Ground sloth

Ground sloths Temporal range: Oligocene - Holocene
Fossil Eremotherium ground sloth skeleton at the National Museum of Natural History, Washington, DC
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Pilosa
Suborder:	Folivora
Families
Rathymotheriidae Ameghino, 1904 Scelidotheriidae Ameghino, 1889 Mylodontidae Gill, 1872 Orophodontidae Ameghino, 1895 Megalonychidae Gervais, 1855 Megatheriidae Gray, 1821

Ground sloths are a diverse group of extinct edentate (Superorder Xenarthra) mammals that are believed to be relatives of tree sloths and three-toed sloths. They may have died out as recently as 1550 in Hispaniola and Cuba (Nowak, 1999), but had long since been extinct on the mainland.

Four of the many identified species found in the United States consist of Harlan's Ground Sloth (Paramylodon harlani), Jefferson's Ground Sloth (Megalonyx jeffersonii), Laurillard's Ground Sloth (Eremotherium laurillardi), and the Shasta Ground Sloth (Nothrotheriops shastensis). All four were massive animals with large claws, and all are believed to have been herbivores.

Families

Paleontologists divide the ground sloths in multiple families. The main families are Mylodontidae, Megalonychidae, Megatheriidae and Nothrotheridae.

Megalonychidae

The Megalonychid ('giant claw') ground sloths first appeared in the early Oligocene, about 35 million years ago, in southern Argentina (Patagonia). With the rise of the land bridge at Panama, these ground sloths began to migrate north as part of the Great American Interchange. Eventually the Shasta giant ground sloth (Nothrotheriops shastensis) reached the Yukon. Megalonychids increased in size as time progressed. The first species were small and may have been partly tree dwelling, whereas the Pliocene (about 5 to 2 million years ago) species were already approximately half the size of the late Pleistocene Megalonyx jeffersonii. Some West Indian island species were as small as a large cat; their dwarf condition typified both tropical adaptation and their restricted island environment.

The earliest known North American megalonychid, Pliometanastes protistus, lived in Florida about 8 million years ago. Several species of Megalonyx have been named; in fact it has been stated that "nearly every good specimen has been described as a different species". A broader perspective on the group, accounting for age, sex, individual and geographic differences, indicates that only three species are valid (M. leptostomus, M. wheatleyi, and M. jeffersonii) in the late Pliocene and Pleistocene of North America.

Closeup of skull

Jefferson's ground sloth has a special place in modern paleontology, for Thomas Jefferson's letter on Megalonyx ("great claw"), read before the American Philosophical Society of Philadelphia, in August 1796, marked the beginning of vertebrate paleontology in North America. When Lewis and Clark set out, Jefferson instructed Meriwether Lewis to keep an eye out for ground sloths. He was hoping they would find some living in the Western range. Megalonyx jeffersonii was appropriately named after Thomas Jefferson.

Megalonyx, a widespread North American genus, lived past the close of the last (Wisconsinan) glaciation, when so many large mammals died out. Remains have been found as far north as Alaska.

There were rumours during the 19th Century from Patagonia that some Ground Sloths had survived with one explorer noting that a very large hairy beast that looked like a giant armadillo trotted past them and disappeared into the undergrowth during an expedition. The local Guaraní Indians of the area said that the Ground Sloth buried itself during the day in burrows it dug with its claws and usually only came out at night.

Closeup of hand, showing claws

Megatheriidae

The Megatheriid ground sloths appeared later in the Oligocene, some 30 million years ago, also in South America. The group includes the heavily-built Megatherium ( given its name 'great beast' by Richard Owen) and Eremotherium. Eremotherium eomigrans, which has been found in 2.2 million year-old sediments in Florida, reached a length of 6 meters and had the bulk of a bull elephant. Bones of a newly discovered ground sloth that is the oldest of its kind ever found in North America have been uncovered by a University of Florida research team. Weighing more than five tons and able to reach as high as 17 feet, the 2.2 million-year-old prehistoric creature was larger than today's African bull elephants, said UF paleontologist David Webb. Unlike other large-bodied ground sloths, the new species had an extra claw, representing a surprisingly primitive stage of evolution, Webb said. While all other giant sloths had four fingers with only two or three claws, this one had five fingers, four of them with large claws, the biggest being nearly a foot long, he said. Other ground sloths, such nothrotheres as the more slightly built Hapalops and Nothrotheriops line, reached a length of about 1.2 meters.

The last ground sloths in North America belonging to Nothrotheriops died so recently that their dung ('coprolites') remains in caves. One of the skeletons, found in a lava tube (cave) at Aden Crater, adjacent to Kilbourne Hole, New Mexico, still had skin and hair preserved, and is now at the Yale Peabody Museum. The American Museum of Natural History in New York City has a sample of dung with a note attached to it that reads "deposited by Theodore Roosevelt". The largest samples of Nothrotheriops dung can be found in the collections of the Smithsonian Museum.

The skeletal structure of ground sloths indicates that the animals were massive. Their thick bones and even thicker joints (especially those on the hind legs) gave their appendages tremendous power that, combined with their size and fearsome claws, provided a formidable defense against predators.

Cryptozoologists believe that a forest creature of the upper Amazon basin called the Mapinguari may be a surviving tropical ground sloth.

La Brea Tar Pits

Two species of ground sloths have been found in the assemblage at the La Brea Tar Pits. Harlan's ground sloth (Paramylodon harlani) was six feet tall when standing. The most common Paramylodon fossil found at La Brea are dermal ossicles: small, oblong spheroids of bone. These small bones were embedded deep in the skin around the neck, shoulders and back of the sloth, and may have served as armor against attacking predators. The smaller ground sloth, less common at the La Brea Lagerstätte is the Shasta ground sloth (Nothrotheriops shastensis).

Taxonomy of Ground Sloths

Harlan's ground sloth (Paramylodon harlani), National Museum of Natural History

Another specimen of Harlan's ground sloth (Paramylodon harlani), Texas Memorial Museum, University of Texas at Austin

Modified from McKenna and Bell (1997). "†" indicates an extinct group. Ground sloths consist of 6 families and 88 genera. Note that ground sloths do not form a monophyletic group. Some extinct ground sloths are more related to today's tree sloths than they are to other ground sloths.

• Infraorder Phyllophaga - sloths
  • †Diellipsodon incertae sedis
  • †Amphiocnus incertae sedis
  • †Pseudoglyptodon incertae sedis
  • Family †Rathymotheriidae incertae sedis
    • †Rathymotherium
  • Parvorder †Mylodonta - mylodont ground sloths
    • Superfamily †Mylodontoidea
      • †Pseudoprepotherium incertae sedis
      • Family †Scelidotheriidae
        • Subfamily †Chubutheriinae
          • †Chubutherium
        • Subfamily †Scelidotheriinae
          • †Scelidotheriops
          • †Analcitherium
          • †Nematherium
          • †Neonematherium
          • †Elassotherium
          • †Scelidotherium
          • †Catonyx
          • †Proscelidodon
      • Family †Mylodontidae
        • †Urumacotherium incertae sedis
        • Subfamily †Mylodontinae
          • †Glossotheriopsis
          • †Promylodon
          • †Strabsodon
          • †Megabradys
          • †Pleurolestodon
          • †Mylodon
        • Subfamily †Lestodontinae
          • Tribe †Thinobadistini
            • †Thinobadistes
            • †Sphenotherus
          • Tribe †Glossotheriini
            • †Acremylodon
            • †Ranculcus
            • †Glossotherium
            • †Paramylodon
            • †Mylodonopsis
          • Tribe †Lestodontini
            • †Lestodon
            • †Lestodontidion
    • Superfamily †Orophodontoidea
      • Family †Orophodontidae
        • †Proplatyarthrus
        • †Orophodon
        • †Octodontotherium
        • †Octomylodon
        • †Octodontobradys
  • Parvorder Megatheria
    • Superfamily Megatherioidea
      • Family †Megatheriidae
        • Subfamily †Megatheriinae
          • Tribe †Megatheriini
            • Subtribe †Prepotheriina
              • †Proprepotherium
              • †Planops
              • †Prepotherium
            • Subtribe †Megatheriina
              • †Megathericulus
              • †Promegatherium
              • †Plesiomegatherium
              • †Megatheridium
              • †Pyramiodontherium
              • †Megatherium
              • †Eremotherium
              • †Ocnopus
              • †Perezfontanatherium
          • Tribe †Nothrotheriini
            • †Pronothrotherium
            • †Xyophorus
            • †Chasicobradys
            • †Gilsolaresia
            • †Diheterocnus
            • †Synhapalops
            • †Nothropus
            • †Thalassocnus
            • †Nothrotherium
            • †Nothrotheriops
        • Subfamily †Schismotheriinae
          • †Hapaloides
          • †Schismotherium
          • †Hapalops
          • †Pelecyodon
          • †Parapelecyodon
          • †Analcimorphus
          • †Hyperleptus
          • †Neohapalops
      • Family Megalonychidae - ground sloths and two-toed tree sloths
        • †Imagocnus
        • Subfamily †Ortotheriinae
          • †Proschisomotherium
          • †Eucholoeops
          • †Pseudortotherium
          • †Megalonychotherium
          • †Paranabradys
          • †Pliomorphus
          • †Torcellia
          • †Ortotherium
          • †Menilaus
          • †Diodomus
          • †Habanocnus
          • †Paulocnus
        • Subfamily Megalonychinae
          • †Pliometanastes
          • †Sinclairia incertae sedis
          • †Megalonychops incertae sedis
          • †Valgipes incertae sedis
          • †Meizonyx incertae sedis
          • Tribe †Megalonychini
            • Subtribe †Megalonychina
              • Infratribe †Megalonychi
                • †Protomegalonyx
                • †Megalonyx
              • Infratribe †Megalocni
                • †Megalocnus
                • †Neomesocnus
            • Subtribe †Mesocnina
              • †Neocnus
              • †Parocnus - greater Haitian ground sloth
          • Tribe Choloepodini
            • Subtribe †Acratocnina
              • †Miocnus
              • †Acratocnus
              • †Synocnus - lesser Haitian ground sloth
            • (Subtribe Choloepodina - modern two-toed sloths)
    • (Superfamily Bradypodoidea - modern three-toed sloths)

References

• McKenna, M. C, and S. K. Bell. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp.
• Nowak, R. M. 1999. Walker's Mammals of the World, Vol. 2. Johns Hopkins University Press, London.
• White, J.L. & MacPhee, R.D.E. 2001. The sloths of the West Indies: a systematic and phylogenetic review. pp 201-235 in Woods, C.A. & Sergile, F.E. (eds.). Biogeography of the West Indies: Patterns and Perspectives

External links

• Picture and information about a ground sloth skeleton on display at the University of Georgia's Science Library.
• Academy of Natural Sciences ground sloth page.
• Illinois State Museum ground sloth page.
• Ground sloths at La Brea.
• Eremotherium in Florida.
• Have some ground sloths survived in Argentina?
• Ground sloths in general.
• Online Sloth Bibliography.