Xenarthra
Sloths, Armadillos and Anteaters
Xenarthra is a superorder of animals that originated in South America and includes anteaters, sloths and armadillos. During the Pleistocene Epoch, the Midwest was home to several types of xenarthrans. The most widespread varieties of these ancient animals were the giant ground sloths, huge extinct relatives of today’s tree sloths, some of which could weigh as much as a grizzly bear. In addition to these sloths, the Midwest was also home to different types of armadillos, including glyptodonts, giant plant-eaters protected by thick bony armor, as well as large pampatheres and relatives of the living nine-banded armadillos. These animals spread into North America from South America as part of the Great American Biotic Interchange after the two continents became joined by the Isthmus of Panama. Over time, unique lineages of xenarthrans began to evolve in their new homes, becoming new endemic species only found in certain parts of North America.
Anatomy
(BELOW) A labeled diagram of the skeletal anatomy of the ground sloth Northrotheriops. Drawing is based on a skeletal mount on display at the KU Natural History Museum.
Skeletal Anatomy
Xenarthrans are a group of animals whose earliest members were adapted for digging burrows. As a result, many features of their skeletons are connected to their fossorial past. One common feature seen in the skeletons of xenarthrans is the presence of specialized articulation points between the lumbar vertebrae which are sometimes called xenarthrous joints. These extra connections add extra strength to the spine during activities which put a lot of strain on this part of the body such as digging or climbing. In most xenarthrans the forelimbs of these animals are heavily built and their arm bones are covered in enlarged surfaces for muscle attachments. This allows these animals to impart a lot of force from their arms and hands when digging nests or reaching up into trees to grab ahold of leafy branches. In armadillos, the underlying skeleton supports a protective covering of bony dermal plates which together make up the animal’s shell segments. The teeth of xenarthrans can vary a lot as well. Megalonyx, for instance, has a set of four large incisors at the front of its mouth which is a feature not seen in most other types of sloths. Anteaters have no teeth at all and armadillos have somewhat pointed peg-shaped teeth.
(BELOW) A labeled diagram showing the key bones and features of the skeletal anatomy of a nine-banded armadillo. For the purposes of this diagram the dorsal shell and cephalic shield are shown only in profile view as a cross-section
Shell Anatomy
Armadillos are a group defined by their shells. These keratinous plates are a form of body covering that serves as a line of defense against predators and environmental hazards. The shells of armadillos are made of hundreds of small bony plates called osteoderms, which are embedded within the skin and coated by a durable layer of keratin. This armor is organized into distinct sections, usually a set of rigid shields, one over the shoulder and the other over the pelvic region, connected by a series of flexible bands that run across the back. In some glyptodonts the bands along the middle of the shell are absent and the dorsal armor instead forms a single large dome covering the animal's entire back. The tails of armadillos are surrounded by a set of overlapping ring-shaped bands. Some giant South American armadillos like Doedicoerus had bony clubs at the ends of their tails which they could swing around when they felt threatened. An armadillo’s armor is completely attached to the animal's body and is supported by the underlying skeleton. In addition to helping to deter attacks from predators.
(BELOW) The plates composing the shell of a nine-banded armadillo. The number of plates in between the pelvic and pectoral shield can vary a lot between genera and some armadillos lack these bands altogether
Sloths (Folivora)
Ancient giants with huge claws
During the Pleistocene Epoch, North America was home to several species of giant ground sloths. Much like their tree-dwelling relatives, the giant ground sloths of the past had long claws on their hands and feet. These claws were used to pull leafy branches down from trees so that they were easier to reach, and possibly as defensive tools to ward off potential predators. The largest sloths of the Pleistocene, animals like the elephant-sized Megatherium and Eremotherium, did not make it far enough to the north to reach the Midwest, but even the relatively mid-sized sloths from this region like Megalonyx and Paramylodon could easily end up weighing half a ton or more.
Jefferson's Ground Sloth
Megalonyx jeffersonii
Temporal Range: 5 million years ago to approximately 13,000 years ago
Geographic Range: Found across North America, Alaska to Mexico
Species: M. jeffersonii, M. leptostomus, M. matthisi, M. obtusidens, M. wheatleyi
Diet: Herbivorous browser, leaves, twigs, buds
Megalonyx, commonly known as Jefferson’s ground sloth, was an extinct ground sloth that lived in North America from the Pliocene epoch to the end of the Pleistocene epoch. It was one of the continent’s largest herbivores, reaching about 3 meters (10 feet) in length and weighing up to nearly 1,300 kilograms (2,900 pounds). Its most distinctive features were its powerful limbs and large curved claws, which it likely used to pull down branches and gather vegetation. Fossils of Megalonyx have been found across much of the United States and even as far north as Alaska during warmer periods, making it the most widely distributed North American ground sloth.
Megalonyx is also unique in terms of the anatomy of its hind feet. In most ground sloths, the feet are positioned sideways such that the weight of the animal is supported by the sides of their feet, while the soles face towards one another. Megalonyx seems to have retained plantigrade feet, where the sole is placed on the ground.
(BELOW) A cranium of the ground sloth Megalonyx on display at the KU Museum. These ground sloths had particularly unique enlarged tusk-like incisors on their upper and lower jaws used to take bites of leafy vegetation
Nothrotheriops texanus
Temporal Range: 2.5 million years ago to approximately 10,000 years ago
Geographic Range: Found in the southern parts of North America
Species: N. shastensis, N. texanus
Diet: Herbivorous, grasses, fruits, roots, tubers, twigs, leaves
Nothrotheriops was one of the most distinctive ground sloths of Ice Age North America. It was relatively small, about the size of a black bear, with a long, slender skull and a narrow mouth. Its powerful forelimbs carried large curved claws used to pull down branches, forage for food, and defend against predators. It had a narrower torso and a longer tail than those seen in most other North American ground sloths, giving it a more slender frame. The species commonly referred to as the “Shasta ground sloth” is Nothrotheriops shastensis, mostly known from the southwestern US. Another species, Nothrotheriops texanus, was found in parts of the southern Great Plains in Oklahoma and Texas.
One of the most remarkable features of Nothrotheriops is the exceptional preservation of its soft tissues and integumentary remains. In dry caves of the American southwest, scientists have discovered not only bones but also preserved hair, skin, tendons, and even dung from these animals, providing rare insights into their appearances and diets. These finds show that it fed on a variety of desert plants, including yucca, cacti, and Joshua tree fruits.
(BELOW) A Nothrotheriops skull from a mounted skeleton on display at the KU Natural History Museum. Note the shape of the cheekbones. The long lower flange of bone seen in this skull is a feature found in the skulls of many types of ground sloths.
Paramylodon harlani
Temporal Range: 4.9 million years ago to approximately 12,000 years ago
Geographic Range: Found throughout North America as far south as Guatemala
Species: P. harlani
Diet: Herbivorous, grasses, twigs, leaves
Paramylodon was one of the largest ground sloths found in North America. It seems to have been a close relative of the South American sloth genus Glossotherium and its name means “near-Mylodon”, referring to its shared skeletal features with other mylodontid ground sloths. Like other mylodontids, Paramylodon had a long snout with well-developed chewing teeth at the back of its mouth and smaller somewhat sharpened teeth further forward. It is well represented in the tar pits at Rancho La Brea, with the remains of over 70 individual sloths known from this site alone.
Mylodontids are suggested to have been better equipped for feeding on grasses than some of their contemporaries like Megalonyx, possibly also using their long claws to dig through the soil in search of roots or tubers. Footprint trackways from New Mexico at White Sand National Monument preserve tracks of a Paramylodon which overlapped with human footprints, possibly left behind by people following the sloth as it moved through the area.
Armadillos (Cingulata)
Armored hulks and flexible burrowers
During the Pleistocene, North America was home to several extinct types of armadillos, including giant glyptodonts and pampatheres. These animals were relatives of modern armadillos but were far larger and more heavily built. Glyptodonts, such as Glyptotherium, were protected by a massive dome-shaped shell made of fused bony plates that covered nearly their entire body. Some species from South America also had heavily armored tails that could be used for defense. Pampatheres like Holmesina were smaller than glyptodonts with their armor formed into more flexible bands instead of rigid domes. Additionally, slightly larger relatives of today’s nine-banded armadillos were also found in the Midwest.
Beautiful armadillo
Dasypus bellus
Temporal Range: 1.8 million years ago to approximately 12,000 years ago
Geographic Range: Found across North America
Species: D. bellus
Diet: Omnivorous, insects, lizards, amphibians, tubers, roots
Dasypus bellus, also known as the beautiful armadillo, was a larger relative of today’s nine banded armadillo, both falling within the genus Dasypus. Their shells were arranged with two large half-dome shaped plates, one behind their heads and one in front of their tails. In between these larger pieces of armor they had a set of bony rings which added a degree of flexibility to their shells. Like their modern relatives the tops of their skulls were covered in a thick layer of reinforced shell material, and the underside of Dasypus bellus was not covered in armor. Their tails were long and thin, surrounded by overlapping bony rings. Their feet were tipped with long claws, used for digging in the process of excavating their burrows and they had long thin snouts and peg-like teeth suggesting that they fed on small animal prey such as insects, mollusks and small vertebrates, as well as a small amount of plant matter. This sort of diet is also seen in the living members of their genus.
Glyptotherium texanum
Temporal Range: 3.9 million years ago to approximately 15,000 years ago
Geographic Range: Found in parts of North and South America
Species: G. texanum, G. cylindricum
Diet: Herbivorous, grasses, twigs, leaves
Glyptotherium was a glyptodont and was one of the largest armadillos ever to live in North America. Like all armadillos, glyptodonts were originally a South American group. The majority of their species diversity remained in the southern hemisphere even during the Pleistocene, but some intrepid populations of these animals managed to reach the southern parts of what would one day become the US. Glyptotherium was found much further north than many of its relatives with its fossils being found in parts of Texas, Oklahoma and Florida. It seems to have preferred living in warmer climates and its spread further north may have been limited by lower winter temperatures.
The dome-like shell of Glyptotherium was composed of a huge number of small bony plates called osteoderms. These osteoderms displayed a characteristic rosette pattern with a central raised area surrounded by smaller peripheral elements. Their tails did not end in a tail club like those of some of their South American relatives but were instead surrounded by a series of 8-9 rings of armor which were further covered in short spines.
Holmesina septentrionalis
Temporal Range: 5 million years ago to approximately 12,000 years ago
Geographic Range: Genus found in North and South America
Species: H. septentrionalis, H. floridanus, H. major, H. occidentalis
Diet: Omnivorous, grasses, twigs, leaves, some invertebrates
Holmesina was a genus of pampathere, a type of large extinct armadillo who could grow to be around the same size as the living giant armadillo, with maximum lengths of around 6 feet. Their shells were divided into smaller plates instead of forming large rigid domes of armor like those seen in glyptodonts. Most pampatheres remained in South America where their fossil record is the most extensive, but animals like Holmesina were able to cross into North America where they became very successful in the Great Plains and the southern parts of what is now the US. Based on their teeth they are thought to have had a greater preference for plant matter as part of their diet than living omnivorous or insectivorous armadillos. Most of their teeth are fairly flat with broad chewing surfaces which were well developed for processing leaves or grass, unlike the peg-shaped teeth seen in nine-banded armadillos.
Media Gallery
Suggested References: Xenarthra
De Iuliis, Gerardo; Bargo, María S.; Vizcaíno, Sergio F. (2001). "Variation in skull morphology and mastication in the fossil giant armadillos Pampatherium spp. and allied genera (Mammalia: Xenarthra: Pampatheriidae), with comments on their systematics and distribution". Journal of Vertebrate Paleontology. 20 (4): 743–754.
Delsuc, F.; Gibb, G.C.; Kuch, M.; Billet, G.; Hautier, L.; Southon, J.; Rouillard, J.-M.; Fernicola, J.C.; Vizcaíno, S.F.; MacPhee, R.D.E.; Poinar, H.N. (2016). "The phylogenetic affinities of the extinct glyptodonts". Current Biology. 26 (4): R155–R156.
Fari a, R. A., & Vizcaíno, S. F. (1997). “Allometry of the bones of living and extinct armadillos (Xenarthra, Dasypoda).” Zeitschrift fur Saugetierkunde, 62, 65-70.
Gillette, David D.; Carranza-Castañeda, Óscar; White, Richard S.; Morgan, Gary S.; Thrasher, Larry C.; McCord, Robert; McCullough, Gavin (2016). "Ontogeny and Sexual Dimorphism of Glyptotherium texanum (Xenarthra, Cingulata) from the Pliocene and Pleistocene (Blancan and Irvingtonian NALMA) of Arizona, New Mexico, and Mexico". Journal of Mammalian Evolution. 23 (2): 133–154.
McDonald, H.G. (2022). "Paleoecology of the extinct Shasta ground sloth, Nothrotheriops shastensis, (Xenarthra, Nothrotheriidae): The physical environment". New Mexico Museum of Natural History and Science Bulletin. 88: 33–43.
Naples, Virginia L. (1987), “Reconstruction of Cranial Morphology and Analysis of Function in the Pleistocene Ground Sloth Nothrotheriops shastense (Mammalia, Megatheriidae)”, Contributions in Science 389, October 1987, Natural History Museum of Los Angeles County
Schubert, Blaine W.; Graham, Russell Wm.; McDonald, H. Gregory; Grimm, Eric C.; Stafford, Thomas W. (2004). "Latest Pleistocene paleoecology of Jefferson's ground sloth ( Megalonyx jeffersonii ) and elk-moose ( Cervalces scotti ) in northern Illinois". Quaternary Research. 61 (2): 231–240.
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Vizcaíno, S. F.; De Iuliis, G.; Bargo, M. S. (1998). "Skull Shape, Masticatory Apparatus, and Diet of Vassallia and Holmesina (Mammalia: Xenarthra: Pampatheriidae): When Anatomy Constrains Destiny". Journal of Mammalian Evolution. 5 (4): 291–322.