Smilodon and the dire wolves. But, of course, these were hardly the only carnivorous mammals on the continent at the time. For one thing, it's worth remembering that something like half of the species in North America in the mid Pleistocene are still around today. Even where modern species had yet to evolve, their immediate ancestors often had, and would have looked very similar to modern forms. So Ice Age North America would have had its share of coyotes, bobcats, and cougars, not to mention smaller creatures like badgers. And let's not forget jaguars, which entered South America from the north early on in the epoch, and are still found in parts of Mexico today.
But surely Smilodon was the most fearsome predator of its day? Well, probably... kind of. But it most certainly has a contender for that crown.
The largest predator in America today is the polar bear, with adult males typically weighing on the order of 450 kg (1000 lbs). You might think that polar bears must have absolutely loved the Ice Ages, but, in fact, they only appeared just before the last one about 0.15 million years ago, and so missed the great majority of them. In fact, they appear to be descended from brown bears (and therefore, technically, probably shouldn't be a species, just a subspecies), which are rather smaller and much more inclined to eat plant matter. Although that 'rather smaller' part should really be qualified with the statement that it only applies as an average for the species as a whole. That's because the Kodiak bear subspecies, found only on islands off the south-west coast of Alaska, is at least the size of a polar bear, and maybe slightly larger.
But, compared with the largest bears of Pleistocene times, even Kodiaks were wusses. Europe had its cave bears, which were certainly large, but North America had the giant short-faced bear (Arctodus simus), quite possibly the largest bear, and, indeed, the largest carnivoran, that ever lived. (I should add, though, that it had a very close relative in South America that may have been about the same size, so exactly which one gets that crown is debatable).
Both brown bears and polar bears originated in Asia, and only entered North America once the last Ice Age was well under way. In contrast, the giant short-faced bear is home grown, descended from a slightly-less-giant short-faced bear (A. pristinus) that had lived on the continent during the previous epoch. In fact, the giant short faced bear is one of the very few extinct Pleistocene animals for which we have a complete mitochondrial genome, enabling us to analyse its genetics, and determine exactly how it's related to bears that are alive today.
It turns out that, much as we'd expected, this giant bear's closest living relative is not the Kodiak or polar bears, but the spectacled bear, a species that inhabits the Andes. Spectacled bears are about medium sized, as bears go, much like the familiar American black bear. The giant short-faced bear was anything but. We can tell from their skeletons that were close to 2.5 metres (8 feet) in length, but estimating their weight is a little trickier. Estimates from the 1990s, based on the thickness of the leg bones and how much weight they seem to have evolved to bear, give a value of around 780 kg (1,700 lbs) for adult males. (Females, as is usual for bears, would presumably have been quite a bit smaller). However, a more recent analysis, from 2010, suggests that this may have been an underestimate, and that weights of 1,000 kg (1.1 US tons) may actually have been quite common.
Remains of giant short-faced bears have been recovered from Alaska to Mexico, and, while they seem to have been more common outside the southeast, at least some fossils have recently been recovered from Florida. Since they lived through, and between, many different Ice Ages, it's likely that their exact range changed quite a lot over the course of their existence, but, even so, they seem to have been fairly adaptable.
But just how they managed that is a subject of far more debate than how big they happened to be. The traditional view is that the animal was a hyper-carnivore, a deadly predator feeding entirely on meat, just as polar bears do today. Evidence in favour of this came from the relatively long legs of the bear, suggesting that it could run faster than living bears can, as well as such things as the large size of the teeth. But we're no longer so sure that this view was correct.
For example, the legs may not have been - relative to the rest of the bear - quite as long as we previously thought. Comparison of the skeleton with that of living pursuit predators shows that it really didn't have the adaptations necessary to chase anything for a long period of time, rather than just making the short (if terrifying) dash that many living bears can. Nor do they appear to have been particularly great at leaping out at things from hiding - even leaving aside the fact that they can't have been that easy to hide, given their size.
However, even if we accept that, it doesn't necessarily mean that they didn't eat a lot of meat. It's entirely possible, for example, that they were scavengers. After all, the sudden arrival of something the size of a giant short-faced bear at the site of a recently killed carcass would have been more than enough to give even a Smilodon pause for thought. In fact, analysis of the isotopic composition of giant bear skeletons in the 1990s showed exactly what you'd expect from an animal that ate little but meat. Taken together with the physical shape of the animal, this suggested a bear that wandered about over a wide area, searching for carrion, and scaring off other predators from their meals.
In the twenty years since those studies, however, opinion has begun to swing the other way. Examining the wear patterns on the teeth indicates that the bears certainly didn't crunch up bone, as a hyena might, and, perhaps more significantly, they quite closely resemble those of its living relative, the spectacled bear. Since that's the second most herbivorous living bear (after the bamboo-munching giant panda), with meat forming less than 10% of its diet, it's not a great sign. Detailed computer analysis of the shape of the skull and teeth, and comparison with other living animals, also shows that this was a creature that could well have eaten a lot of plant matter.
Most likely, then, they were omnivores, eating whatever they could find, just as modern brown bears do. Certainly, they would have eaten meat, and probably more than modern spectacled bears do, but they were probably at least as much of a menace to berries and acorns as they were to the local herbivores.
As with many other large animals of the time, giant short-faced bears died out about the time that the last Ice Age ended, with the most recent fossil currently known dating from somewhere around 11,000 years ago. It was found in Kansas, and would certainly have lived alongside humans.
But, if the giant short-faced bear wasn't quite the predator it's sometimes cracked up to be, there were still plenty of predators that were. And where there are predators, there are herbivores wanting to outrun them. Indeed, possibly the second-fastest animal on Earth today is the American pronghorn antelope. Because it also has better endurance, given a sufficient head start it could probably outrun a cheetah.
But therein lies a problem: why does it run quite that fast? After all, it doesn't need to outrun cheetahs, it just needs to outrun the fastest thing in America that's likely to want to eat it. Once it had already evolved to reach that speed, why was there any evolutionary pressure to go faster still?
The answer, we now believe, is that the pronghorn evolved to escape from something that no longer exists. Something that was incredibly fast, and that once tore across the Great Plains of America in pursuit of its prey. What was this animal that was once as fast as a cheetah?
Well, actually, it was the American cheetah (Miracinonyx spp.) Despite the name, though, it wasn't really a cheetah, at least not in the sense of being an especially close relative of the animal that now lives in Africa. It's easy to see why we used to think otherwise, though. From the admittedly incomplete skeletons we have, the animals seem to have had a similar body-form to living cheetahs, with a slender, flexible, body and legs built for speed. The nostrils are also enlarged, presumably to increase the amount of air that can be breathed into the lungs, and there is some skeletal evidence that, like cheetahs, they may have had short whiskers.
There are differences, such as the claws being more retractable than the running spikes of a true cheetah, but those could have been explained by this simply being an earlier species that had yet to reach its modern form. Indeed, of the two species we know of - both of which are Pleistocene - the younger is the more cheetah-like, which would fit with this being a possible ancestor of an animal that subsequently snuck away to Africa.
But, when we finally got enough DNA to do at least a partial genetic analysis, we discovered what some scientists had already expected: it was actually a really fast puma. Or at least, while it's not literally a puma, it is closer to that animal than anything else alive today, making it a representative of a native American lineage of cats that also includes the jaguarundi. Pumas and American cheetahs likely last had a common ancestor about 3 million years ago, shortly before the Pleistocene, before one evolved for speed, and the other to become the cougar/mountain lion that we know today.
We don't, of course, know exactly how fast they could run, although the fact that a pronghorn can reach 50 mph suggests they weren't exactly slow. That they look so similar to cheetahs, at least from what we have of their skeletons, also suggests that they adapted for high speed in essentially the same way that their namesakes in Africa did. They lived on and around the Great Plains, from Texas to Wyoming and Pennsylvania, ideal terrain for chasing down prey. Presumably, like living cheetahs, they were fast sprinters, but not endurance runners, not being able to maintain their top speed for long.
Like the giant short-faced bear, they died out around 11,000 years ago, and, as with so many other animals of the late Pleistocene that died out around that time, that's unlikely to be a coincidence.
The earliest indisputable human culture in the Americas is that of the Clovis people, who first arrived on the northern continent around 11,000 BC, and colonised the south not much later, lasting for about 4,000 years before fragmenting into a range of local cultures, from which most of today's Native Americans descend. The date of 11,000 BC is not long before the end of the Pleistocene, a date formally set at 9,700 BC (give or take a century), by which time the melting ice sheets had made Alaska a place that you might at least have some motive to visit, even if you didn't know there was a land of plenty on the other side of it.
There is increasing evidence that the Clovis people weren't really the first to reach the Americas, and that some may have even have walked there before the Bering Straits opened up around 15,000 years ago. These early people don't seem to have left many signs of their existence, though, and perhaps weren't very numerous. Even the oldest estimates for their arrival, though, leave America an untouched land for 98% of the Pleistocene, and it may well have been more.
Debate still rages over whether it was the Clovis people or the changing climate that played the biggest role in wiping out North America's Pleistocene animals; it seems hard to argue that they didn't both play at least some part. At any rate, it is with the arrival of humans on the continent that I leave it behind, to head elsewhere...
[Picture by "sergiodlarosa", from Wikimedia Commons]