When did the last of the ground sloths disappear? The standard answer is “about 10,000 years ago”. That’s the oft-repeated cutoff date for when much of the world’s Ice Age megafauna – from mastodons to Megatherium – faded away. It’s nice and neat, falling just after the close of the last Ice Age and during a time when humans were spreading to new continents. In fact, it’s too clean a cutoff. The shaggy, ground-dwelling sloths that inhabited almost the entire span of the New World didn’t all topple over at once. They very last of their kind, both protected and made vulnerable by life on islands, were still shuffling 4,200 years ago.

Calling the time of death for any species or lineage is always complicated by definitions and details. Should a species be considered extinct when its very last member perishes, or when the population sinks below a level from which they can recover? And in these fading families, should the explanation for extinction be the cause of death of the last individual, or do we assemble a more complex picture that considers factors that made the population vulnerable in the first place? Both science and storytelling influence our answers to these questions, but one thing is abundantly clear. Extinction is a process, not a single fell swoop.

Consider the times when the giant ground sloths disappeared. They were one of the great success stories of the Ice Age – with 19 genera ranging through South, Central, and North America, as well as Caribbean islands at the end of the Pleistocene – but, as reported by paleontologist David Steadman and colleagues in a 2005 study, 90% of the existing Ice Age sloths disappeared within the last 11,000 years.

Megalonyx and other giants from North America were some of the first to go. While Steadman and colleagues stressed that the dates represent “last appearance dates” rather than actual time of species death, the youngest known sloth remains from North America date to about 11,000 years ago. South America’s ground sloths, such the enormous Eremotherium, soon followed – the youngest dung and tissue samples found on the continent date between 10,600 and 10,200 years ago.

But for another 5,000 years, ground sloths survived. They weren’t on the continents, but scattered through the islands of the Caribbean. I had not even heard about these sloths until paleo geneticist Ross Barnett told me about them in a Twitter exchange long ago, and, as reviewed in the paper by Steadman and colleagues, there were at least five genera and thirteen species of large ground sloths that were unique to these islands.

View Images Cuba’s extinct ground sloth Megalocnus rodens. Photo by Ghedoghedo.

The largest of all was Megalocnus. This sloth hasn’t received nearly as much attention as the other “mega”-prefixed sloths, but, as you can see from the bones on display at the American Museum of Natural History’s fossil mammal hall, this 200-pound sloth was still an impressive beast. Based on remains found in a limestone cave on Cuba, Steadman and colleagues determined that Megalocnus lived until at least 6,250 years ago.

Other smaller sloths persisted even longer. Parocnus, also found on Cuba, lived until about 4,960 years ago, and the small ground sloth Neocnus trundled over Hispaniola until about 4,500 years ago. There’s no direct evidence that people were hunting or eating the sloths, but, based on tentative evidence for human occupation of Caribbean islands around 5,000 years ago, Steadman suggest that the arrival of Homo sapiens tipped the sloth into extinction.

Of course, last appearance dates are often revised with new finds and updated techniques. Two years after the Steadman study, Ross MacPhee and coauthors published a new, youngest date for Cuba’s Megalocnus. From a tooth found on the island, the researchers estimated that the ground sloth survived to at least 4,200 years ago.

Through the lens of geologic time – wherein millions of years are thrown around because the numbers are too big to truly comprehend – extending the lifetime of a ground sloth another 2,000 years might not sound like much. But MacPhee and colleagues underscore the importance of getting good dates for when Ice Age creatures vanished. If people really showed up on Cuba and other sloth-bearing islands around 5,500 years ago, then humans and ground sloths coexisted for over a thousand years and the “blitzkrieg” model of extinction starts to crumble. Humans may have still been responsible for the extinction of the sloths and other species, but the record doesn’t show the pattern of rapid die-off that has sometimes been used to pin our species as the chief cause of megafaunal extinctions.

In time, we may get a clearer picture of why such a diverse and widespread ground of mammals disappeared. Assuming that humans, climate change, or any of the other traditional suspects without more detailed evidence masks the complexity of how extinction happens. But even if paleontologists eventually puzzle together what happened to these great beasts, I’ll still be saddened by the fact that I just missed the ground sloths. Especially because there are habitats – such as vast stretches of desert in the basin and range I call home – that could still host them. Sometimes, when hours of rolling over the interstate starts to addle my brain, I start to imagine them out among the Joshua trees – reminders that we still live in the shadow of the Ice Age world.

References:

MacPhee, R., Iturralde-Vinent, M., Vázquez, O. 2007. Prehistoric sloth extinctions in Cuba: Implications of a new “last” appearance date. Caribbean Journal of Science. 43, 1: 94-98.