joint articulations (xenarthrous articulation) bracing the lumbar vertebrae (xenarthra means "strange joints"). In most mammals, the vertebrae articulate with each other by two dorsal bony processes (zygapophyses), but xenarthran vertebrae also have lateral vertebral articular processes with dorsal and ventral arms. Some xenarthrans, primarily anteaters and armadillos, play an important ecological role in insect control.

The most current classification of mammals arranges the twenty-nine living species of Magnorder Xenarthra into thirteen genera, four families, and two orders. Order Cingulata contains the eight genera and twenty living species of armadillos (Dasypodidae). Order Pilosa consists of Suborder Vermilingua, containing the giant anteater, the two species of tamanduas, and silky anteater (Myrme-cophagidae); and Suborder Phyllophaga for the two species of two-toed tree sloths (Mega-lonychidae) and the three species of three-toed tree sloths (Bradypodidae). Pangolins were once thought to be closely related to xenarthrans, but that association has been rejected.

Xenarthrans are highly specialized and are morphologically isolated from all other pla-cental mammals. They likely separated from ancestral placental stock before the divergence of other modern placental orders. The modern xenarthran fauna represent but a small fraction of the rich xenarthran evolutionary diversity. Fossils dating from the Late Pale-ocene up to the Late Pleistocene and even prehistoric times have been described representing 108 extinct genera and 1 extinct family in Order Cingulata, and 96 extinct genera and 6 extinct families of Order Pilosa. The earliest fossils are from Late Paleocene strata in South America and consist of bony plates that once formed the armor of an extinct armadillo (Dasypodidae). That recognizable armadillos existed at that early period of the Cenozoic suggests that xenarthrans originated much earlier, possibly in the Cretaceous. The greatest evolutionary diversity of xenarthrans occurred between Late Paleocene and Pliocene times, when South America was an island continent; it included evolutionary lines of armadillos, huge armadillolike glyptodonts, anteaters, ground sloths, arboreal sloths, and even sloths exploiting aquatic habitats. The origin and evolutionary history of xenarthrans was confined to South America between Late Paleocene and Pliocene times, with secondary centers of radiations in Central American, North America, and Caribbean Islands after the Pliocene, when North and South America were tectonically connected.

Xenarthrans share other features besides their specialized lumbar vertebrae. In many species, especially armadillos, lumbar and caudal vertebrae are fused to the pelvis (ilium and ischium), some cervical (neck) vertebrae are fused to the axis forming a single vertebralike structure, and in some glyptodonts all backbone elements, from the pelvis to the neck, are fused. An extra cranial bone (sep-tomaxilla) occurs in some species and is found elsewhere among living mammals only in monotremes. Scapular processes are prominent, and a clavicle is present. All living species lack incisors and canines (two-toed sloths have caninelike teeth, but no true canines; a few extinct forms had true canines). Cheek teeth lack enamel, and each has an open root, allowing continuous growth throughout the life of the animal. The number of teeth varies in armadillos and sloths (the giant armadillo has up to 100 small teeth), but anteaters lack teeth.

Most species have five clawed hind digits and three to five front digits with two or three of them bearing very long, sharp, and strong claws. A double vena cava (returning blood from posterior body regions to the heart) is common; this is a large single vein in most other placental mammals.

Living armadillos range from the Strait of Magellan at the southern tip of South America north through Central America and Mexico into the southeastern United States. The twenty species are found in a broad range of habitats: deserts, savannas, pampas (grasslands), temperate deciduous forests, and tropical ever green rain forests. Their most conspicuous and unique trait is the protective, jointed, armorlike covering (carapace) over the head and body that is formed by bony scutes covered by horny epidermis. The scutes are arranged into a rigid plate on the head, and bands on the body connected by flexible skin. Sparse hair projects between the bands and covers the limbs and undersurface of the body. The tail of most species is also covered by bony scutes. Because the armor covering is sufficiently flexible in some species, they can roll into a ball, protecting their vulnerable soft underparts. Leathery ears range from small to large. The snout is usually elongate, and all armadillos have a long, protrusible tongue. Upper cheek teeth occur in the maxillary bone only, except for one species that also has premaxillary teeth. Sections of the vertebral column are fused (some cervical vertebrae are fused; lumbar and caudal vertebrae are fused with the pelvis), producing a rigid vertebral column that braces the carapace. The front limbs are powerful, and the digits are armed with long, strong digging claws. Armadillos walk on the tips of the front digits but on the soles of the hind feet (plantigrade).

There is a great range in body size among armadillos. The smallest are the two species of pichiciegos (Chlamyphorus), which are about the size of a small rat, with a head and body up to 117 mm long, tail up to 35 mm, and weight about 85 gm. The giant armadillo (Priodontes maximus) is the largest (about the size of a large dog), with a head and body up to 100 cm long, a tail up to 50 cm, and weighing up to 32 kg (zoo animals may reach 60 kg). All armadillos are terrestrial, some are nocturnal, others diurnal. They use their powerful front limbs and claws to dig for food and excavate burrows, where they stay when not active. Most species eat insects, but other invertebrates, small vertebrates, plants, and sometimes carrion are also sought. Armadillos may be solitary, travel in pairs, or sometimes form small bands, depending upon the species. After a gestation period extended by delayed implantation, armadillos bear a litter (up to twelve, usually two to four) of identical young produced from a single egg.

The armadillo traits reflected in the fused vertebrae and carapace reached their greatest specialization in members of the extinct Glyptodontidae, the other family in Order Cingulata. This group reached its greatest diversity between the Miocene and Pleistocene, perhaps in response to the spread of pampas in South America during that time. The largest was 2.5 to 3 m long and had a huge, turtlelike, inflexible carapace supported by an arched backbone in which the pelvis and all the sacral, lumbar, and thoracic vertebrae were fused into a single, immobile unit. The very heavy carapace was also supported by massive limbs. These ponderous animals were probably herbivores and grazed slowly over the pampas.

Living anteaters range from southern Mexico through Central America and into South America as far as Paraguay and northern Argentina. Habitat includes savannas, pampas, and tropical forests. The specializations of these xenarthrans reflect their ability to capture and eat social insects, primarily ants, termites, and bees. The skull consists of an elongate cranium, long and tapered rostrum, and a long, delicate mandible. The mouth is tubular. All anteaters lack teeth. Jaw musculature is reduced, but muscles controlling the tongue are well developed and strong. The highly specialized tongue is long, slender, covered with backward-directed, spinelike papillae, protrusible, and attaches by muscles to the sternum (breastbone), rather than to the hyoid bones in the throat (the site in almost all other mammals). Salivary glands secrete a sticky saliva that covers the tongue. Powerful front limbs end in four digits, three of them bearing large, robust, and recurved claws (the fourth digit has a small claw).

Tamanduas (Tamandua) and silky anteaters (Cyclopes) walk on the side of the front foot, with the digits and claws pointing inward; the giant anteater (Myrmecophaga) walks on its knuckles, with its digits partly flexed (the claws are protected in both stances). The four or five hind digits bear small claws, and the animals walk on the soles of the hind feet. The strong front limbs are used to tear apart ant and termite nests. Eggs, larvae, and adults are picked up by the sticky tongue and swallowed whole. Giant anteaters are terrestrial and mostly diurnal; tamanduas are arboreal and terrestrial and active during day and night; and the silky anteater is strictly nocturnal and terrestrial, rarely descending from tree crowns. Tamanduas and the silky anteater have strongly prehensile tails. The giant anteater does not construct burrows, but rests curled up in tall grass and forest underbrush; tamanduas shelter in large tree holes; and the silky anteater rests during the day in vine tangles or on branches in the tree crown. The silky anteater, about the size of a tree squirrel, is the smallest living anteater, with a head and body up to 230 mm long, tail up to 295 mm long, and weighing up to 295 gm. The largest is the giant anteater, about the size of a wolf, with a head and body up to 120 cm long, a tail up to 90 cm, and weighing up to 60 kg. Anteaters are solitary or go about in pairs; a female and her young may form a small band. All living anteaters bear a single young; gestation ranges from 130 to 190 days.

The five species of living tree sloths occur only in tropical evergreen rain forests from Honduras in Central America south to northern Argentina. About the size of monkeys, these animals are highly specialized for arboreal life and a folivorous diet (young leaves, tender twigs, and buds). For most of their lives they hang from limbs in the crowns of trees, a position they maintain while eating, sleeping, mating, and giving birth. They descend to the ground once or twice a week to urinate and defecate, and will occasionally descend and move awkwardly along the ground to another tree. Two-toed sloths (Choloepus) have long limbs, with the front limbs being only slightly longer than the hind limbs. Their feet are narrow and curved, with two digits on the front feet and three on the hind. All digits of each foot are bound together by skin. The claws are very long, laterally compressed, and recurved. Three-toed sloths (Bradypus) have three digits on both front and hind feet, and each digit bears a long, recurved claw. The front limbs are much longer than the hind. Both kinds of sloths hang from branches by their long limbs and grappling claws. The tail is absent or vestigial in Choloepus but short and blunt in Bradypus. Tree sloths have shaggy, coarse fur consisting of long overhairs and short underfur. The overhairs are roughened by transverse cracks or longitudinal fluting, providing habitat for green algae and cyanobac-teria in the fur of Choloepus and algae in the fur of Bradypus—giving the sloths a greenish cast if the algae is prolific. The coat of Brady-pus also provides refuge for some species of moths and beetles. Like other strictly herbivorous mammals, tree sloths have a chambered stomach in which digestion is enhanced by micro-organisms that break down cellulose. Cervical vertebrae vary from five to nine (seven is usual in nearly all other mammals), and allow a greater range of head movement, which is important to a sedentary, hanging ani mal. Tree sloths bear a single young after gestation of about 11 months in Choloepus and up to 106 days in Bradypus.

Living tree sloths appear very similar, but aspects of their morphology, physiology, and ecology are convergent. The two species of two-toed tree sloths are the only survivors of the Megalonychidae, which reached its greatest diversity during the Miocene-Pleistocene (more than twenty-five extinct genera) in South America, and included arboreal and giant terrestrial species. One of these ground sloths, Megalonyx, was the size of a large cow, evolved in North America, and once occurred as far north as Alaska. The Family Bradypo-didae contains only three-toed tree sloths and is not represented by fossils. Anatomy of the living species indicates a very distant relationship to both Megalonychidae and the extinct Megatheriidae.

—Mary Ellen Holden

See also: Arthropods, Terrestrial; Bacteria; Mammalia; Protoctists


Carroll, Robert. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman; Gardner, Alfred L. 1993. "Order Xenarthra." In Mammal Species of the World, 2d ed., edited by Don E. Wilson and DeeAnn M. Reeder, pp. 63-68. Washington, DC: Smithsonian Institution Press; McKenna, Malcolm C., and Susan K. Bell. 1997. Classification of Mammals above the Species Level. New York: Columbia University Press; Montgomery, G. Gene. 1985. The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas. Washington, DC: Smithsonian Institution Press; Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th ed. Vol. 2. Baltimore: Johns Hopkins University Press; Vaughan, Terry A., James M. Ryan, and Nicholas J. Czaplewski. 2000. Mammalogy, 4th ed. Orlando, FL: Harcourt.

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