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Extinct Ground Sloths (Paramylodon harlani, Nothrotheriops shastensis, and Megalonyx jeffersoni) Fact Sheet: Physical Characteristics

Extinct Ground Sloths (Paramylodon harlani, Nothrotheriops shastensis, and Megalonyx jeffersoni)

How Do We Know This?

Careful study of fossil bone or tooth anatomy yields much
exact information about placement and strength of muscles, tendons, ligaments, nerves,
and blood vessels. In rare cases, skin and hair impressions or actual skin or hair remains.
Body weight is more difficult to gauge because fat leaves no impression on the skeleton.

Physical Characteristics

AttributeParamylodon harlani
Nothrotheriops shastensis
Megalonyx jeffersoni
Estimated Body Weight
varied estimates from 1,000 kg (2,205 lb) to 1,089 kg (2,400 lb) estimated 250 kg (551 lb)
estimated near 1,000 kg (2,205 lb)
Body Length
3 m (9.8 ft) - bison sized black bear-sized 3 m (9.8 ft) - bison sized, slightly smaller than Paramylodon

General Appearance

General description

  • Giant ground sloths were a diverse groups of large, hairy extinct sloths with massive jaws, blunt snouts, and powerful clawed limbs.
    • One huge North American ground sloth from Florida, Eremotherium, grew up to 6 m (20 ft) in length
    • Megatherium americanum grew to nearly 11 feet standing height, weighing 6 metric tons (13,228 lb) (Vizcaíno et al 2008)
  • Forelimbs are shorter than hindlimbs.
  • Feet are twisted inward when walking ( McDonald 1977)
  • Living tree sloths are not good analogues for studying physiology, behavior, or morphology of extinct ground sloths; the two groups are very different.


  • Teeth are high crowned (hypsodont) and open rooted
    • The extreme height of the teeth may have been necessary for grazing abrasive grass and withstanding large amounts of grit from the soil. (Bargo et al 2006)
  • Teeth all resemble molars (molariforn) (Bargo et al 2006)
  • Teeth lack enamel; are composed of dentine held together by cementum. (Bargo et al 2006)
  • Nothrotheriops has no teeth towards the front of the jaw but Paramylodon and Megalonyx did (Bargo et al 2006)
  • All ground sloths lack incisors. (Naples 1989)


  • Hair length of South American mylodontid sloth fossil preserved in dry cave in Argentina: up to 50 mm on back of head, 70-100 mm on flank, 220 mm on rear limbs
  • Hair length of Nothrotheriops: 45 mm average. (McNab 1985)

Sexual Dimorphism

  •  Two sizes of skeletal elements; not known which sex is larger (McDonald 2004), but perhaps the male in Paramylodon harlani (McDonald 2006).

Other Physical and Physiological Characteristics

  • Sloths in the mylodontid family had small simple isolated bony deposits (osteoderms) imbedded in their skin (Hill 2006)
    • Osteoderms preserve well after the animal dies
    • Living pangolins and armadillos also have osteoderms, but theirs are highly modified and fit together in complex patterns (Hill 2006)
    • Many other groups of animals have osterderms, including lizards, some dinosaurs, crocodiles
  • Three large powerful claws on sloths' hands; probably for digging as well as defense.
    • Claws of Paramylodon wide, somewhat triangular; presumably for digging
    • Claws of Megalonyx are somewhat sharp; perhaps for grasping or defense

Page Citations

Bargo et al. (2000)
Hill (2006)
McDonald (2004, 2006)
Bargo et al. (2006)
McNab (1985)
Naples (1989)
Vizcaíno (2008)

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