Megalonyx

Bones and claw cores of Megalonyx jeffersonii, illustrated by Wistar, 1799

In 1796, Colonel John Stuart sent Thomas Jefferson, shortly before he took office as Vice President of the United States, some fossil bones: a femur fragment, ulna, radius, and foot bones including three large claws. The discoveries were made in a cave in Greenbrier County, Virginia (presently West 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 the American Philosophical Society in Philadelphia on March 10, 1797.[4] The paper was published in 1799, in the same volume as an accompanying paper by his colleague Caspar 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 animal Megalonyx ("great-claw") and compared each recovered bone to the corresponding bone in a lion.[6] 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 sloth Megatherium, discovered in Argentina (mistakenly referred to as Paraguay), Jefferson revised his interpretation and compared Megalonyx to Megatherium.

Contrary to Baron Cuvier's view that extinction 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 asked Lewis and Clark, as they planned their famous expedition in 1804–1806, to keep an eye out for living specimens of Megalonyx, as this would support his case. His idea made no headway and was later shown to be incorrect.[7] However, Jefferson's notion that humans and Megalonyx co-existed in North America has been shown to be correct, as some bones of Megalonyx show marks made by flint tools.[8]

His presentation to the American Philosophical Society in 1797 is often credited as the beginning of vertebrate paleontology in North America. In 1799, Caspar Wistar correctly identified the remains as those of a giant ground sloth. In 1822, Desmarest named the species Megalonyx jeffersonii in honor of the former statesman and scientist. Richard Harlan in 1825[9] provided additional taxonomic description of Megalonyx, and is sometimes incorrectly cited as the one who formally named the genus.

Recent research confirms that the sloth bones were discovered in Haynes Cave in Monroe County, West Virginia. For many decades in the twentieth century, the reported origin of Jefferson's "Certain Bones" was Organ Cave in what is now Greenbrier County, West Virginia. This story was popularized in the 1920s by a local man, Andrew Price of Marlinton.[10] The story came under scrutiny when in 1993 two fragments of a Megalonyx scapula were found in Haynes Cave in neighboring Monroe County. Smithsonian paleontologist Frederick Grady presented evidence in 1995 confirming[11] Haynes Cave as the original source of Jefferson's fossil.

Jefferson reported that the bones had been found by saltpeter 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 contained saltpeter vats. An investigation of property ownership records revealed "Frederic Crower" to be an apparent misspelling of the name Frederic Gromer.[12]

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 near Second 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.[13]

M. jeffersonii is still the most commonly identified species of Megalonyx. It was designated the state 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 of M. jeffersonii. It evolved into M. wheatleyi, the direct ancestor of M. jeffersonii. Species gradually got larger, with different species mostly based on size and geologic age.

A skeleton of Megalonyx jeffersonii on display in the Orton Geological Museum. This skeleton was mounted in 1896.

Megalonyx jeffersonii was a large, heavily built herbivore about 3 m (9.8 ft) long.[3] It was comparable in size to a cow, with some specimens estimated to exceed 1,000 kg (2,200 lb) in mass.[18] 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.[18] The hands had three large claws, which were likely used for grasping and defense.[3] The teeth of Megalonyx jeffersonii were hypselodont (high crowned).[18]

During excavations at Tarkio Valley in southwest Iowa, an adult Megalonyx jeffersonii was found in direct association with two juveniles of different ages, suggesting that adults cared for young of different generations.[21][22] 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, that gestation time was around 14 months, and the interval between births was approximately 3 years.[18] Megalonyx is thought to have been a browser.[18]

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.[18] Their remains have been found as far north as Alaska[23] and the Yukon during interglacial intervals.[24]

In 2010, a specimen was discovered at the Ziegler Reservoir site near Snowmass Village, Colorado, in the Rocky Mountains at an elevation of 8,874 feet (2,705 m).[25] The habitat of Megalonyx jeffersonii was highly variable, but often associated with spruce-dominated, mixed conifer-hardwood forest.[18]

Megalonyx jeffersonii became extinct at the end of the Pleistocene, as part of the Quaternary extinction event, in which all other mainland ground sloths and most other large mammals of the Americas became extinct.[18] The youngest confirmed radiocarbon date is in Ohio, dating to 13,180–13,034 calibrated years Before Present.[2] This timing was co-incident with both the Younger Dryas and a major growth in population of recently arrived Paleoindians. In Ohio, a specimen of Megalonyx jeffersonii, dubbed the "Firelands Ground Sloth", dating to around 13,738 to 13,435 calibrated years Before Present (~11,788 to 11,485 BCE) was found with cut marks indicative of butchery, suggesting that hunting may have played a role in its extinction.[8]

• Paramylodon a mylodontid ground sloth native to North America during the Pliocene and Pleistocene
• Nothrotheriops a nothrotheriid ground sloth native to North America during the Pleistocene

• Sloth Central
• Smithsonian National Zoological Park: At the Zoo - Slow and Steady Sloths