Jefferson’s Ground Sloth

Jefferson's Ground Sloth, Megalonyx jeffersonii

Jefferson’s ground sloth, Megalonyx jeffersonii, is a Megalonychid ground sloth and one of two types of ground sloth that have been recovered from Ice Age sites in the Midwest. You can read about the other type, Harlans' ground sloth, here.  Ground sloths were large relatives of the modern two-toed sloths (Choloepus spp.) and three-toed sloths (Bradypus spp.). However, unlike modern sloths, which spend most of their time in trees, the ground sloths spent all of their time on the ground. 

Jefferson's ground sloth, Megalonyx jeffersonii

Mounted skeleton of Jefferson's ground sloth, Megalonyx jeffersonii, on display in the Changes exhibit at the Illinois State Museum. 

Order: 
Xenarthra (Sloths and Armadillos)
Family: 
Megalonychidae (Megalonychid ground sloths)
Statistics: 

Length: 2.4-3.0 m (8-10 ft)
Weight: 1000-1100 kg (2200-2425 lbs); McDonald 2005

Status: 
Extinct
Physical Description: 

Jefferson's ground sloth was a large, heavily built animal. It had a large skull with blunt snout, massive jaw, well-developed chewing muscles, and large, blunt, peg like teeth.  As with other sloths, the teeth had an outer layer of dentine, rather than enamel, and thus were softer than those of other mammals. Soft teeth wear faster than hard teeth, and to compensate for this, their teeth continued to grow throughout their lifetime (MacFadden et al. 2010). While most ground sloths walked on the outsides of their hind feet, Jefferson's ground sloth kept their feet flat while walking. Furthermore, the shape of the hip bone indicates that this animal could also stand on its hind legs, using its stout tail for support. They also had very large claws on their forelimbs, which were probably used to strip leaves or move tree branches. Although very few specimens have been recovered with preserved soft tissue, it is thought that Jefferson's ground sloth was covered with thick hair, as has been found for better preserved species of late Pleistocene ground sloths (e.g., Shasta ground sloth). 

Habitat: 

Jefferson's ground sloth dwelled primarily in woodlands and forest, although they likely occupied a variety of habitats within these broad systems. Two recent studies were able to link directly-dated specimens from the terminal Pleistocene with regional paleoenvironmental records, demonstrating that these particular animals were associated with spruce dominated, mixed conifer-hardwood habitats (Hoganson and McDonald 2007; Schubert et al. 2004). However, this is likely only one of several types of woodland habitats utilized by Jefferson's ground sloth over the course of the Pleistocene. 

Diet: 

The shape of this animal's teeth suggests that it was a browser who primarily ate leaves, twigs and possibly nuts (Kurten and Anderson 1980; McDonald 2005), and results from a recent isotopic study are consistent with this interpretation (Kohn et al. 2005).  Furthermore, a recent analysis of the bone chemistry of an individual recovered in Virginia strongly indicates that this animal had an herbivorous diet (France et al. 2007), which is in keeping with a previous study conducted for an individual recovered from Alberta (Bocherens et al. 1994). 

Behavior: 

Unlike other ground sloths, this species walked flat on the soles of their hind feet, and they were able to stand on their hind legs. Jefferson's ground sloths are one of two species of late Pleistocene ground sloths that are commonly found in caves (~22% of sites; McDonald 2003). It is unclear, however, whether this association reflects denning behavior due to physiology (e.g., caves provide a relatively stable environment that would reduce the animal's cooling or warming needs) and/or the need for a protected location to give birth. Juveniles have been recovered from numerous caves, but this may be due to the better overall preservation afforded by cave environments (McDonald 2003). 

North American Ice Age Distribution: 

Jefferson's ground sloth had the widest range of all North American ground sloths. They have been recovered from over 150 sites across the United States, as well as from northwestern Canada and western Mexico (Hoganson and McDonald 2007; McDonald et al. 2000). Furthermore, they are the only ground sloth that has been recovered from the Yukon and Northwest Territories in Canada. Scarcity in the Great Plains has been noted and interpreted as reflecting the paucity or absence of forested areas, the species' preferred habitat, within the region (Hoganson and McDonald 2007). 

Status at the end of the Pleistocene: 

As with other Pleistocene megafauna in North America, Jefferson's ground sloth went extinct towards the end of the Pleistocene. Despite the large number of specimens that have been recovered from across the continent, only a handful have been directly dated through radiocarbon. Fortunately, most of the directly dated specimens are from the terminal Pleistocene and provide good temporal estimates for the species' extinction window. Currently, the youngest widely-accepted radiocarbon dates were obtained from the lower jaw of an individual recovered from northern Illinois. Two dates were obtained on purified bone collagen from this specimen, returning a combined age of 13,800-13,160 cal BP (Schubert et al. 2004). A slightly older direct date of 13,830-13,560 cal BP was obtained on purified bone collagen from a toe bone (ungual) recovered from a site in North Dakota (Hoganson and McDonald 2007). 

Midwestern Paleontological Finds: 

Megalonyx jeffersonii remains have been recovered from late Pleistocene sites, primarily caves, across the Midwest. These sites include West Cave and Brynjulfson Cave #1 in central Missouri, a site in the Galena Lead region of northwestern Illinois, the Hanson Megalonyx from Minneapolis, Minnesota, the Carter site in western Ohio, Gillenwater near Bowling Green, Kentucky, Robinson Cave in northeastern Tennessee, and Cheek Bend Cave and Darks Mill in central Tennessee. 

Fun Facts: 
  • Jefferson's ground sloth is named for Thomas Jefferson, who recorded fossil bones from this animal that had been recovered from a cave in West Virginia in the late eighteenth century. Originally, Jefferson thought the remains belonged to a giant cat, based on the size of the large claws recovered from the cave, and the name he assigned to the animal, "Megalonyx", means "giant claw". He soon realized, however, that the animal was closely related to South American tree sloths.
  • Jefferson presented a scientific paper about his research on Megalonyx to the American Philosophical Society in 1797, marking the beginning of vertebrate paleontology in North America. In fact, Megalonyx was the subject of the first two scientific articles ever published in the U.S. on fossils.
Web Resources: 

https://www.beringia.com/research/ground_sloth.html
https://www.isgs.uiuc.edu/education/ice-age-res/sloth.shtml
https://www.dmr.nd.gov/ndfossil/poster/PDF/Megalonyx%20jeffersonii.pdf
https://www.monticello.org/site/research-and-collections/megalonyx-jeffersonii-fossils
 

References: 

Bocherens, H., Fizet, M., Mariotti, A., Gangloff, R.A., Burns, J.A., 1994. Contribution of isotopic biogeochemistry (13C, 15N, 18O) to the paleoecology of mammoths (Mammuthus primigenius). Historical Biology 7:187–202.

France, C., Zelanko, P., Kaufman, A., Holtz, T. R. 2007. Carbon and nitrogen isotopic analysis of Pleistocene mammals from the Saltville Quarry (Virginia, USA): Implications for trophic relationships. Palaeogeography, Palaeoclimatology, Palaeoecology 249:271–282.

Hoganson, John W. and H. Gregory McDonald. 2007. First Report of Jefferson’s Ground Sloth (Megalonyx Jeffersonii) in North Dakota: Paleobiogeographical and Paleoecological Significance. Journal of Mammalogy, 88(1):73–80

Kohn, M.J., McKay, M.P., and Knight, J.L. 2005. Dining in the Pleistocene: who’s on the menu? Geology 33:649–652.

Kurtén, B. and Anderson, E. 1980. Pleistocene Mammals of North America. Columbia University Press, New York.

MacFadden, B., DeSantis, L., Labs-Hochstein, J., Kamenov, G. 2010. Physical properties, geochemistry, and diagenesis of xenarthran teeth: Prospects for interpreting the paleoecology of extinct species. Palaeogeography, Palaeoclimatology, Palaeoecology 291:180–189.

McDonald, H. G. 2003. Sloth Remains from North American Caves and Associated Karst Features. In Ice Age Cave Faunas of North America, edited by B. W. Schubert, J. I Mead, and R. W. Graham, pp. 1-16. Indiana University Press, Indianapolis.

McDonald H. G. 2005. The paleoecology of extinct xenarthrans and the Great American Biotic Interchange. Bulletin of the Florida Museum of Natural History 45:313-333.

McDonald, H. G., Harington, C. R., De Iuliis, G. 2000. The Ground Sloth Megalonyx from Pleistocene Deposits of the Old Crow Basin, Yukon, Canada. Arctic 53(3):213–220

Schubert, B. W., R. W. Graham, H. G. McDonald, E. C. Grimm and T. W. Stafford, Jr. 2004. Latest Pleistocene paleoecology of Jefferson’s ground sloth (Megalonyx jeffersonii) and elk-moose (Cervalces scotti) in northern Illinois. Quaternary Research 61:231– 240.