President’s Day was last week, and that got me thinking about Megalonyx. That’s because the name Megalonyx – which means “great claw” – was coined by President Thomas Jefferson in 1797. Jefferson used the term in a most eloquent manner to refer to some large limb bones, including several claws, that had been collected from a cave in western Virginia:

“…to avoid the embarrassment of designating our animal always by circumlocution and description, I will venture to refer to him by the name of the Great-Claw or Megalonyx, to which he seems sufficiently entitled by the distinguished size of that member.”

Jefferson thought that “the Megalonyx” was a giant, lion-like cat that might still be roaming the wilder areas of the continent.* Jefferson didn’t think necessarily this animal was extinct and that its bones could be considered fossils, but he is generally credited as founding vertebrate paleontology in the United States. Jefferson also did not officially name the animal. That honor goes to Richard Harlan, who first used the genus name Megalonyx in a taxonomic (rather than shorthand) sense, in 1925. The specific epithet jeffersonii – in honor of Thomas Jefferson, of course – was coined several years earlier by A. G. Desmarest (Desmarest 1822). But Desmarest placed the species in the pre-existing ground south genus Megatherium rather than creating a new one to accommodate it. Harlan, by contrast, recognized that Jefferson’s ground sloth was very different from the truly gigantic, long-snouted Megatherium americanum of South America. For that reason, he created the genus Megalonyx by converting Jefferson’s informal name into a formal scientific one. The informal name for this animal is simply Jefferson’s ground sloth.

M. wheatleyi.jpeg
Skeleton of Megalonyx wheatleyi, a slightly older (and smaller) relative of Jefferson’s ground sloth (Megalonyx jeffersonii), on display at the American Museum of Natural History in New York. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

The discovery of Megalonyx is intriguing from an historical standpoint and eventually resulted in Jefferson’s ground sloth being named the official state fossil of West Virginia, but this extinct sloth is quite interesting in its own right. Its paleobiological claim to fame is that it had one of the greatest North American ranges of any South American participant in the Great American Biotic Interchange or GABI, the name given to the dispersal of terrestrial animals north and south via the Panamanian Isthmus that mainly took place during the last few million years. Remains of Jefferson’s ground sloths have been found throughout most of the United States and Canada, as far north as northern Yukon territory (nearly 70° N latitude; McDonald et al. 2000). The only other native South America mammal to achieve a similarly broad distribution is the modern North American porcupine (Erethizon dorsatum), which reaches Alaska’s northern Arctic coast (on the Beaufort Sea). The Virginia opossum (Didelphis virginiana) is a distant third among native South American mammals, with a distribution that reaches the southern extreme of Ontario (about 45° N), though another extinct sloth, Paramylodon harlani, had a similar geographic range during the late Pleistocene (McDonald 2005).

Distibution Maps Megalonyx.jpg
Geographic distributions of Megalonyx (yellow and blue dots; from NeoMap), the Virginia opossum, Didelphis virginiana (dark purple; from NatureServe), and the North American porcupine, Erethizon dorsatum (from The Smithsonian Institution).

Why have most native South American mammal groups met with such limited success in expanding northward? An important factor seems to be that most of them originated and flourished in tropical areas of South America and were restricted to similar habitats elsewhere. Since very little of North and Central America is located in tropical latitudes, few representatives of native South American mammals have made it past Mexico and the Gulf Coast Plain of the United States, and most do not make it even that far. Nevertheless, many of these Neotropical natives are characteristic and charismatic members of Central American faunas and are no longer restricted to South America sensu stricto. These include monkeys, armadillos, sloths, anteaters, opossums, and several groups of caviomorph rodents.

But if modern sloths are only found in tropical forests, how was Jefferson’s ground sloth able to live comfortably in the Arctic Circle? Differences in physiology and ecology undoubtedly resolve this apparent conundrum. The large size of Jefferson’s ground sloth (about one ton) probably helped it tolerate a much greater temperature range than modern sloths, and it evolved to feed on the leaves and twigs of temperate shrubs and other plants rather than relying on fruits and leaves of tropical trees (McDonald 2005). Interestingly, the North American porcupine and the Virginia opossum show a similar pattern, as each is the largest member of its respective family and less frugivorous than many of its close relatives. Jefferson’s ground sloths were able to live at high elevations in addition to high latitudes; remains of this species have been discovered in the Rocky Mountains near Snowmass, Colorado at more than 2,700 m above sea level (nearly 9,000’) (Sertich et al. 2014).

Rusty Megalonyx Med.jpg
“Rusty,” the fleshed-our Jefferson’s ground sloth on display at the Museum of Natural History on the campus of my alma mater, the University of Iowa. Image from here.

The skull anatomy of Jefferson’s ground sloth is also quite distinctive compared to other members of its family (Megalonychidae). Members of this group, which include modern two-toed sloths (Choloepus spp.), typically have canine-like teeth (“caniniforms”) in their upper and lower jaws that are triangular in cross section and wear against one another to create sharp, beveled edges. In Megalonyx, these teeth are rounded in cross-section and proportionately much larger than in other megalonychids. Furthermore, these caniniforms did not slide past one another to form sharp tips for shearing vegetation; instead, they occluded at their tips, creating large, flat surfaces for crushing vegetation (McDonald 2005). Although it isn’t clear how Jefferson’s ground sloth used its unusual caniniforms, it was certainly doing something different with them than other megalonychids. This idea is supported by the fact that the area of the jaw in front of the caniniforms was much shorter in Jefferson’s ground sloth than in other members of its family. This suggests that its lips and snout were also structured (and presumably used) differently.

Megalonychid Caniniforms.jpg
Front (anterior) portions of lower jaws of megalonychid sloths in occlusal view, including a modern two-toed sloth (left; CMNH 18972), the extinct sloth Pliometanastes protistus, a close relative of Megalonyx, (middle; from Hirschfeld and Webb 1968:fig. 14) and Megalonyx (right; modified from McDonald 2005:fig. 7). Note the large, rounded caniniform teeth in Megalonyx (highlighted in green), as well as its short predental spout (the area in front on them). Image by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

Why is Jefferson’s ground sloth no longer roaming the forests of temperate North America? Jefferson’s ground sloth went extinct along with a variety of other large mammals in North and South America near the end of the Pleistocene, around 12,000 years ago. Since this extinction primarily affected large mammals (those larger than about 100 lbs. or 44 kg), it is known as the Pleistocene (or late Quaternary) megafaunal extinction (Barnosky et al. 2016). Humans almost certainly played a major role in this extinction, but direct evidence of human-megafauna interaction (specifically predation) is lacking for most of the dozens of species that went extinct. Jefferson’s ground sloth has the dubious honor of being among those species for which human exploitation has been documented (Redmond et al. 2012).

Jefferson’s ground sloth was one of several types of giant sloths that went extinct in the megafaunal extinction. It is probably no coincidence that the only sloths that survived were the small, cryptic ones. Fortunately, these are relatively abundant in many parts of their range; it just takes some good luck or a good guide to spot one!

*For additional historical details about Thomas Jefferson and Megalonyx, check out this nice summary from Discovering Lewis & Clark®. Jefferson’s address to the American Philosophical Society was published in the society’s Transactions in 1799 and is available here, but a transcribed version that is easier to read is available here.

References Cited:

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