Editing Ground sloth (section)

=== Megalonychidae ===
{{Main|Megalonychidae}}
The [[Megalonychidae|megalonychid]] ground sloths first appeared in the [[Eocene|Late Eocene]], about 35&nbsp;million years ago, in Patagonia. Megalonychids first reached North America by island-hopping, prior to the formation of the [[Isthmus of Panama]]. Some lineages of megalonychids increased in size as time progressed. The first species of these were small and may have been partly tree-dwelling, whereas the [[Pliocene]] (about 5 to 2&nbsp;million years ago) species were already approximately half the size of the huge [[Late Pleistocene]] ''[[Megalonyx jeffersonii]]'' from the [[Quaternary glaciation|last ice age]]. Some West Indian island species were as small as a large cat; their dwarf condition typified both tropical adaptation and their restricted island environment.  This small size also enabled them a degree of arboreality.<ref name="J.L. White 1993">J.L. White (1993)</ref>

''[[Megalonyx]]'', which means "giant claw", was a widespread North American genus that lived past the close of the [[Last glacial period|last (Wisconsin) glaciation]], when so many large mammals died out.  Remains have been found as far north as Alaska<ref>{{cite journal |last=Stock |first=C. |title=A ground sloth in Alaska |journal=[[Science (journal)|Science]] | volume = 95 |issue = 2474 |pages = 552–553 |publisher=[[American Association for the Advancement of Science|AAAS]] |date=29 May 1942 |doi=10.1126/science.95.2474.552 |pmid=17790868 |bibcode=1942Sci....95..552S }}<!--| access-date = 10 October 2009--></ref> and the [[Yukon]].<ref name=Harrington/><ref>{{cite journal | last = McDonald | first = H.G. | author2 = Harington, C.R. | author3 = de&nbsp;Iuliis, G. | title = The Ground Sloth Megalonyx from Pleistocene Deposits of the Old Crow Basin, Yukon, Canada | journal = Arctic | volume = 53 | issue = 3 | pages = 213–220 | publisher = The Arctic Institute of North America | location = Calgary, Alberta | date = September 2000 | url = http://pubs.aina.ucalgary.ca/arctic/Arctic53-3-213.pdf | access-date = 16 August 2008 | doi = 10.14430/arctic852 | archive-date = 3 July 2020 | archive-url = https://web.archive.org/web/20200703013709/http://pubs.aina.ucalgary.ca/arctic/Arctic53-3-213.pdf | url-status = dead }}</ref> Ongoing excavations at Tarkio Valley in southwestern [[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.<ref>Semken and Brenzel, http://slothcentral.com/?page_id=2 {{Webarchive|url=https://web.archive.org/web/20090101073804/http://slothcentral.com/?page_id=2 |date=2009-01-01 }}</ref><ref>{{cite journal | last1 = Semken | last2 = Brenzel | year = 2007 | title = One Sloth Becomes Three | journal=Newsletter of the Iowa Archeological Society | volume = 57 | page = 1 }}</ref>

The earliest known North American megalonychid, ''[[Pliometanastes|Pliometanastes protistus]]'', lived in the southern U.S. about 9&nbsp;million years ago and is believed to have been the predecessor of ''Megalonyx''. 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".<ref name=Harrington>Harrington (1993)</ref> 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,<ref>Kurtén & Anderson, 1980, p. 136.</ref> although work by McDonald lists five species.
Jefferson's ground sloth has a special place in modern [[paleontology]], for [[Thomas Jefferson]]'s letter on ''Megalonyx'', read before the [[American Philosophical Society]] of [[Philadelphia]] in August&nbsp;1796, marked the beginning of [[vertebrate]] paleontology in North America.<ref name=Harrington/> 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.<ref name=Harrington/>