| Megalonyx | |
|---|---|
 | |
| M. wheatleyi skeleton inAMNH | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Pilosa |
| Family: | †Megalonychidae |
| Subfamily: | †Megalonychinae |
| Genus: | †Megalonyx Jefferson, 1799[1] |
| Type species | |
| †Megatherium jeffersonii[1] Desmarest, 1822 | |
| Species | |
Megalonyx (Greek, "great-claw") is an extinctgenus ofground sloths of thefamilyMegalonychidae, native to North America. It evolved during thePliocene Epoch and became extinct at the end of theLate Pleistocene, living from ~5 million to ~13,000 years ago.[3] Thetype species,M. jeffersonii (also calledJefferson's ground sloth), the youngest and largest known species, measured about 3 m (9.8 ft) in length and weighed up to nearly 1,300 kg (2,900 lb).
Megalonyx is suggested to have descended fromPliometanastes, a genus of ground sloth that had arrived in North America during the LateMiocene around 9 million years ago, prior to the main phase of theGreat American Interchange.Megalonyx had the widest distribution of any North American ground sloth, having a range encompassing most of the contiguous United States, extending as far north as Alaska during warminterglacial periods.
Megalonyx is notable for having been originally described by future U.S. PresidentThomas Jefferson in 1799 based on remains found in West Virginia; the speciesM. jeffersonii was described later, named in honor of him.[1]
Megalonyx became extinct as part of theend-Pleistocene extinction event, simultaneously with all other mainland ground sloths and most other large mammals native to the Americas. These extinctions followed thearrival of humans in the Americas, and there is evidence that humans interacted withMegalonyx, including butchering its remains shortly prior to its extinction.
In 1796, ColonelJohn Stuart sentThomas Jefferson, shortly before he took office asVice President of the United States, some fossil bones: afemur fragment,ulna,radius, and foot bones including three large claws. The discoveries were made in a cave inGreenbrier County,Virginia (presentlyWest Virginia). Jefferson examined the bones and presented his observations in the paper "A Memoir on the Discovery of Certain Bones of a Quadruped of the Clawed Kind in the Western Parts of Virginia" to theAmerican Philosophical Society inPhiladelphia on March 10, 1797.[1][4] The paper was published in 1799, in the same volume as an accompanying paper by his colleagueCaspar Wistar, who provided detailed anatomical information about the bones, and illustrated them.[5] Together these two papers are considered the first North American publications devoted to paleontology.
In the 1799 paper, Jefferson named the then-unknown animalMegalonyx ("great-claw") and compared each recovered bone to the corresponding bone in alion.[1][4] In his original draft of the paper, Jefferson thought the animal was a carnivore, one of the large cats, writing "Let us only say then, what we may safely say, that he was more than three times as large as the lion". In a postscript, composed after learning of Baron Georges Cuvier's description and illustration of the giant ground slothMegatherium, discovered in Argentina (mistakenly referred to as Paraguay), Jefferson revised his interpretation and comparedMegalonyx toMegatherium.
Contrary to Baron Cuvier's view thatextinction had played an important role in natural history, an idea that would reach scientific consensus decades later, Jefferson wrote about a "completeness of nature" whose inherent balance did not allow species to go extinct naturally. He askedLewis and Clark, as they planned their famous expedition in 1804–1806, to keep an eye out for living specimens ofMegalonyx, as this would support his case. His idea made no headway and was later shown to be incorrect.[6] However, Jefferson's notion that humans andMegalonyx co-existed in North America has been shown to be correct, as some bones ofMegalonyx show marks made by flint tools.[7]
His presentation to the American Philosophical Society in 1797 is often credited as the beginning ofvertebrate paleontology inNorth America. In 1799,Caspar Wistar correctly identified the remains as those of a giant ground sloth. In 1822, Desmarest named the speciesMegatherium jeffersonii in honor of the former statesman and scientist, although he classified it in the genusMegatherium instead.Richard Harlan in 1825 revived the genusMegalonyx with the type speciesM. jeffersonii, and provided additional taxonomic description.[8] Scientific papers variously give theauthority for the genus as Jefferson 1799 (after Jefferson's original naming of the genus),[9] or Harlan 1825.[10] Most authors gave Jefferson as the authority on the genus until a 1942 paper byGeorge Gaylord Simpson, who suggested that the attribution of the authority to Jefferson "is certainly erroneous,", and suggested that Harlan "may have been the first to use the name in a valid Linnaean form". A 2024 review paper byLoren E. Babcock found that Simpson's views were mistaken, and clarified that Jefferson was the valid author of the genus, as the description was done in accordance with the rules of taxonomic nomenclature at the time, despite him not assigning a species to it.[1]
Recent research confirms that the sloth bones were discovered inHaynes Cave inMonroe County, West Virginia. For many decades in the twentieth century, the reported origin of Jefferson's "Certain Bones" wasOrgan Cave in what is now Greenbrier County, West Virginia. This story was popularized in the 1920s by a local man, Andrew Price ofMarlinton.[11] The story came under scrutiny when in 1993 two fragments of aMegalonyxscapula were found in Haynes Cave in neighboring Monroe County.Smithsonianpaleontologist Frederick Grady presented evidence in 1995 confirming Haynes Cave as the original source of Jefferson's fossil.[12]
Jefferson reported that the bones had been found bysaltpeter workers. He gave the cave owner's name as Frederic Crower. Correspondence between Jefferson and Colonel Stuart, who sent him the bones, indicates that the cave was located about five miles from Stuart's home and that it containedsaltpeter vats. An investigation of property ownership records revealed "Frederic Crower" to be an apparent misspelling of the name Frederic Gromer.[12]
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Organ Cave was never owned by Gromer, but Haynes Cave was. Two letters written by Tristram Patton, the subsequent owner of Haynes Cave, indicate that this cave was located in Monroe County nearSecond Creek. Monroe County had originally been part of Greenbrier County; it became a separate county shortly after the discovery of the bones. In his own letters Patton described the cave and indicated that more fossil bones remained inside.[12]
M. jeffersonii is still the most commonly identified species ofMegalonyx. It was designated thestate fossil of West Virginia in 2008.
M. leptostomus, named by Cope (1893), lived from the Blancan to the Irvingtonian. 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 ofM. jeffersonii. It evolved intoM. wheatleyi, the direct ancestor ofM. jeffersonii. Species gradually got larger, with different species mostly based on size and geologic age.
The first wave of Megalonychids came to North America by island-hopping across theCentral American Seaway fromSouth America, where ground sloths arose, prior to formation of thePanamanian land bridge. Based on molecular results, its closest living relatives are thethree-toed sloths (Bradypus); earlier morphological investigations came to a different conclusion.[13][14]Megalonyx is thought to be descended fromPliometanastes, a ground sloth that arrived in North America during the late Miocene, around 9 million years ago.[15][16]
The earliest representatives ofMegalonyx appeared during the Pliocene.[16]M. jeffersonii lived from the lateMiddle Pleistocene/ lateIrvingtonian (250–300,000 years ago)[10] through to theRancholabrean of theLate Pleistocene (11,000 BP[17]).M. jeffersonii was probably descended fromM. wheatleyi.[18] TheMegalonyx lineage increased in size with time, with the last speciesM. jeffersonii being the largest.[10]
AdultM. jeffersonii measured around 3 m (9.8 ft) long,[19] and weighed up to 1,090–1,295 kg (2,403–2,855 lb).[10][7] The hind limbs wereplantigrade (flat-footed) and this, along with its stout tail, allowed it to rear up into a semi-erect position to feed.[10] The hands had three large claws, which were likely used for grasping and defense.[20] The teeth ofMegalonyx jeffersonii were hypselodont (high crowned).[10]
During excavations at Tarkio Valley in southwestIowa, an adult (presumably female)Megalonyx jeffersonii was found in direct association with two juveniles of different ages, the oldest suggested to be around 3-4 years old, suggesting that adults cared for young of different generations.[21][22][10] A 2022 study estimated, based on the ages of the adult and the two juveniles, that the average lifespan was approximately 19 years, sexual maturity occurred at about 6 and a half years, thatgestation time was around 14 months, and the interval between births was approximately 3 years.[10]Megalonyx is thought to have been abrowser.[10] Stable isotopic study consisting of pairedδ13C andδ15N analysis ofM. jeffersoniispecimens found in Saltville Quarry in Virginia has found that they predominantly relied onC3 plants for sustenance.[23]
Megalonyx jeffersonii ranged over much of North America, with its range spanning nearly the whole contiguous United States and parts of southern Canada, with some remains known as far south as central Mexico.[10] Their remains have been found as far north asAlaska[24] and theYukon duringinterglacial intervals.[25] The sloth ranged as far northeast asNew York.[26]
In 2010, a specimen was discovered at theZiegler Reservoir site near Snowmass Village, Colorado, in the Rocky Mountains at an elevation of 8,874 feet (2,705 m).[27] The habitat ofMegalonyx jeffersonii was highly variable,[28] but often associated withspruce-dominated, mixedconifer-hardwood forest.[10]
Megalonyx jeffersonii became extinct at the end of the Pleistocene, as part of theQuaternary extinction event, in which all other mainland ground sloths and most other large mammals of the Americas became extinct.[10] The youngest confirmed radiocarbon date is in Ohio, dating to 13,180–13,034calibrated yearsBefore Present.[3] This timing was co-incident with both theYounger Dryas and a major growth in population of recently arrivedPaleoindians. In Ohio, a specimen ofMegalonyx jeffersonii, dubbed the "Firelands Ground Sloth", dating to around 13,738 to 13,435 calibrated yearsBefore Present (~11,788 to 11,485BCE) was found with cut marks indicative of butchery, suggesting that hunting may have played a role in its extinction.[7]
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