Ground Sloths - An American Survivor of the Megafaunal Extinction

Giant ground sloth (Megatheriinae) is the common name for several species of large bodied mammals (megafauna) who evolved and lived exclusively on the American continents. The superorder Xenarthrans--which includes anteaters and armadillos--emerged in Patagonia during the Oligocene (34-23 million years ago), then diversified and dispersed throughout South America. The first giant ground sloths appeared in South America at least as long ago as the late Miocene (Friasian, 23-5 mya), and by the Late Pliocene (Blancan, ca. 5.3-2.6 mya) arrived in North America. Most of the large forms died out during the late Pleistocene, although there is recently discovered evidence of ground sloth survival in central America as recently as 5,000 years ago.

There are nine species (and up to 19 genera) of giant sloths known from four families: Megatheriidae (Megatheriinae); Mylodontidae (Mylodontinae and Scelidotheriinae), Nothrotheriidae, and Megalonychidae. Pre-Pleistocene remains are very sparse (except for Eremotheriaum eomigrans), but there are lots of fossils from the Pleistocene, especially Megatherium americanum in South America, and E. laurillardi in both South and North America. E. laurillardi was a large, intertropical species known as the Panamanian giant ground sloth, who may well have survived into the late Pleistocene.

Life as a Ground Sloth

Ground sloths were mostly herbivores. A study on over 500 preserved feces (coprolites) of the Shasta ground sloth (Nothrotheriops shastense) from Rampart Cave, Arizona (Hansen) indicate that they mainly dined on desert globemallow (Sphaeralcea ambigua) Nevada mormontea (Ephedra nevadensis) and saltbushes (Atriplex spp). A 2000 study (Hofreiter and colleagues) found that the diet of sloths living in and around Gypsum Cave in Nevada changed over time, from pine and mulberries around 28,000 cal BP, to capers and mustards at 20,000 years bp; and to saltbushes and other desert plants at 11,000 years bp, an indication of changing climate in the region.

Ground sloths lived in a variety of ecosystem types, from treeless scrublands in Patagonia to wooded valleys in North Dakota, and it seems that they were fairly adaptive in their diets. Despite their adaptability, they almost certainly were killed off, as with other megafaunal extinctions, with the assistance of the first set of human colonists into the Americas.

Ranking by Size

Read More

Giant Ground Sloth (Megalonyx)

By Bob Strauss

Giant ground sloths are loosely categorized by size: small, medium and large. In some studies, the size of the various species seems to be continuous and overlapping, although some juvenile remains are definitely larger than the adult and subadult remains of the small group. Cartell and De Iuliis argue that the difference is size is evidence that some of the species were sexually dimorphic.

• Megatherium altiplanicum (small, femur length about 387.5 mm or 15 inches), and about 200 kilograms or 440 pounds per adult individuals)
• Megatherium sundti (medium, femur length about 530 mm, 20 in)
• Megatherium americanum (large, femur length between 570-780 mm, 22-31 in; and up to 3000 kg, 6600 lb per individual)

All of the extinct continental genera were "ground" rather than arboreal, that is to say, lived outside of trees, although the only survivors are their small (4-8 kg, 8-16 lb) tree-dwelling descendants.

Recent Survivals

Most of the megafauna (mammals with bodies greater than 45 kg, or 100 lbs) in the Americas died out at the end of the Pleistocene after the retreat of the glaciers and about the time of the first human colonization of the Americas. However, evidence for ground sloth survival into the late Pleistocene has been found in a handful of archaeological sites, where research indicates that humans were preying on ground sloths.

One of the very old sites thought by some scholars to be evidence of humans is the Chazumba II site in Oaxaca state, Mexico, dated between 23,000-27,000 calendar years BP [cal BP] (Viñas-Vallverdú and colleagues). That site includes a possible cutmark--butchery mark--on a giant sloth bone, as well as a few lithics such as retouched flakes, hammers, and anvils.

Shasta ground sloth (Nothrotheriops shastense) dung has been found in several caves in the southwestern United States, dated to as late as 11,000-12,100 radiocarbon years before the present RCYBP. There are also similar survivals for other members of the Nothrotheriops species found in caves in Brazil, Argentina, and Chile; the youngest of those are 16,000-10,200 RCYBP.

Solid Evidence for Human Consumption

Evidence for human consumption of ground sloths exists at Campo Laborde, 9700-6750 RCYBP in the Talpaque Creek, Pampean region of Argentina (Messineo and Politis). This site includes an extensive bone bed, with over 100 individuals of M. americanum, and smaller numbers of glyptodons, panamanian hare (Dolichotis patagonum, vizcacha, peccary, fox, armadillo, bird, and camelid. Stone tools are relatively sparse at Campo Laborde, but they include a quartzite side-scraper and a bifacial projectile point, as well as flakes and micro-flakes. Several sloth bones have butchery marks, and the site is interpreted as a single event involving the butchery of a single giant ground sloth.

In North Dakota in the central US, evidence shows that Megalonyx jeffersonii, Jefferson's ground sloth (first described by the U.S. President Thomas Jefferson and his physician friend Caspar Wistar in 1799), were still fairly widely distributed across the NA continent, from Old Crow Basin in Alaska to southern Mexico and from coast to coast, about 12,000 years RCYBP and just before most of the sloth extinction (Hoganson and McDonald).

The most recent evidence for ground sloth survival is from the West Indian islands of Cuba and Hispaniola (Steadman and colleagues). Cueva Beruvides in Matanzas Province of Cuba held a humerus of the largest West Indies sloth, the Megalocnus rodens, dated between 7270 and 6010 cal BP; and the smaller form Parocnus brownii has been reported from the tar pit Las Breas de San Felipe in Cuba between 4,950-14,450 cal BP. Seven examples of Neocnus comes have been found in Haiti, dated between 5220-11,560 cal BP.

Sources and Further Information

• _{Cartelle C, and De Iuliis G. 2006. Eremotherium Laurillardi (Lund) (Xenarthra, Megatheriidae), the Panamerican giant ground sloth: Taxonomic aspects of the ontogeny of skull and dentition. Journal of Systematic Palaeontology 4(2):199-209.}
• _{Hansen RM. 1978. Shasta ground sloth food habits, Rampart Cave, Arizona. Paleobiology 4(3):302-319.}
• _{Hofreiter M, Poinar HN, Spaulding WG, Bauer K, Martin PS, Possnert G, and Pääbo S. 2000. A molecular analysis of ground sloth diet through the last glaciation. Molecular Ecology 9(12):1975-1984.}
• _{Hoganson JW, and McDonald HG. 2007. First Report of Jefferson's Ground Sloth (Megalonyx jeffersonii) in North Dakota: Paleobiogeographical and Paleoecological Significance. Journal of Mammalogy 88(1):73-80.}
• _{Iuliis GD, Pujos F, and Tito G. 2009. Systematic and Taxonomic Revision of the Pleistocene Ground Sloth Megatherium (Pseudomegatherium) Tarijense (Xenarthra: Megatheriidae). Journal of Vertebrate Paleontology 29(4):1244-1251.}
• _{Messineo PG, and Politis GG. 2009. New Radiocarbon Dates from the Campo Laborde Site (Pampean Region, Argentina) Support the Holocene Survival of Giant Ground Sloth and Glyptodonts. Current Research in the Pleistocene 26:5-9.}
• _{Pereira ICdS, Dantas MAT, and Ferreira RL. 2013. Record of the giant sloth Valgipes bucklandi (Lund, 1839) (Tardigrada, Scelidotheriinae) in Rio Grande do Norte state, Brazil, with notes on taphonomy and paleoecology. Journal of South American Earth Sciences 43:42-45.}
• _{Steadman DW, Martin PS, MacPhee RDE, Jull AJT, McDonald HG, Woods CA, Iturralde-Vinent M, and Hodgins GWL. 2005. Asynchronous extinction of late Quaternary sloths on continents and islands. Proceedings of the National Academy of Sciences 102(33):11763-11768.}
• _{Viñas-Vallverdú R, Arroyo-Cabrales J, Rivera-González II, Xosé Pedro R-Á, Rubio-Mora A, Eudave-Eusebio IN, Solís-Torres ÓR, and Ardelean CF. 2015. Recent archaeo-palaeontological findings from Barranca del Muerto site, Santiago Chazumba, Oaxaca, México. Quaternary International in press.}