Perhaps the most obvious difference would be that the Great Lakes didn't exist yet, since they were carved out by the advancing glaciers of the Ice Ages - which have yet to happen. With sea levels higher, Florida (then, as now, not a place known for its mountain ranges) is largely underwater, and there were probably many other changes around the coast, too.
Arguably the most important difference, however, is further south. Mexico is not so different, at least in its general outline, but beyond that things start to change. Depending on the exact point within the Pliocene we're talking about, you could perhaps have walked as far as Nicaragua without getting your feet wet. Beyond that, however, the Central American peninsula tapers to a point, and where Costa Rica and Panama should be, there is nothing but a chain of tropical islands, a southern counterpoint to the much larger chain of the Caribbean further north.
So, in order to reach South America from the north, you would have to cross the sea, hopping past tropical islands as you do so. This was not impossible for animals, which can, given enough time, cross quite remarkable amounts of sea, but, on the other hand, it isn't exactly easy, either. As a result, South America remained an island continent with its own, unique, animal life, and relatively few animals made the crossing between it and the North. (Although not, it should be noted, none at all).
As the Pliocene wore on, that would change, as the two continents slowly shuffled into their current positions, and the ridge connecting them rose bit by bit above the surface of the sea. Panama formed around 2.7 million years ago, just a hundred thousand years or so before the official end of the epoch, and, when it did, the effects on American wildlife would be dramatic. For the most part, it was the animals that headed south that were successful, but a few would make the opposite trip.
The Pliocene then, lies almost entirely before the Great American Interchange, when the two American continents collided, and their respective faunas would merge, walking across Panama to create roughly the distribution that we have now. Before that time, North America is a land devoid of armadillos and opossums, and of the giant ground sloths that would be such a part of the following, Pleistocene, epoch.
For much of the Pliocene, North America as a whole seems to have been fairly dry. The Great Plains were covered by open steppe-land, grassy and largely devoid of trees. Even within this, there was obviously a fair amount of variation across the continent - our two best fossil sites from the time are in Idaho and Texas, which were no more similar in temperature then than they are now. And there were exceptions, too, with both Mexico and the Pacific Northwest being well-forested for most of the epoch. But, for the most part, the heart of America, at least, was dominated by grassy wastes.
This did not bode well for many of the creatures that had lived in North America in the previous, Miocene, epoch, and many died out, if not straight away, at least not too far into the Pliocene. For example, there had once been a profusion of animals related to the modern pronghorn "antelope" on the continent, but they became far less diverse with the dawning of the Pliocene, and even less so later on as temperatures began their slide into the long autumn of the later part of the epoch.
One that did well was the dwarf pronghorn (Capromeryx spp.), an animal whose fossilised teeth tell us that it ate tough foods like grasses and low-lying herbs, making it ideally suited to the growing grassland habitat. Early dwarf pronghorns, some of which lived as far south as Mexico, were not that much smaller than the living species. Unusually, however, they got smaller as time went on, and they lived well into the Ice Ages, where they were apparently quite numerous. While there are also some species known from no more than a single specimen, the related Tetrameryx, whose horns divided in two so close to the base that it actually looked as if it had four, was the only other genus we know to have been widespread at the time, and it, too, was a grass-eater that survived into the Pleistocene, dying out some way into the Ice Ages.
As the pronghorns declined in diversity, however, new animals arrived to take their place. At or around the very beginning of the Pliocene, the first deer entered the continent, heading across from their original homeland in Asia. Deer being woodland animals, the fact that the Pacific Northwest was wetter and more fertile than the heart of the continent doubtless helped, as that's the first place they would have encountered, and Washington state does, indeed, hold some of the earliest deer fossils in America.
These early arrivals included the immediate ancestors of the white-tailed deer that are so numerous across the continent today, and, in fact, were essentially indistinguishable from the living species by the time the epoch ended. Other known deer of the time, such as Eocoileus and Bretzia, look very similar, and are probably close relatives. All of the deer unique to the New World are descended from these early arrivals, although some more widespread species (such as elk) belong to a separate lineage that arrived much later.
If the deer preferred the more forested habitats around the great steppe-lands, however, the latter perfectly suited the horses. At the beginning of the Pleistocene, many of the local horses still had three toes, even if two of them were mere stubs that did not reach the ground (much like the outer toes of most cloven-hoofed animals today). These included Nannippus, a close relative of the more famous Hipparion.
However, it was here, in North America, that the first horses of the modern genus Equus evolved. The oldest such fossil currently known is of the Hagerman horse (Equus simplicidens), named for the Hagerman bone beds in Idaho, where many of the fossils have been found. Hagerman horses lived across much of North America, with fossils also known from Texas, and would have been about the size of a small pony. Physically, their skeletons more closely resembled those of zebras than domestic horses, so it's possible that, if they weren't the common ancestor of all later horses, they may have been closer to the zebra lineage than to its counterpart. However, the differences between the skeletons of living members of the horse family are sufficiently slight that it's hard to know for sure, and we certainly can't say whether or not they would have been striped.
It has even been argued that the grazing habits of the Hagerman horse may have contributed to the growth of the steppe-lands that they inhabited, chewing away at the lusher vegetation on the fringes, and then taking advantage of the open terrain that they themselves had helped create. It would have been a matter of accelerating change, rather than directly causing it, but either way, the range expansion that this led to for horses of all kinds later led to them crossing over the land bridge to Asia, and giving rise - presumably via more than one evolutionary path - to all the species alive today. Ironically, of course, come the end of the Ice Ages, horses died out in North America, and by the time the white man arrived they had been gone for thousands of years.
Another animal that was absent from pre-Columbian America was the pig; all the ones there now are either domesticated or introduced. America does, however, have peccaries, animals that look remarkably similar to pigs, and, to be fair, aren't exactly distant relatives. Also known as javelinas, peccaries are no longer found outside the New World, although they were once much more widespread.
Perhaps the most common American peccary of the Pliocene was Platygonus, which was somewhat larger and longer-legged than any of the modern species. The long legs suggest an animal suited for running on open plains, rather than inhabiting dense forest. Of course, this is all relative, and the animal was still generally pig-shaped, and not some fast-running antelope; the savannah-dwelling warthogs of Africa might well be a close analogy. Analysis of microscopic scratch marks on its teeth suggest that it ate relatively little grass, however, and would, like many modern peccaries have had an omnivorous diet, including invertebrates as well as herbs and forage.
A close relative, Mylohyus, was thought to be unique to the Ice Ages until fairly recently. In 2010, however, a new fossil assigned to the genus was discovered from 5-million year old fossil beds in Florida, pushing it back to the dawn of the Pliocene. It was smaller than Platygonus, but likely had a similar lifestyle.
However, there were also much larger herbivores in North America at the time, and I'll look at those in part 8.
[Photo by "Reynosa Blogs" from Wikimedia Commons.]