The Great Claw:
More about Ground Sloths
Jefferson's ground sloth (Megalonyx jeffersonii) was about 8-10 ft (2.5-3 m) in length. It was a heavily built animal, with an estimated maximum weight of 800 lbs (360 kg). The massive skull had a blunt snout, massive jaw, and large peg-like teeth. The hind legs were plantigrade (flat-footed) and together with the stout tail allowed it to rear up into a semi-standing posture to feed on foliage in trees. The forelimbs had three well-developed claws that probably allowed it to strip leaves or manipulate branches.
This species was one of the more widely distributed ground sloths of North America. Its fossils have been recorded from woodland and forest sites in most of the American states east of the Rocky Mountains and along the West Coast. It was the only ground sloth to be widely distributed in Canada and the only species to extend into eastern Beringia (Yukon and Alaska). It appeared in the fossil record during the Illinoian glaciation (more than 150,000 years ago) until the end of the Wisconsinan glaciation (about 10,000 years ago).
Megalonyx jeffersonii was the last member of the family Megalonychidae. The earliest records for this family occur in Oligocene (approximately 35 million years ago) of Patagonia. Several South American genera are known from the middle Miocene, but the record for later members on that continent is poor. On the other hand, immigrants from South America evolved into a diverse array of species in the Caribbean from the late Miocene through the Pleistocene. Two genera, Pliometanastes and Megalonyx, colonized North America via the Caribbean. As a general trend, the megalonychids progressed from small arboreal forms (in South America) to larger, ground-dwelling forms in North America; many of the Caribbean species were relatively small, apparently because of the ecological restrictions of living on small islands.
The remaining ground sloths belong to two other families, the Megatheriidae and the Mylodontidae. Unlike the Megalonychidae, most of the evolutionary history of these two families occurred in South America.
The Megatheriidae consist of two main groups, megatheres and nothrotheres. The megatheres were huge animals reaching as much as 20 ft (6 m) in length and weighing up to 3 tons (2700 kg). The most common North American species was Eremotherium rusconii, which inhabited parts of Texas and the southeastern United States from the late Pliocene through the end of the Pleistocene. Another megathere, Megatherium americanum, was the first ground sloth to be discovered by Europeans (1). In contrast to the megatheres, the nothrotheres were smaller and more gracile (lightly-built). The most common and long-lived North American species, the Shasta ground sloth (Nothrotheriops shastensis), extended from South America into southwestern Canada. It's the best known of the ground sloths. Numerous specimens have been recorded from the La Brea tarpits of California and exceptional specimens from caves in the Colorado Plateau have informed us of its diet, hair, age structure and anatomy. Despite their differences in build, megatheres and nothrotheres share many characteristics, including a relatively long snout and other specializations for browsing shrubs and trees. Unlike Megalonyx, the toes on their hind limbs curved under their feet. Presumably, they were less capable walkers.
The remaining family of ground sloths is the Mylodontidae. These were heavily built animals with short necks and massive chests. They had dermal ossicles (small boney plates underneath the thick hide) and like the Megatheriidae, the toes on their hind limbs curved underneath the feet. The most common North American mylodontidid was the Harlan's ground sloth (named by different authorities as either Paramylodon harlani or Glossotherium harlani). It was about 6 ft (1.8 m) in length and 3500 lbs (1600 Kg) in weight.
The last of the ground sloths became extinct at the end of the Pleistocene. Their closest living relatives are the three-toed tree sloths (Bradypodidae) and the two-toed tree sloth (Choloepidae). Each tree sloth family is represented by a single genus, Bradypus and Choloepus, respectively, and both are restricted to South America. Detail of the teeth, hair and number of vertebrae indicate that the three-toed sloths are closely related to the Megatheridae, whereas Megalonyx is most closely allied with the two-toed sloths.
All of the sloths belong to the order Xenarthra (or edentates), Other members of the order include armadillos, anteaters and extinct group of large, armored forms called glyptodonts. As a group, they're characterized by peg-like teeth that lack enamel (the anteaters have secondarily lost their teeth) and by a uniquely complex joint in their vertebrate (backbone). Xenarthrans are also notable for their metabolism. They're less capable of regulating their body temperature than are other placental mammals. This inability means that they are much less active than most other mammals. As it turns out, a sloth is slothful because of physiology, not attitude.
Webpages:
- The Academy of Natural Sciences' web page on Joseph Leidy's study of Megalonyx and other ground sloths:
www.ansp.org/museum/leidy/paleo/ground_sloths.php - Illinois
State Museum's web page on ground sloths:
www.museum.state.il.us/exhibits/larson/sloth.html - Tarpits.org's web page on ground sloths:
www.tarpits.org/education/guide/flora/sloth.html - Yukon Berangia's web page on Megalonyx jeffersonii:
www.beringia.com/02/02maina1.html
Print Resources:
- Bru, Juan Bautista. 1796. "Descripción del esqueleto en particular, segun las observaciones hechas al tiempo de amarle y cococarle en esta Real Cabinente." pp. 1-16, Pl 1-5. in: José Garriga (ed.) Descripción del esqueleto de un quadropedo myy porpulento y rara, que se conserva en Real Cabinente de Historia Natural de Madria. Madrid: Viude del Ibarra.
- Carroll, R.L. 1988. Vertebrate Paleontology and Evolution. New York: W.H. Freeman and Company.
- Cuvier, Geoges. 1796. "Notice sur le squelette d'une très-grande espèce de quadrupède onconnue jusqu'à présent, trouvé au Paraguay, et déposé au cabinet d'historie naturelle de Madrid, redigée par G. Cuvier". Magasin encyclopédique, 2e anée, 1: 303-310.
- Kurtén, B. and E. Anderson. 1980. Pleistocene Mammals of North America. New York: Columbia University Press.
- Lang, I.A. 2002. Ice Age Mammals of North America: A Guide to the Big, the Hairy, and the Bizarre. Missoula: Mounain Press.
- Webb. S.S. 1994. "Successful in Spite of Themselves." Natural History 103(4): 50-53.
Notes:
- A nearly complete skeleton of Megatherium was discovered in present-day Paraguay in 1789. The bones were sent to Spain where they were reassembled by Juan-Bautista Bru, a conservator at the Royal Museum in Madrid. A copy of Bru's unpublished illustration and a short description of the skeleton was taken to France and examined by Georges Cuvier. In 1796, Cuvier published a paper describing this startlingly large and bizarre animal. He identified it as a relative of the sloth and armadillo, and named it Megatherium. (=giant beast). Bru also published a description of the animal in 1796. [go back]
Image Credits:
- Leidy, Joseph. 1855. "A Memoir of the Extinct Sloth Tribe of North America." Smithsonian Contributions to Knowledge 7(5). [go back]
- Cuvier, Georges. 1812. Researches sur les ossemens fossiles de quadrupèdes, òu l'on rétablit les caractères de plusieurs espèces d'animaux que les révolutions du globe paroissent avoir détruites. 4 vol. Paris: Deterville. [go back]