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By Phil Myers
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.
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.
Contributors
Phil Myers (author), Museum of Zoology, University of Michigan.
