Evolution and systematics

Evolved in South America, this diverse order first appears in the fossil record in the Paleocene, about 60 million years ago (mya). It has two main groups, the Pilosa and the Cin-gulata. The Pilosa contains sloths and anteaters, also known as the hairy xenarthrans, and the Cingulata includes the extinct glyptodonts and armadillos, the animals with bony carapaces. The group name "Xenarthra" refers to the additional articulations between the lumbar vertebrae, called xe-narthrous processes. These extra articulations provide an unusually large amount of stability to the pelvic region, and have been hypothesized to confer advantages on sloths and arboreal anteaters in reaching out from one support to another with the body held horizontally, and to the armadillos for digging with great strength and speed. The hands in xenarthrans usually have two or three digits larger than the others, although the number of digits in tree sloth hands is reduced to two or three. All fingers have sharp claws, which are often long and laterally compressed. Other than the tree sloths, which have three toes on the hind foot, most xenarthrans have five toes, each bearing a sharp claw. Sloths and anteaters have a tendency toward supination of the forearm and hands. Freedom to rotate the wrist is useful for climbing in trees as well as allowing anteaters to turn the sharp claws upward and inward for protection while walking terrestrially. The hind foot in some extinct sloths was also capable of supination, and the animals walked on the outsides of the feet, again to protect long, sharp claws. The radius and ulna are separate, which also contributes to supination ability. The scapulae are large and have a second spinous process posterior to and parallel to the first. This characteristic is particularly important for increasing the surface area for attachment of the muscles used in retracting the forelimb as would be necessary in digging, and is most striking in the armadillos and anteaters although also present in the sloths, contributing to their ability to climb.

The earliest xenarthrans were small, resembling primitive armadillos more than sloths or anteaters. Presently, no direct ancestral fossil lineage leading to the xenarthran groups is known. By the time sloths, anteaters, armadillos, and glyptodonts occur in the fossil record, the distinctions among lineages as well as those within the lineages are clear. Past authors considered Paleanodonts to be xenarthran ancestors, but they are now recognized instead as ancestors to the Pholidota, a group that includes the living pangolins. Even though the Paleanodonts were not of the direct lineage leading to the Xe-narthra, they probably were of a body form similar to xenarthran ancestors and may be the equivalent of a sister group. These animals were small, armadillo-like, but lacked the bony armor seen in the earliest xenarthrans. They had reduced dentitions, lacking enamel, as was probably a primitive characteristic of

Choloepus Didactylus
The southern two-toed sloth (Choloepus didactylus) uses all four limbs to move along a tree branch. (Photo by Kate McDonald. Bruce Coleman, Inc. Reproduced by permission.)

the ancestral xenarthran. Both groups arose from Insectivores in the late Cretaceous, over 70 mya.

The earliest sloths appear in the fossil record in the Oligocene. These are the mylodonts, the family Mylodonti-dae, the group that retains more primitive characters than the other two, including dermal ossicles. The other two lineages, the family Megatheriidae and the family Megalonychidae, are first recorded in the fossil record in the Miocene. Although some extinct sloths were very large, all three early lineages were small to moderate and increased size through time, culminating in the giant Pleistocene megatheres and eremoth-eres. Although the largest sloths went extinct at the end of the Pleistocene, some may have persisted to less than 13,000 years ago. Speculations that extinct large sloths coexisted with humans and may have been driven to extinction by hunting activities are refuted by differences in the level of the strata in which sloth remains and human artifacts are found in caves. These differences indicate that the sloths used the caves at some time prior to their occupation by humans. To date, there is no conclusive evidence of human and sloth interaction.

The fossil record of the anteaters is the most fragmentary of the xenarthrans, and the earliest species do not significantly differ from the more recent ones. No fossil anteater had teeth, and even the earliest had elongate heads and, presumably, long tongues. Together with a body form reminiscent of the living anteaters, it is likely that by the time they appear in the fossil record, they were restricted to eating insects.

The Cingulata, or the armored xenarthrans, are closer in body form to the earliest of the xenarthrans. Of the three main groups, the chlamytheres or giant armadillos, the glyptodonts, and the armadillos, the first two are entirely extinct. The chlamytheres were similar to living armadillos in body form, although some were much larger. The cingulates that differed the most from the other lineages were the glyptodonts. As did the sloths, these animals increased in body size through time, and the largest survived into the Pleistocene. Not only did the glyptodonts have solid armor on their bodies, but their tails were encased in bony plates and some had solid bone club-like expansions at the ends. They probably were used very effectively for defense.

The dentition in xenarthrans is typically reduced in tooth types and numbers and all lack enamel. There is no milk dentition, and the teeth are ever-growing. No xenarthran has identifiable incisors. Sloths are the only xenarthrans with canine-shaped teeth, and in these animals they occlude upper in front of lower, opposite from the pattern in other mammals, making their relationships to true canine teeth uncertain. Therefore, in sloths those teeth are called "caniniform." Likewise, neither premolars nor molars can be distinguished in sloths, armadillos or glyptodonts, and the cheek teeth are all similar in appearance and all called "molariform." The anteaters are the only edentulous xenarthrans, although the

Almost all of the Hoffmann's two-toed sloth (Choloepus hoffmanni) typical behavior happens while the animal is upside down. (Photo by Tom Brakefield. Bruce Coleman, Inc. Reproduced by permission.)
A pichi armadillo (Zaedyus pichiy) is also known as pygmy armadillo. (Photo by Tom McHugh/Photo Researchers, Inc. Reproduced by per-

ture of having additional vertebral articular surfaces that allow them to stretch themselves horizontally from a vertical support while holding on only by the hind limbs.

Anteaters are recognized by their long, tapered snouts, remarkably long and thin tongue, and large, powerful fore-claws. The foreclaws are used both for defense and for the purpose of tearing open ant and termite mounds. The pelage is long and thick enough to temporarily protect the animals from invading insects. All but one species has a grasping prehensile tail.

Armadillos are the only living mammals with a protective bony skin armor. Unlike reptile shells, an armadillo's mail is interrupted by several folds of skin to assist with agility. The skin's surface is gray or brown, and quite soft. They are stocky, medium-sized mammals that walk low to the ground. Head and ear shape varies among species, and powerful limbs bear enlarged claws for digging burrows and gathering food.

group was previously known as the Edentata. The teeth in sloths erupt as simple cones, and acquire the cusp pattern characteristic of each species through wear caused by movements of the masticatory muscles. The generation of tooth wear patterns in other xenarthrans has not been studied.

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