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Extinct Ground Sloths (Paramylodon harlani, Nothrotheriops shastensis, and Megalonyx jeffersoni) Fact Sheet: Taxonomy & History

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

How Do We Know This?

Like living animals, fossil remains of once-living animals are
classified and grouped according to their relationships to each other and to their ancestors.
Some fossils yield DNA which helps scientists determine these animals' similarity to living animals.

Taxonomy and Nomenclature

  • Giant ground sloths were some of the first animals to be recognized as extinct
    • In 1796 George Cuvier described a ground sloth skeleton of Megatherium ("Great Beast") from South America
    • Cuvier saw characters in the bones and teeth of these fossils that were shared with the group of living animals such as tree sloths, anteaters and armadillos
    • Sloths, both living and extinct, armadillos, anteaters, and extinct glyptodonts are now termed xenarthrans ("strange joints" referring to the extra points of articulation in their vertebrae)
  • The diversity of sloths in the fossil record is high; over 100 genera are recognized (McKenna & Bell 1997) with 19 genera in late Pleistocene sediments (Steadman et al 2005)
  • McKenna and Bell (1997) established a new sloth Infraorder, Megatheria, with the nothrotheres in a separate tribe, Nothrotheriini
  • DeMuizon et al (2004) formally recognized nothrotheres in their own family, Nothrotheriidae; this convention is followed by most researchers today
  • Mckenna et al (2006) suggest Tardigrada should be used for "the most recent common ancestor of Bradypus and Choleopus plus all of its descendants".

Evolutionary History

  • Earliest record of a possible sloth is of a single tooth from Eocene rocks on Seymour Island, near Antarctica; other sloth fossils also from Eocene rocks, in Patagonia (McDonald & De Iuliis 2008)
  • The divergence of the Bradypus and Choloepus lineages may be ancient, dating back as much as 40 million years. (Gaudin 2004)
  • Sloths are known to have been in Patagonia 35 million years ago (Oligocene) (McDonald & I2008)
  • By 8 or 9 million years ago (Late Miocene) sloths had dispersed into islands in the Caribbean and North America.
  • Pliometanastes is the earliest known ground sloth in North America,
    • Dated to 8 million years ago in Nuevo Leon, Mexico. (Castañeda & Miller 2004)
    • Found in 7 million year-old (Miocene) Florida rocks.
  • Megalonyx recorded from 4.8 million year old (Early Pliocene) rocks in Mexico.(Flynn et al 2005)
    • This date is earlier than known previously and suggests the so-called Great American Biotic Interchange (= North American species going south and South American species moving north) saw South American species like Megalonyx arriving well before North American animals dispersed south.
  • Megalonyx survived until between 11-12,000 years ago. (Steadman et al 2005).
  • Nothrotheriops survived until 10,400 to 11,480 years ago. (Steadman et al 2005)
  • 90% of sloth genera became extinct at the end of the Pleistocene (Steadman et al 2005)
  • Dwarf megalonychid sloth species the size of a cat survived the megafauna extinction on West Indian islands
    • These remains have been dated at 4,400 years ago, just after the arrival of humans on those islands.
  • Two-Toed Tree Sloth (Choloepus) may be descended from the extinct megalonychid sloths from the West Indies (Gaudin 2004)
  • Three-toed Tree Sloth (Bradypus) may be a sister-taxon to all other sloths. (Gaudin 2004)
  • All giant ground sloths are extinct today.


Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Pilosa (anteaters and sloths)

Family: Mylodontidae (family is extinct)

Genus: Paramylodon                              

Species: Paramylodon harlani (Owen, 1840)

Family: Nothrotheriidae (family is extinct)

Genus: Nothrotheriops

Species: Nothrotheriops shastensis (Sinclair, 1905)

Family: Megalonychidae (family has extinct and living/extant members)

Genus: Megalonyx (extinct)

Species: Megalonyx jeffersoni (Wistar, 1822)

Page Citations

Castañeda & Miller (2004)
DeMuizon et al. (2004)
Flynn et al. (2005)
Gaudin (2004)
McDonald (1996)
McDonald & De Iuliis (2008)
McKenna & Bell (1997)
McKenna et al. (2006)
Steadman et al. (2005)

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