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Temporal range: late Miocene to Pleistocene, 10.3–0.011 Ma
Megalonyx wheatleyi - AMNH - DSC06327.JPG
M. wheatleyi skeleton.
Megalonyx size.svg
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Superorder: Xenarthra
Order: Pilosa
Family: Megalonychidae
Gervais, 1855
Subfamily: Megalonychinae
Genus: Megalonyx
Harlan, 1825

Megalonyx (Greek, "large claw") is an extinct genus of giant ground sloths of the family Megalonychidae endemic to North America from the Hemphillian of the Late Miocene through to the Rancholabrean of the Pleistocene, living from ~10.3 Mya—11,000 years ago, existing for approximately 10.289 million years. Type species, M. jeffersonii, measured about 3 m (9.8 ft) and weighted up to 1000 kilograms.[1]


Megalonyx jeffersonii skeleton cast produced and distributed by Triebold Paleontology Incorporated
Megalonyx jeffersonii skeleton

Megalonyx was a large, heavily built animal about 9.8 feet (3 m) long. Its maximum weight is estimated at 1,000 kg (2,205 lb).[1] This is medium-sized among the giant ground sloths. Like other ground sloths it had a blunt snout, massive jaw, and large peg-like teeth. The hind limbs were plantigrade (flat-footed) and this, along with its stout tail, allowed it to rear up into a semi-erect position to feed on tree leaves. The forelimbs had three highly developed claws that were probably used to strip leaves and tear off branches.


Ongoing excavations at Tarkio Valley in southwest Iowa may reveal something of the familial life of Megalonyx. An adult was found in direct association with two juveniles of different ages, suggesting that adults cared for young of different generations.[2][3]


Megalonyx ranged over much of North and Central America.[4] Their remains have been found as far north as Alaska[5] and the Yukon.[6]

M. jeffersonii was apparently the most wide ranging giant ground sloth. Fossils are known from many Pleistocene sites in the United States, including most of the states east of the Rocky Mountains as well as along the west coast. It was the only ground sloth to range as far north as the present-day Yukon[7] and Alaska.[8]

In the fall of 2010, the first specimen ever found in Colorado was discovered at the Ziegler Reservoir site near Snowmass Village (In the Rocky Mountains at an elevation of 8,874 feet).[9] The Firelands ground sloth fossil dated between 11,727 and 11,424 B.C. represents the earliest known hunting activity by Ohioan paleoindians.[10]

Taxonomy and evolution[edit]

The generic name Megalonyx was proposed by future U.S. President Thomas Jefferson in 1797, based on fossil specimens of what later came to be called Megalonyx jeffersonii that he had received from western Virginia. His presentation to the American Philosophical Society that year is often credited as the beginning of vertebrate paleontology in North America. However, Jefferson's name has no validity in zoological nomenclature, and Megalonyx was first formally named by Richard Harlan in 1825.[11]

Megalonyx evolved from ancestors that island-hopped across the Central American Seaway from South America, where ground sloths arose, prior to formation of the Panamanian land bridge. Its appearance in North America thus predates the bulk of the faunal exchange between North and South America. Its immediate predecessor was Pliometanastes and its closest living relatives are the two-toed sloths (Choloepus).

M. jeffersoni lived from the Illinoian Stage during the Middle Pleistocene (150,000 years BP) through to the Rancholabrean of the Late Pleistocene (11,000 BP[12]). It belongs to the genus Megalonyx, a name proposed by Thomas Jefferson, future president of the United States, in 1797. (Jefferson's surname was appended to the animal as the specific epithet in 1822 by the French zoologist Anselme Gaëtan Desmarest.) M. jeffersoni was probably descended from M. wheatleyi, which was in turn was probably derived from M. leptostomus.

M. leptostomus was named by Cope (1893). This species lived from Florida to Texas, north to Kansas and Nebraska, and west to New Mexico, Nevada, Oregon, and Washington. It is about half the size of M. jeffersoni.

Discovery and history[edit]

Claws of M. jeffersonii

In 1796, Colonel John Stuart sent Thomas Jefferson some fossil bones (a femur fragment, ulna, radius, and some foot bones including three large claws) from a cave in Greenbrier County, Virginia (present-day West Virginia). In 1797, Jefferson (who was serving at the time as Vice President) presented a paper on "Certain Bones" to the American Philosophical Society in Philadelphia. He theorized that they were the remains of a lion which he named Megalonyx ("giant claw").[13] Jefferson believed that this lion was still alive (i.e., still extant), which is of particular importance because the validity of his argument would directly be used in the argument for or against evolution. Jefferson believed in a "completeness of nature" worldview which contrasted the new theory of evolution, where no species have or ever will go extinct naturally. When Lewis and Clark embarked on their expedition (1804–1806), he asked them to keep an eye out for this "Megalonyx", as he insisted this species was still in existence. If the creature was found, he could use this to support his argument against the concept of the extinction of species. His argument, of course, was later not supported by the scientific community and shown to be incorrect.[14]

Jefferson is considered to have initiated the science of vertebrate paleontology in the United States with the reading of this paper. In 1799, Dr. Caspar Wistar correctly identified the remains as belonging to a giant ground sloth. In 1822, Wistar proposed naming it Megalonyx jeffersonii in honor of the former president.[15][16] Desmarest then published it as such.

For many decades, Jefferson's "Certain Bones" had been assumed to have come from Organ Cave in what is now Greenbrier County, West Virginia. This theory originated in the early 20th century when a local man, Andrew Price of Marlinton, decided this and popularized the idea.[17] In 1993, two fragments of a Megalonyx scapula were found in Haynes Cave in neighboring Monroe County. In 1995, Smithsonian paleontologist Frederick Grady proposed[18] that Haynes Cave was the true original source of Jefferson's fossil.

Grady’s reasons had to do with historical land-ownership records. Jefferson reported that the bones had been found by saltpeter workers and gave the cave owner’s name as Frederic Crower, an apparent misspelling (according to Grady) of Frederic Gromer. Correspondence between Jefferson and Colonel Stuart, who sent him the bones, indicates that the cave was about five miles from Stuart’s home and contained saltpeter vats. Grady believes that Organ Cave can be eliminated as the source cave as it was never owned by Gromer. In addition, two letters written by the subsequent owner of the cave—Tristram Patton—indicate that the cave was in Monroe County near Second Creek. (Monroe County, originally part of Greenbrier County, was separated shortly after the discovery of the bones.) In the letters, Patton described the cave and indicated that more bones were there.[19]

M. jeffersonii is still the most commonly identified species of Megalonyx. It was designated the state fossil of West Virginia in 2008.

(incomplete listing)



  1. ^ a b
  2. ^ Semken and Brenzel,
  3. ^ Semken; Brenzel (2007). "One Sloth Becomes Three". Newsletter of the Iowa Archeological Society 57: 1. 
  4. ^ "Megalonyx". Paleobiology Database. Retrieved 2011-07-16. 
  5. ^ Stock, C. (1942-05-29). "A ground sloth in Alaska". Science (AAAS) 95 (2474): 552–553. doi:10.1126/science.95.2474.552. PMID 17790868. 
  6. ^ McDonald, H. G.; Harington, C. R.; De Iuliis, G. (September 2000). "The Ground Sloth Megalonyx from Pleistocene Deposits of the Old Crow Basin, Yukon, Canada" (PDF). Arctic (Calgary, Alberta: The Arctic Institute of North America) 53 (3): 213–220. doi:10.14430/arctic852. Retrieved 2008-08-16. 
  7. ^ McDonald, H. G.; Harington, C. R.; De Iuliis, G. (September 2000). "The Ground Sloth Megalonyx from Pleistocene Deposits of the Old Crow Basin, Yukon, Canada" (PDF). Arctic (Calgary, Alberta: The Arctic Institute of North America) 53 (3): 213–220. doi:10.14430/arctic852. Retrieved 2008-08-16. 
  8. ^ Stock, C. (1942-05-29). "A ground sloth in Alaska". Science (AAAS) 95 (2474): 552–553. doi:10.1126/science.95.2474.552. PMID 17790868. 
  9. ^ "Snowmass tally: 10 mastodons, 4 mammoths, one "once-in-a-lifetime" find". The Denver Post. November 18, 2010. 
  10. ^ "Research reveals first evidence of hunting by prehistoric Ohioans". Cleveland Museum of Natural History. Feb 2012. Retrieved 12 Apr 2012. 
  11. ^ McKenna, M.C.; Bell, S.K. (1997). Classification of Mammals: Above the Species Level. Columbia University Press. p. 100. ISBN 0-231-11013-8. 
  12. ^ Fiedal, Stuart (2009). "Sudden Deaths: The Chronology of Terminal Pleistocene Megafaunal Extinction". In Haynes, Gary. American Megafaunal Extinctions at the End of the Pleistocene. Springer. pp. 21–37. doi:10.1007/978-1-4020-8793-6_2. ISBN 978-1-4020-8792-9. 
  13. ^ Jefferson, Thomas, "A Memoir on the Discovery of Certain Bones of a Quadruped of the Clawed Kind in the Western Parts of Virginia", Read before the American Philosophical Society, March 10, 1797.
  14. ^ Rowland, Steve, "The Fossil Record", Published lecture notes for "The Fossil Record" course at the University of Nevada, Las Vegas, August 2010
  15. ^ Jefferson, Thomas (1799). "A Memoir on the Discovery of Certain Bones of a Quadruped of the Clawed Kind in the Western Parts of Virginia". Transactions of the American Philosophical Society 4: 246–260. doi:10.2307/1005103. JSTOR 1005103. 
  16. ^ Wistar, Caspar (1799), “A Description of the Bones Deposited, by the President, in the Museum of the Society, and Represented in the Annexed Plates", Transactions, pp. 526-531, plates.
  17. ^ Humphreys, Blanche (1928), History of Organ Cave Community (Greenbrier County, West Virginia); Agricultural Extension Division.
  18. ^ Grady, Fred (1995), "The Search for the Cave from which Thomas Jefferson Described the Bones of the Megalonyx" [Abstract], In: "Selected Abstracts from the 1995 National Speleological Society National Convention in Blacksburg, Virginia"; In: Journal of Cave and Karst Studies, April 1997, pg 57.
  19. ^ Grady (1995), Op. cit.
  20. ^ Williams, David B. (2010). Construction at Sea-Tac Airport unearths an extinct giant sloth on February 14, 1961.

Other sources[edit]

  • Cope, ED. (1871) Preliminary report on the vertebrata discovered in the Port Kennedy Bone Cave. American Philosophical Society, 12:73-102.
  • Cope, ED. (1893) A preliminary report on the vertebrate paleontology of the Llano Estacado. 4th Annual Report on the Geological Survey of Texas: 136pp.
  • Hirschfeld, SE. and SD. Webb (1968) Plio-Pleistocene megalonychid sloths of North America. Bulletin of the Florida State Museum Biological Sciences, 12(5):213-296.

External links[edit]