Physical characteristics

Xenarthrans living today range in size from the smallest, Chlamyphorus, the fairy armadillo, at about 5 in (12.5 cm) head and body length, to Myrmecophaga, the giant anteater, at about 47 in (120 cm) head and body length. Some extinct forms were larger; the extinct glyptodonts were over 6.5 ft (2 m) in head and body length, and the largest of the extinct sloths probably exceeded 10 ft (3 m) and were as heavy as modern elephants. It is probable that all early xenarthrans had some form of bony armor. Dermal ossicles occur in some extinct sloths, although living sloths and anteaters have entirely lost this tendency.

Tree sloths have pear-shaped bodies with large abdomens allowing a large cecum and long, slender limbs ending in elongated curved claws. Their fingers are bound together by skin, so the claws are often mistaken for their fingers or toes. All tree sloths have three claws on their hind feet, but the common names of "two-toed" and "three-toed" are based on the number of digits present on their front feet. Their outer pelage is long and coarse, and there is a short, soft, dense undercoat. Their heads are small and rounded. The eyes in Bradypus are small and dark; both sloths show only pinhole opening for the pupil and are not believed to see well. Tree sloths have external ears, but these are typically hidden in the elongate guard hair of the outer fur coat. Sloths are extremely unusual among mammals in having a variable number of cervical vertebrae (six in Choloepus and eight or nine in Brady-pus). These additional vertebrae cause the neck to be longer in Bradypus and may contribute to the ability of this sloth to turn its head around to a greater distance than is typical for a mammal. The sloths also share the unusual xenarthran fea-

A female brown-throated three-toed sloth (Bradypus variegatus) with green algal growth on her hair. (Photo by Michael P. L. Fogden. Bruce Coleman, Inc. Reproduced by permission.)
A female three-toed sloth (Bradypus tridactylus) sleeping. (Photo by Michael P. L. Fogden. Bruce Coleman, Inc. Reproduced by permission.)

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