Ground sloths were large quadrupedal mammals that were predominantly herbivorous (more on that later). They evolved in South America, before continental drift had joined North America to South America, and then, managed to cross the land bridge in Central America and make it all the way into North America. In fact, remains of ground sloths have been found in Alaska. Not bad for slow-moving, ponderous vegetarians.
There were many, many species of ground sloths; something like 80 genera, and above that, at least six families. As you may recall from the last entry; a species is a single type of animal, a genus a grouping of closely related species, and a family a grouping of several different, but related genera. So ground sloths, although ungainly looking, as will be discussed below, were quite successful – not a fluke, or trick of evolution. I should note, of course, that there is still some confusion about just which fossil remain of a given sloth falls into which family, and genus. Nonetheless, the ground sloths, as a whole, were quite successful, and evolved into many different shapes, sizes, and habitats.
Perhaps the best known of the now extinct ground sloths was megalonyx jeffersonii, best known, if it’s known at all, because of its association with President Thomas Jefferson, for whom the species is named. President Jefferson was an avid naturalist, and paleontologist, who received fossil specimens of the ground sloth that bears his name, in 1796-97. These included some gigantic claws (of which, more later). He suggested that they were a species of lion, and suggested that the as-yet undiscovered animal be named megalonyx, or giant claw. In fact, when Lewis and Clark set out to explore the Louisiana purchase in 1804, Jefferson asked them to look out for megalonyx, which he thought might still be alive somewhere in the unknown west. He was wrong. The claws were not from a lion, but from the sloth, and the sloths were extinct. Nonetheless, his boundless curiosity, and suggestion that discoveries of this sort were worthwhile, remain commendable.
M. jeffersonii, was enormous – eight to ten feet long. And it weighed up to 2000 pounds. Like the other ground sloths, it was herbivorous, and ate leaves, branches and bark it stripped from trees.
Big as it was, however, megalonyx was dwarfed by another giant sloth – Megatherium. This animal was formidable. It grew at long at twenty feet, and weighed up to four metric tons. A metric ton is roughly 2,204 pounds, so an adult megatherium might have weighed almost 9,000 pounds. You know what else weighs that much? Elephants.
Megatherium was confined to South and Central America, but its close cousin, the slightly smaller Eremotherium ranged into southern North America.
I wrote that they were herbivorous, and that is undoubtedly true. There is some thought, however, that they may also have been opportunistic carnivores, perhaps flipping glyptodonts (think Volkswagen-sized, turtle-shaped mammals) over, to get to their soft underbelly, or even chasing active predators away from kills, in order to scavenge the carcass. These theories are still quite controversial, and await further testing or discovery for clarification.
These were big, ungainly, slow-moving creatures. And yet, they thrived. They walked on all four legs, but could sit upright, to reach up into trees. Some could stand on their hind legs like bears. And they were armed with long, sharp claws on their front legs.
They apparently lived in family groups, and presumably the parents would have protected the young. But it seems unlikely that they were herd animals.
So why are these animals interesting? There are several reasons:
First, as will be discussed in a later post, when South and North America joined, more animals native to North American spread into South America, than did animals coming north from South America. Sloths were among the relatively few species that migrated north, out of South America. Why?
Second, why did they, like so many of the other animals of this time period, become so large? Presumably, in North America, they were expanding into an otherwise vacant ecological niche, so they had no direct competition. But they had evolved in South America, where there was competition, and still they grew to enormous size. Why?
Third – how did they evolve? They don’t seem like a likely candidate for evolutionary success, these big, slow-moving herbivores. But they were very successful, for thousands and thousands of years. Were those claws that deadly? Did no animal selectively prey upon them?
Finally, as with so many other species of Pleistocene animals, we are left to wonder – what happened to them? Remains of giant sloths have ben found in association with human hunting – so evidently humans were a species that preyed upon them. And perhaps, human hunting pressure, combined with a low or slow reproductive rate were sufficient to drive them to extinction. Climate change, too, may have played a role. Whatever the reason, they are all gone now – extinct.
But imagine how happy President Jefferson would have been if Lewis and Clark had found a Megalonyx for him.