Harlan's Ground Sloth

Harlan's ground sloth, Paramylodon harlani

Harlan's ground sloth is a mylodont ground sloth and one of two types of ground sloth that has been recovered from late Pleistocene sites in the Midwest. You can read about the other type, Jefferson's ground sloth, here.  Ground sloths were large relatives of the modern two-toed sloths (Choloepus spp.) and three-toed sloths (Bradypus spp.). Unlike modern sloths, which spend most of their time in trees, ground sloths spent all of their time on the ground.

For nearly two centuries there has been some controversy regarding how Harlan's ground sloth relates to other Pleistocene mylodont ground sloths, and particularly whether it belongs to the Glossotherium genus or Paramylodon genus. Consequently, Harlan's ground sloth is alternately identified as Glossotherium harlani or Paramylodon harlani within scientific literature and across the internet. This disagreement stems primarily from early confusion about how to name the different ground sloth skeletons assigned to the Mylodont family, with the result that a single species, such as Harlan's ground sloth, might have had as many as 15 different scientific names! Over time, this confusion led to disagreement about whether Paramylodon was truly a different genus than Glossotherium, with some scientists suggesting that all mylodont sloths should be assigned to the genus Glossotherium (Hoffstetter 1952; Kurtén and Anderson 1980; Simpson 1945) and others arguing that Paramylodon is a valid genus for North American mylodont sloths (Cabrera 1936; Kraglievich 1928; McDonald 1995).  A recent evaluation of physical characteristics and measurements of different mylodont species, particularly skull and jaw measurements, supports the latter argument and demonstrates that the two genera are, in fact, distinguishable from each other (McAfee 2009). For that reason, we refer to Harlan's ground sloth as P. harlani throughout this web exhibit.

 

Harlan's ground sloth, Paramylodon harlani

Complete Paramylodon harlani skeleton recovered from the Rancho la Brea formation in California and in the Paleobiology collections of the National Museum of Natrual History, Smithsonian Institution. Information provided with the permission of the National Museum of Natural History, Smithsonian Institution, 10th and Constitution Ave. N.W., Washington, DC 20560-0193. (https://www.nmnh.si.edu/).

Order: 
Xenarthra (Sloths and Armadillos)
Family: 
Mylodontidae (Mylodont ground sloths)
Statistics: 

Length: 3 m (9.8 ft); McDonald 2005

Weight: 1100-1400 kg (2425-3085 lbs); McDonald 2005

Status: 
Extinct
Physical Description: 

Harlan's ground sloth was a massive, powerfully built animal, with an enormous chest, extremely large forelimbs, stout hind limbs, strong claws, short neck and a robust tail. It also had an elongated head with blunt nose and flat grinding 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 down faster than hard teeth, and to compensate for this, their teeth continued to grown throughout their lifetime (MacFadden et al. 2010). A fairly unique characteristic of mylodont ground sloths is the presence of small, pebble-like bones, called osteoderms or dermal ossicles, embedded deep within the skin. In Harlan's ground sloth, these osteoderms were located around the neck, shoulders and back, and they likely provided some protection against predators. It is also thought that this animal was covered with coarse, shaggy hair, based on preserved hair recovered from a cave site in Utah (Davis et al. 1984; McDonald et al. 2004; Mead et al. 1986).

Habitat: 

The remains of this animal are generally found in areas that were once open grasslands or parklands, suggesting that Harlan's ground sloth preferred this type of habitat. They are not found in association with dry, desert habitats, and, in fact, the few remains that have been recovered from desert areas are found in direct association with some source of permanent water, such as rivers or lakes.  It has been suggested that the riparian vegetation associated with these permanent water sources would have served as important migration corridors, allowing these animals to move through otherwise hostile areas in search of their preferred habitat (McDonald et al. 2004).

Diet: 

Harlan's ground sloth is thought to have been an herbivore that grazed primarily on grasses and sedges. This interpretation is based in large part on the shapes of their jaw and teeth (McDonald 2005) as well as the close association of P. harlani remains with grassland habitats. Recent carbon isotope (d13C) analysis of P. harlani teeth from a site in Texas supports the idea that these animals primarily ate grasses, although the results from this study also suggest that they may have eaten small amounts of shrubs and trees as well (Ruez 2005). These findings are similar to those obtained from a bone chemistry study conducted on Paramylodon remains from Rancho La Brea (Coltrain et al. 2004), and they are in line with previous suggestions, based on skull morphology, that this animal may have included small amounts of browse (leaves, twigs, roots, etc.) in its diet (Naples 1989).

Behavior: 

Typically, records of P. harlani represent remains of single animals, suggesting that they were solitary animals with limited social interactions (McDonald and Pelikan 2006). Based on their physiology, it is thought that these animals were slow moving and clumsy; they most likely walked on the outside of their hind feet and the backs of their fore feet. A South American study (Bargo et al. 2000) demonstrated that the forearms of mylodont ground sloths were particularly suited to strenuous activities, such as digging, although a similar study has yet to be conducted on North American mylodont sloths.

North American Ice Age Distribution: 

During the late Pleistocene, Harlan's ground sloth was widely distributed across the U.S., with a range that extended from coast to coast and from Florida to Washington. Their remains are fairly common over much of this range, and particularly so in the western U.S.  In fact, more than 76 individuals have been recovered from the Rancho La Brea tar pits in California (Kurtén and Anderson 1980).  However, they are noticeably absent from desert areas (McDonald et al. 2004), as well as the upper Midwest and Northeast (McDonald and Pelikan 2006).

Status at the end of the Pleistocene: 

Based on associated radiocarbon dates, Harlan's ground sloth went extinct during the terminal Pleistocene (roughly 11,500 years ago), along with many other large mammals in North America. The timing of this extinction will be better understood once radiocarbon dates are obtained directly on P. harlani remains.

Midwestern Paleontological Finds: 

Although Harlan's ground sloth is missing from the paleontological record of the northern Midwest, remains of this animal have been recovered from several sites in the lower Midwest, including Boney Springs, Jones Springs, Kimmswick and Autolite Cave in Missouri and Big Bone Lick in Kentucky. 

Fun Facts: 
  • Sloths have a very hard time regulating their body temperature, and as a consequence they are much less active than other mammals. Thus, while sloths may be "slothful" by nature, this characteristic is due to physiology rather than attitude.
Web Resources: 

https://library.sandiegozoo.org/factsheets/_extinct/sloth_extinct/extinct_sloth.htm
https://slothcentral.com/

References: 

Bargo, S., Vizcaino, S., Archuby, F., Blanco, E. 2000. Limb Bone Proportions, Strength and Digging in Some Lujanian (Late Pleistocene-Early Holocene) Mylodontid Ground Sloths (Mammalia, Xenarthra). Journal of Vertebrate Paleontology 20(3):601–610.

Cabrera, A. 1936. The species of the genus ‘Glossotherium’. Notas del Museo de La Pata 1: 193–206.

Coltrain, J., Harris, J., Cerling, T., Ehleringer, J., Dearing, M.D., Ward, J., Allen, J. 2004. Rancho La Brea stable isotope biogeochemistry and its implications for the palaeoecology of late Pleistocene, coastal southern California. Palaeogeography, Palaeoclimatology, Palaeoecology 205 (2004) 199– 219

Davis, O.K., Agenbroad, L.D., Martin, P.S., Mead, J.I., 1984. The Pleistocene dung blanket of Bechan Cave, Utah. Carnegie Museum of Natural History Special Publication 8, 267–282.

Hoffstetter R. 1952. Pleistocene mammals from the Republic of the Equator. Mémoires de la Société Géologique de France, Nouvelle Série 31: 1–391.

Kraglievich L. 1928. ‘Mylodon Darwin’ Owen is the type species of ‘Mylodon’ Owen. Revista de la Sociedad Argentina de Ciencias Naturales, Physis 9: 169–185.

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

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 (2010) 180–189.

McAfee, R. 2009. Reassessment of the cranial characters of Glossotherium and Paramylodon (Mammalia: Xenarthra: Mylodontidae). Zoological Journal of the Linnean Society 155: 885–903

McDonald, H.G. 1995. Gravigrade Xenarthrans from the Early Pleistocene Leisey Shell Pit 1A, Hillsborough County, Florida. Bulletin of the Florida Museum of Natural History 37 Pt. II (11): 345–373.

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., Pelikan, S. 2006. Mammoths and mylodonts: Exotic species from two different continents in North American Pleistocene faunas. Quaternary International 142–143: 229–241.

McDonald, H. G., Agenbroad, L. D., Manganaro Haden, C. 2004. Late Pleistocene Mylodont Sloth Paramylodon harlani (Mammalia: Xenarthra) from Arizona.  The Southwestern Naturalist 49(2): 229-238

Mead, J.I., Agenbroad, L.D., Davis, O.K., Martin, P.S., 1986. Dung of Mammuthus in the arid Southwest, North America. Quaternary Research 25:121–127

Naples, V. L. 1989. The feeding mechanism in the Pleistocene ground sloth, Glossotherium. Contributions in Science 415:1-23.

Ruez, D. 2005. Diet of Pleistocene Paramylodon Harlani (Xenarthra: Mylodontidae): Review of Methods and Preliminary Use of Carbon Isotopes. The Texas Journal of Science 57(4):329-344.

Simpson, G. 1945. Principles of classification and a classification of mammals. Bulletin of the American Museum of Natural History 85.