Megalonychidae is an extinct family of

sloths Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their l ...

including the extinct ''

Megalonyx ''Megalonyx'' (Greek, "large claw") is an extinct genus of ground sloths of the family Megalonychidae, native to North America during the Pliocene and Pleistocene epochs. It became extinct during the Quaternary extinction event at the end of th ...

''. Megalonychids first appeared in the early Oligocene, about 35 million years (Ma) ago, in southern Argentina (

Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and g ...

). There is actually one possible find dating to the

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

, about 40 Ma ago, on

Seymour Island Seymour Island or Marambio Island, is an island in the chain of 16 major islands around the tip of the Graham Land on the Antarctic Peninsula. Graham Land is the closest part of Antarctica to South America. It lies within the section of the isla ...

in Antarctica (which was then still connected to South America). They first reached North America by

island-hopping Leapfrogging, also known as island hopping, was a military strategy employed by the Allies in the Pacific War against the Empire of Japan during World War II. The key idea is to bypass heavily fortified enemy islands instead of trying to ca ...

across the Central American Seaway, about 9 million years ago, prior to formation of the Isthmus of Panama about 2.7 million years ago (which led to the main pulse of the Great American Interchange). Some megalonychid lineages increased in size as time passed. The first species of these were small and may have been partly tree-dwelling, whereas the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Late Pleistocene ''

Megalonyx jeffersonii ''Megalonyx'' (Greek, "large claw") is an extinct genus of ground sloths of the family Megalonychidae, native to North America during the Pliocene and Pleistocene epochs. It became extinct during the Quaternary extinction event at the end of the ...

'' from the last ice age.J.L. White (1993) It was formerly believed, based on morphological comparisons, that Greater Antilles sloths and extant arboreal

two-toed sloth ''Choloepus'' is a genus of xenarthran mammals of Central and South America within the monotypic family Choloepodidae, consisting of two-toed sloths, sometimes also called two-fingered sloths. The two species of ''Choloepus'' (which means "la ...

s were part of this family. However, molecular results based on sequences from collagen and mitochondrial DNA have shown that the former represent a basal branch of the sloth radiation, while the latter are more closely related to

mylodontid Mylodontidae is a family of extinct South American and North American ground sloths within the suborder Folivora of order Pilosa, living from around 23 million years ago (Mya) to 11,000 years ago. This family is most closely related to another fa ...

sloths. The megalonychids plus nothrotheriid and

megatheriid Megatheriidae is a family of extinct ground sloths that lived from approximately 23 mya—11,000 years ago. Megatheriids appeared during the Late Oligocene (Deseadan in the SALMA classification), some 29 million years ago, in South Amer ...

sloths, together with living

three-toed sloth The three-toed or three-fingered sloths are arboreal neotropical mammals . They are the only members of the genus ''Bradypus'' and the family Bradypodidae. The four living species of three-toed sloths are the brown-throated sloth, the maned slot ...

s, make up the sloth superfamily

Megatherioidea Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their li ...

. Megalonychid ground sloths became extinct in North and South America around the end of the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in ...

Evolution

'', which means "giant claw", is a widespread North American genus that lived past the close of the last (Wisconsin) glaciation, when so many large mammals died out. Remains have been found as far north as

Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S. ...

and the

Yukon Yukon (; ; formerly called Yukon Territory and also referred to as the Yukon) is the smallest and westernmost of Canada's three territories. It also is the second-least populated province or territory in Canada, with a population of 43,964 as ...

. 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. The earliest known North American megalonychid, ''

Pliometanastes ''Pliometanastes'' is an extinct genus of ground sloths of the family Megalonychidae endemic to North America during the Late Miocene epoch through very early Pliocene epoch ( Hemphillian in the NALMA classification). Its fossils have been foun ...

protistus'', lived in

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...

and the southern U.S. about 9 million years ago, and is believed to have been the predecessor of ''Megalonyx''. Several species of ''

'' have been named; in fact, a 2000 article by Harington et al. in ''

Arctic The Arctic ( or ) is a polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenland), Finland, Iceland, N ...

'' claimed that "nearly every good specimen has been described as a different species".Harrington (1993) 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. Although work by McDonald lists five species. Jefferson's ground sloth has a special place in modern

paleontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

, for

Thomas Jefferson Thomas Jefferson (April 13, 1743 – July 4, 1826) was an American statesman, diplomat, lawyer, architect, philosopher, and Founding Father who served as the third president of the United States from 1801 to 1809. He was previously the natio ...

's letter on ''Megalonyx'', read before the

American Philosophical Society The American Philosophical Society (APS), founded in 1743 in Philadelphia, is a scholarly organization that promotes knowledge in the sciences and humanities through research, professional meetings, publications, library resources, and communit ...

Philadelphia Philadelphia, often called Philly, is the List of municipalities in Pennsylvania#Municipalities, largest city in the Commonwealth (U.S. state), Commonwealth of Pennsylvania, the List of United States cities by population, sixth-largest city i ...

, in August 1796, marked the beginning of

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

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 consequently named after Thomas Jefferson.

Phylogeny

The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data (see Fig. 4 of Presslee ''et al''., 2019). Here is a more detailed phylogenetic tree of the Megalonychidae, based on the work of Stinnesbeck and colleagues (2021).

References

External links

Smithsonian National Zoological Park: At the Zoo - Slow and Steady Sloths
{{Taxonbar, from=Q3259942 Prehistoric sloths Prehistoric mammal families Rupelian first appearances Holocene extinctions Taxa named by Paul Gervais

Evolution

Phylogeny

References

Further reading

External links