|A modern spectacled bear|
But not, it has to be said, all of them - at least, from a human perspective. There were, as you might expect, many small carnivores in North America at the time, including badgers, weasels, skunks, raccoons and early wolverines and otters. While we can't know the details of their coat colours, it's quite likely that many of these creatures would have looked not too different from their modern relatives and possibly would have been hard to tell apart from them on casual inspection.
The same may well have been true of many of the wild dogs of the time. There were no true wolves in America at the time, since those came later, but coyotes were a different matter. For much of the Pliocene, Johnston's coyote (Canis lepophagus) was widespread across what is now the southern and western United States, from Washington and California on the Pacific coast, down to Florida in the west. While there are enough differences in the skull to mark it as a different species, the younger the fossils are the more they come to resemble the modern coyote. It's on these grounds that we're pretty confident that this animal was the direct ancestor of its modern counterpart, and that, some time around 2 million years ago, as the Ice Ages started, it would have imperceptibly blurred into its descendant.
For a long time Johnston's coyote was the oldest species of Canis known, and was therefore regarded as a possible ancestor of wolves and jackals as well - albeit by a more circuitous route, with some of its population splitting off to head into Asia. In 1998, however, an even older species of coyote-like animal, Canis ferox, was identified, initially from deposits in central Mexico. It seems likely that this animal, which died out in the middle Pliocene, was not only the direct ancestor of Johnston's coyote, but, whether through that animal or through intermediaries we haven't found yet, of all the other members of the genus.
These were not, however, the only dogs of Pliocene North America. For one thing there would have been foxes, albeit related to the living grey fox rather than the more familiar red one (which, like the modern wolf, originated in Asia). More significantly, though, there was Borophagus diversidens. This was the very last of the borophagine dogs, a lineage paralleling the modern "canine" dogs and dating back over 30 million years, to long before the dawn of the Pliocene.
But now, B. diversidens was the only species left, the terminal product of millions of years of adaptation towards greater carnivory, and arguably the most specialised of its kind. It was a big dog, around 60 cm (2 feet) tall at the shoulder, and very heavily built. To modern eyes, its most unusual features would have been the short snout, domed forehead, and remarkably wide head. These, perhaps, might not look so odd on a domestic rottweiler, but they're not what we expect on wild dogs. These features were accompanied by unusually broad and powerful teeth, that could have delivered an exceptionally strong crushing bite for an animal of its size.
It lived through what is now Mexico and the southern United States, with fossils known from both Florida and Texas, and also up the Pacific coast at least as far north as Washington, and as far inland as Idaho. This is, for the most part, much the same area as its coyote-like counterparts, but the two animals would have had a very different lifestyle. As the scientific name "lepophagus" suggests, we think that Johnston's coyote mostly ate things like rabbits... Borophagus would have gone for much larger prey.
Its teeth, and the short, powerful, jaws, suggest an animal that could easily crack open bones to get at the marrow, much as hyenas do. It was long thought that this meant it was primarily a scavenger, and it seems hard to imagine that it didn't feed on the leftovers of other carnivores when the opportunity arose. But, just as modern spotted hyenas are quite effective hunters when they put their mind to it, it seems likely that even Borophagus - the best bone-crusher of all the "hyena-like dogs" - was also quite a deadly predator. Especially if, like many modern dogs, it hunted in packs.
Borophagus was unique to North America, with no close relatives elsewhere, but many of the other big carnivores of Pliocene North America were much more widely distributed. Among the big cats, the most common was Megantereon, which is also known from Europe, Asia, and Africa. It's unclear whether the North American form was even a different species from its Old World counterparts, but even if it was, they were probably no more different than, for example, leopards and jaguars. At any rate, Megantereon survived longer in North America than it did elsewhere, and it's probably the immediate ancestor of Smilodon, the even more extreme form that would live through the Ice Ages.
Certainly, the presence of a jaguar-sized cat, complete with large sabre-like teeth, would have been a significant difference from modern North American wildlife. But at least modern America has cougars - and, indeed, jaguars, once one counts Mexico. But, in the Pliocene, the continent also had hyenas, animals that are no longer found outside of Africa. The hyenas in question belonged to the species Chasmaporthetes ossifragus, a very close relative of C. lunensis, the European "running hyena." That they apparently had slightly stronger jaws than their European counterparts may suggest that they ate more bones, and perhaps even fought over carcasses with Borophagus, although one suspects that the latter would have had the edge if they did.
The American running hyena first appeared on the continent around 3.5 million years ago, as the climate was turning colder, and is the only hyena known to have crossed the Bering Straits to do so. Perhaps surprisingly, they survived here longer than the Old World forms managed closer to their point of origin, and they were one of many species to be wiped out by the coming of the Ice Ages. Indeed, one of their last known populations was in Florida, which sounds as good a place to escape the ice as any.
In the previous epoch, the largest carnivore in North America - and, indeed, one of the largest land-dwelling mammalian carnivores that ever lived - was the giant bear Agriotherium. While species of this genus lived on in Africa and Eurasia well into the Pliocene, it had long been thought that the American form had gone extinct in, at the latest, the very early part of the epoch. In 2009, however, a fossil from Idaho was squarely dated to the late Pliocene, with a second similarly-dated fossil being uncovered in Washington in 2013.
Agriotherium was not only large, but also more predatory than modern bears, belonging to a group of primitive bears still in the transition from the meat-eating ancestors of the group to the generally more omnivorous modern species (polar bears aside). The bears that live in North America today came across from Asia at a later date, but, even so, Agriotherium was not only the only bear on the continent during the Pliocene.
For much of the Pliocene, it shared North America with Plionarctos. This was a small bear, with even the larger specimens (which were presumably male, if it was like modern bears) comparable to the females of the modern American black bear, and many quite a bit smaller. Already clearly evolving towards what we would think of as a typically bear-like mixed diet of meat and fruit, it is likely the ancestor of modern spectacled bears, and also of a number of other extinct species, one of which, the European Arctodus, rose to rival Agriotherium in size.
Today, spectacled bears live only in South America, their ancestors having headed there after the two continents collided almost at the end of the Pliocene. Before that time, however, the very closely related Florida spectacled bear (Tremarctos floridanus) had also lived across what is now the southern US, from Florida and Tennessee in the east to southern California in the west. They first appeared about 5 million years ago, in the early Pliocene, and went on to survive right through the Ice Ages, only dying out when the climate warmed again and humans arrived.
Casually dismissing the other large carnivore of Pliocene North America, on the grounds that alligators aren't mammals, I've finished my look at the continent, and, indeed, the hemisphere, during that epoch. We know, perhaps, somewhat less about what lived in the more southerly continents at the time. But that's by no means nothing at all, so it's time to turn to Africa...