These creatures were unique to the continent and were medium-sized herbivores. They would have looked vaguely like pigs, although without the flattened nose-disc of those animals, and with four functional toes on each foot. As far as we can tell from their teeth, they were mostly browsing animals and were spread quite widely across the continent.
Early examples, such as Merychyus and Ticholeptus, lived across much of the west, from Oregon to Florida and central Mexico. They were generalised browsers, about the size of sheep, but became less common, and eventually died out, as more brushy terrain began to spread in the Middle Miocene. They were replaced by Ustatochoerus, sometimes regarded as a direct descendant of Merychyus (and even referred to by the same name), but distinguished from it by teeth suited to tougher plant material, and with nostrils placed further back on the skull.
This latter feature is taken to imply the presence of a short proboscis, similar to that of a tapir, with the fleshy part of the nose, not preserved in the fossils, extending over the top of the snout down to what would have been the visible nostrils in the living animal. An even more extreme example of this skull shape is seen in Brachycrus, which originated somewhere in what is now Central America, and spread up into western North America in the Middle Miocene, where it has been found from New Mexico to California and Wyoming. This was at least the size of a modern domestic pig, and heavily built, almost like a small hippopotamus, and seems to have been adapted to living in marshy river beds, and feeding on soft water plants.
Quite what the oreodonts actually were has been the subject of debate over the decades. Clearly, they don't belong to any group that's alive today, and they died out in the Late Miocene, leaving no descendants. They were initially assumed to be related to pigs, given their general appearance, but more modern studies suggest that they may actually have been more closely related to camels, and so conceivably had multiple stomach chambers that enabled them to chew the cud. Their demise suggests that, while some of the later species (such as Ustatochoerus) were capable of eating grasses, they weren't very good at it, and the growth of the prairies eventually spelled their doom.
Rhinoceroses are not the sort of animal we associate with modern-day America, yet, judging from the sheer number of fossils found, the most common large herbivore of Early Miocene North America may have been just such a creature: Menoceras. This entered the continent around the dawn of the epoch from Asia, where it would have encountered the older native rhino Diceratherium, which it continued to live alongside for millions of years. It likely managed this feat in part because the two animals were very different in size, and so presumably had different living requirements; Diceratherium was about the size of the smallest rhinos alive today (so still pretty big, then) while Menoceras was about the size of a domestic pig or sheep.
The two animals, however, had one significant feature in common: they had two horns on their snout, but, unlike in any living rhino, these horns were side-by-side, instead of one behind the other. This was long thought to indicate that the two must have been close relatives, but more recent analysis suggests that this may be an unusual case of parallel evolution, with the two stocks arising on separate continents, and Menoceras being closer to the living sort of rhino.
A major difference between Menoceras and its later relatives, however, is the significant difference between the sexes, so far as we can infer such a thing from fossils alone. While males and females seem to have been about the same size overall, the males had much larger horns, as well as larger teeth. This probably indicates that they used them for fighting, competing with other males for access to mates. While it has been suggested that the sheer number of fossils we have for them at certain sites implies that they may have lived in relatively large herds, the proportion of males that apparently died young otherwise suggests a social life not very dissimilar to that of modern rhinos.
At any rate, around 20 million years ago, both of these two-horned rhinos were replaced by a second wave of immigrants from Asia. These were hornless and two-horned rhinos, belonging to at least five different genera, all of which appear round about the same time, and seem to have been better suited to eating the tougher vegetation that was slowly becoming more common. Some of them did not survive very long - Floridaceras is known only from its namesake state, and doesn't seem to have lasted more than around 4 million years or so - but others went on to ensure that the Late Miocene, too, was a time of American rhinos.
Rather less common in North America, despite living nowhere else in the world at that time, were tapirs. These had a long history on the continent, but, by the Miocene, we know of only one, relatively rare, species, which lived in forested environments from at least Wyoming to California. This was Miotapirus harrisonensis, and, barring some odd coat-colouring we don't know about, it might well have been hard to distinguish from modern species, being almost exactly the same size and shape. Due to the lack of any other known candidates from this time period, it is also quite likely the direct ancestor of those animals, too.
Related to the tapirs (although how closely is not entirely clear) were the chalicotheres. Although primitive examples had previously lived in North America, by the Early Miocene they had been extinct there for millions of years. They had, however, been continuing to evolve in Asia, and, around 23 million years ago, they crossed back over the Bering land bridge to enter the continent once again.
The animals that returned had changed significantly during their time in the Old World, becoming more bizarre as they did so. In fact, the ones that returned to North America were rather less peculiar than those living in Europe at the time, but they still had an odd combination of characters to modern eyes. In size and shape, they were roughly horse-like, but with a sloping back and long forearms with which they loped along the ground. Their head was also at least somewhat horse-like, and its clear from their skeletons that, regardless of their precise affinities, they were clearly related to both horses and tapirs.
But they didn't have hooves; they had claws.
Since horses are not generally known for their claws, when they were first discovered in the 19th century, it was assumed that the claws - which had become detached from the rest of the skeleton - must have come from a different creature. But, even by the end of that century, it had become clear that, despite being herbivores, these animals really had had claws. Or, to be more accurate, they had hooves that happened to be shaped exactly like claws, which is pretty much the same thing, really.
We know of only two genera in Miocene North America, which were similar enough to have likely had a common ancestor (possibly back in Asia) not that long before. The better known of the two is probably Moropus, which lived in open woodland environments across at least the western half of what is now the US. Many internet sites quote Moropus as standing about eight feet tall at the shoulder but this, so far as I can tell, is how high it would have reared while raised partially upright on its haunches; in its more usual four-legged stance it seems to have been about horse-sized.
The name "Moropus" literally translates as "sloth-foot", likely referring to the shape of its claws, and the evidence is that it used the ones on its front feet in the same way that ground sloths probably did, pulling down branches to feed on the leaves. It didn't quite have the extreme gorilla-like adaptations for knuckle-walking that the European Chalicotherium did, although it did have fused bones in the index finger/second toe suggesting that it may have put some weight on the inner side of the foot. Pieces of the upper jaw of juveniles show that these had sharp, clipping teeth at the front that were lost in the adults. It's possible that the resulting gap made room for a long, muscular tongue to flick out, rather like a giraffe's, although, clearly, this is the sort of thing it's hard to know for sure.
In some respects even stranger-looking was the other kind of North American chalicothere: Tylocephalonyx, only discovered in the late 1970s. This seems to have lived in more riverine habitats, or at least denser, moister, woodland, than its better known relative. In shape and size, it was not dramatically different from Moropus, and it had the same adaptations to its feet and claws... but it also had a dramatic dome-shaped bony lump covering its forehead.
The exact purpose of this dome is unclear; although it's tempting to assume that it was used for some sort of head-butting, it doesn't appear solid enough (or the neck strong enough) for that to have been the sort of high-impact bashing seen in sheep and goats. Possibly there was milder head-butting involved, or perhaps they just thought it looked sexy.
Having arrived at the beginning of the Miocene, both kinds of American chalicothere had died out by around 13 million years ago, as the Middle Miocene transitioned into the Late. They left no descendants, although their relatives elsewhere survived as late as the Pleistocene.
The demise of creatures such as Menoceras and Moropus resulted in a distinctly different fauna emerging in Late Miocene North America. But before we look at that, it's time to see what carnivores were preying on the animals we have looked at so far...
[Photo by Ryan Somma, from Wikimedia Commons.]