So – What Happened?

Between 10 and 12,000 years ago, the world changed.  In North America, 35 genera of large mammals became extinct.  Not species, but genera – whole groups of closely related species.  Thirty five genera:  Mammoths, mastodons, giant sloths, horses, saber-toothed cats, the short-faced bear (but not grizzlies), the American lion (but not the Mountain lion), camels (but not llamas and vicunas in South America), the giant beaver, but not the regular little guy.

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So – what happened?  Where’d everybody go?  And why?

For years, paleontologists and archeologists have tried to figure this out.  There have been five main theories to explain this wholesale extinction:

  1. Overkill – to much hunting by the newcomers to North America, the so-called Clovis people;
  2. Environmental change;
  3. Disease;
  4. Some extraterrestrial impact, akin to the comet which is believed to have killed the dinosaurs;
  5. Some combination of any of the above.

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One issue that has come up, because of the paucity of fossil remains, and gaps in the fossil record, is the timing of these extinctions.  Did these genera go extinct at roughly the same time, or were these extinctions staggered, spread out over time? Was this a slow catastrophe, or a fast one?

In 2009, two scientists, J. Tyler Faith and Todd Surovell, took a look at this issue, and published an article titled Synchronous Extinction Of North America’s Pleistocene Mammals, in PNAS, vol. 106, no. 49.  No slouchy journal, either – PNAS means Proceedings of the National Academy of Sciences of the United States of America.

By doing a careful statistic analysis of the fossil remains associated with the extinctions, they concluded that “the combination of these lines of evidence suggests that North American late Pleistocene extinctions are best characterized as a synchronous event.”  Specifically, “our analyses demonstrate that the structure of the chronology of North American late Pleistocene extinctions is consistent with the synchronous extinction of all taxa between 12,000 and 10,000 radiocarbon years. B.P.”

Okay, so what does that mean?

It means that most of these animals all became extinct in a two thousand-year span.  The authors note that:

         “Our simulations do not rule out the possibility that some extinctions may have occurred before 12,000 radiocarbon years B.P. The biogeographic simulation suggests that anywhere from 0 to 8 genera could have disappeared before the terminal Pleistocene . . . Even so, 23–31 genera abruptly disappeared at approximately the same time. Our results leave open the possibility for a small level of background extinctions (0–8 genera) followed by a surge in extinction rates that wiped out the remaining taxa (23–31 genera) between 12,000 and 10,000 radiocarbon years B.P.”

So it is possible, they acknowledge, that of the 35 genera that became extinct, maybe as many as 8 of them had gone extinct earlier that 12,000 years ago.  That still means, however, that 27 of them became extinct in that short – remarkably short – period of time.  As the authors put it,

           “Whether or not background extinctions took place, that a catastrophic event or process occurred at the end of the Pleistocene is abundantly clear.”

The implications for this are important.  Whatever happened, it happened very fast, and was continent-wide.  Europe experienced what the authors call a “long-term, piecemeal extinction process.”  Not so, here.  It happened all across the continent, in what they call “a geologic instant.”

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This conclusion doesn’t necessarily eliminate any of the five possible causes of the mass extinction, but it does put certain constraints on them.  An environmental change, for example, if it was the primary cause of these extinctions, must have been nation-wide and very rapid.  But intriguingly, even those limits – speed and breadth – still fit with three of the possible causes for the extinctions:  “This time period encompasses the earliest secure evidence of human foragers in North America . . . the Younger Dryas cold interval . . . and a possible extraterrestrial impact.”

Well, science marches on.  We still don’t know why these extinctions occurred.  And while two thousand years may be a geologic instant, in the lives of these animals, it encompassed tens or hundreds of generations.  A drought that lasted five hundred years, or seasons so cold that plants wouldn’t grow, could certainly have caused some of these extinctions.  And there may have been a cascade effect, too:  If a given herbivore becomes extinct or vanishingly rare, then the predator that preys on it is in trouble, too.  And family structures may have been disrupted by hunting, too, for that matter:  If the matriarch of the mammoth herd is killed, maybe the young ones don’t know how to survive a particularly harsh winter, or a dry summer.

But as to what happened?  We still don’t really know.  As Faith and Surovell put it, “further research on the biogeographic histories of individual species in relation to detailed paleoclimatic, paleoecological, and archaeological data could help to finally pin down the cause of North American end-Pleistocene extinctions.”

The Giant Sloths – An Unlikely Success Story

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.

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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.

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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.

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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.