Day 309#: Megalonyx jeffersoni
Today's animal of the day is Megalonyx jeffersonii!
Commonly known as Jefferson's ground sloth, this large species lived in what is now North America during the Pliocene and Pleistocene epochs. They were among the last of the ground sloths and were a pretty decent size, too. They are estimated to have grown to around 9.8 ft long and weigh up to almost 2,900 pounds! While this is much smaller than the massive giant ground sloth Megatherium from South America, it was still a pretty large animal (and to be honest, it's my personal favorite species of ground sloth).
Image credit: prehistoric-wildlife.com
They're called Jefferson's ground sloth because their fossils were originally described by Thomas Jefferson himself. The remains of this giant sloth were sent to him in 1796 (shortly before he took office as Vice President) by Colonel John Stuart, who found them in a cave in what is now known as Greenbrier County, West Virginia. The remains consisted of part of a femur, the ulna, radius, some foot bones, and three massive claws. Jefferson originally believed that these claws belonged to a gigantic big cat three times bigger than a lion! He named this beast Megalonyx, meaning "great claw". However, it wouldn't be until he heard of the discovery of Megatherium by Baron Georges Cuvier that Jefferson would realize Megalonyx was also a gigantic type of sloth.
Image credit: Caspar Wistar (illustration of the original fossils of M. jeffersonii)
During this time there was actually a bit of a debate over whether or not species could actually go extinct. Keep in mind, this was still a few decades before creatures like dinosaurs, pterosaurs, and marine reptiles would be discovered by the scientific community, and many people (including Thomas Jefferson) didn't believe that God would let any of His creations die out and that all the strange animals they kept finding bones of still existed somewhere in the unexplored wilderness. In fact, Jefferson specifically told Lewis and Clark to keep an eye out for any Megalonyx when they went on their famous expedition in 1804. Sadly, they didn't find any live Megalonyx. However, they did find several more fossils, including some that had cut marks that could have only been made by flint tools. This discovery, as well as many more like it that would be made later, proved that while Megalonyx is extinct, it did live alongside humans at some point.
Image credit: BBC Studios (Prehistoric Planet Ice Age episode 1: The Big Freeze)
Megalonyx was an incredibly widespread genus of ground sloth, and is believed to have evolved from another species of ground sloth called Pliometanastes, which migrated up north from South America during the Great American Biotic Interchange that occurred when the land bridge between North and South America formed during the Late Miocene epoch (about 9 million years ago). There are currently five known species of Megalonyx that could be found across North America, but for now, I'm just gonna focus on M. jeffersonii. This species ranged all the way from Alaska to central Mexico and was believed to have been able to inhabit a wide variety of different habitats. Though it is most commonly found in areas that would have been mountainous spruce forests mixed with other types of hardwood and conifer trees.
Image credit: BBC Studios (Prehistoric Planet Ice Age episode 1: The Big Freeze)
One of my favorite facts about these sloths is how they raise their young. Some remains were found in Iowa of an adult female Jefferson's ground sloth with two juveniles, which were of different ages. The oldest was somewhere between three and four, while the younger one was about a year old. This would imply that this species, and possibly other species of ground sloth, would've cared for multiple different generations at one time. Many other animals do this, as it allows the older sibling more time to grow up before setting out on their own, while also learning how to take care of their own offspring one day by babysitting their young sibling (which also helps out the mom). Paleontologists have also estimated some potential life expectancies for this species by calculating their body mass as well as the approximate ages of all of the fossils. The first estimate puts their average lifespan at around 19 years old, with them reaching sexual maturity at around 6. However, another calculation puts their max life expectancy at around 49 years. Personally, I hope it's the longer one, just because I love them and want them to have lived long and happy lives.
Image credit: Beth Zaiken
Analysis of their teeth as well as isotopes found within their fossils shows that Megalonyx's diet primarily consisted of plants, and there's currently no evidence of them being opportunistic omnivores like the South American Mylodon darwini (which I covered on day 93#) was. Like other ground sloths, Megalonyx jeffersoni would have used its long claws to reach high up tree branches and pull them down low enough to reach. Sadly, this species went extinct around the end of the Pleistocene, around 11,000-13,000 years ago. While their extinction does coincide with the arrival of humans in North America, I don't think it's fair to blame their extinction (as well as the extinction of many of the other species of Ice Age megafauna) on humans. Yeah, being hunted by humans probably wasn't super good for their population, but climate change and habitat loss probably played a bigger role in the extinction of these beautiful creatures. People tend to blame an animal going extinct on just one reason, when in reality it's often a combination of many different factors.
Also, as a Megalonyx superfan, I would like to say fuck Thomas Jefferson. Not only did he own around 600 slaves, but he also had a long-term sexual relationship with one of his slaves, named Sally Hemings, who was a teenager at the time while he was in his 40s. I don't want to hear anyone say "oh, it was a different time," because there were people who opposed slavery back then, too. I hate that guy, love the sloth though.
Image credit: The Walt Disney Company/Blue Sky Studios (formerly)
Sorry for putting this last fact right after a pretty serious/heavy statement, but Megalonyx is also the type of ground sloth that Sid from the Ice Age movies is.
