| Megalonychidae Temporal range:Early Oligocene-Early Holocene (Hemphillian-Rancholabrean) (NALMA) &Tinguirirican-Lujanian (SALMA) 35–0.009 Ma | |
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| Megalonyx wheatleyi skeleton and restoration | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Pilosa |
| Superfamily: | Megatherioidea |
| Family: | †Megalonychidae Gervais, 1855 |
| Type genus | |
| †Megalonyx Jefferson, 1799 | |
| Subgroups | |
See text | |
Megalonychidae is an extinct family ofsloths including the extinctMegalonyx.[1] Megalonychids first appeared in the earlyOligocene, about 35 million years (Ma) ago, in southern Argentina (Patagonia).[2] There is, however, one possible find dating to theEocene, about 40 Ma ago, onSeymour Island in Antarctica (which was then still connected to South America).[3] They first reached North America byisland-hopping across theCentral American Seaway, about 9 million years ago,[4] prior to formation of theIsthmus of Panama about 2.7 million years ago (which led to the main pulse of theGreat 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 thePliocene (about 5 to 2 million years ago) species were already approximately half the size of the hugeLate PleistoceneMegalonyx jeffersonii from thelast ice age.[5]
It was formerly believed, based on morphological comparisons, thatGreater Antilles sloths and extant arborealtwo-toed sloths were part of this family. However, molecular results based on sequences fromcollagen[6] andmitochondrial DNA[7] have shown that the former represent abasal branch of the sloth radiation, while the latter are more closely related tomylodontid sloths. The megalonychids plusnothrotheriid andmegatheriid sloths, together with livingthree-toed sloths, make up the sloth superfamilyMegatherioidea.[6][7]
Megalonychidae, along with all other mainland ground sloths became extinct in North and South America around the end of theLate Pleistocene, approximately 12,000 years ago, as part of theQuaternary extinction event following the arrival of humans to the Americas.[8]
Megalonyx, which means "giant claw", is a widespread North American genus that lived past the close of thelast (Wisconsin) glaciation, when so many large mammals died out. Remains have been found as far north asAlaska and theYukon.[9][10] Ongoing excavations at Tarkio Valley in southwest Iowa may reveal something of the familial life ofMegalonyx. An adult was found in direct association with two juveniles of different ages, suggesting that adults cared for young of different generations.[11][12]
The earliest knownNorth American megalonychid,Pliometanastes protistus, lived inFlorida and the southern U.S. about 9 million years ago, and is believed to have been the predecessor ofMegalonyx. Several species ofMegalonyx have been named; in fact, a 2000 article by Harington et al. inArctic claimed that "nearly every good specimen has been described as a different species".[9] 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, andM. jeffersonii) in the late Pliocene and Pleistocene of North America.[13] Although work by McDonald lists five species.
Jefferson's ground sloth has a special place in modernpaleontology, forThomas Jefferson's "memoir" onMegalonyx, read before theAmerican Philosophical Society ofPhiladelphia, on 10 March 1797,[1] marked the beginning ofvertebrate paleontology in North America.[9] Jefferson's paper was published, with an accompanying description and illustrations of theMegalonyx fossils byCasper Wistar, in 1799.[1] WhenLewis 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 named after Thomas Jefferson in 1822.[1][9]
The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data (see Fig. 4 of Pressleeet al., 2019).[14]
| Folivora |
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A morphological tree of Megalonychidae, based on the work of Stinnesbeck and colleagues (2021).[15] (Note that this tree does not conform to genetic studies, as it includes the Caribbean slothsNeocnus,Parocnus Megalocnus andArcatocnus which have been placed in the separate familyMegalocnidae, well as the two toed sloths (Choloepus), which are placed in the cladeMylodontoidea).[16]
| Megalonychidae | |
Total evidence phylogeny after Tejada et al. 2023.[16]
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Authority control databases: National |
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