As the climate warmed, sea levels rose, flooding low-lying coastal regions and turning major valleys into bays. For much of the epoch, for instance, Florida was almost entirely underwater. Further north, however, it was still connected to Asia (and Greenland, although that's less significant in the grand scheme of things). Perhaps because the land bridge was both far to the north and relatively mountainous, not many animals seem to have crossed it, but that changed in the Early Miocene as the climate began to improve, and only increased as the epoch wore on. And, for whatever reason, most of them were heading east - into America.
Many of them were, as always, small animals, rather than the sort of thing that tends to grab the attention of the layman. Even this early, North America already had a lot of rodents that at least belonged to the general types that we would recognise today: there were mice, beavers, and gophers, not to mention non-rodent animals such as shrews and rabbits. Some of these benefited from the improvements to the climate, with a number of new species arising to take advantage of new habitats. One particularly ancient group, the aplodontids, which are related to, but clearly distinct from, squirrels, had a great many species across the continent at the time - today, only one survives, the "mountain beaver" or sewellel.
True squirrels also did well, including Nototamias, an animal sometimes described as an early chipmunk - although how closely related it is to the modern forms is unclear - as well as primitive marmots. They were joined by the arrival of the first flying squirrels from Asia during the Mid Miocene. Also somewhere over on the squirrel-like side of the rodent family tree were the mylagaulids, a group that had arisen not long before, and, during the Miocene, were one of the few animals to have crossed the Bering land bridge by heading west, since some are also known from China. For most of the epoch, they were relatively primitive, with early examples, such as Alphagaulus, only beginning to develop the burrowing habits that later forms would eventually improve on.
Naturally, many larger herbivores also made the crossing from Asia. Among them were two groups of deer-like animal - the true deer had only just begun to evolve at the time, and wouldn't arrive in North America until much later. The more familiar of these new arrivals were musk deer, represented by animals such as Blastomeryx, which is known from Saskatchewan to Texas. Physically, while this may have been slightly smaller than living forms, it otherwise probably looked very similar in life, with large sabre-like teeth, but no horns. Presumably, it also had a similar lifestyle, spending much of its time hiding in the forest undergrowth. They died out in the Middle Miocene, long before true deer arrived on the scene.
Slightly stranger to modern eyes were a group called the dromomerycids. More commonly placed in a family of their own, these are sometimes considered to instead be members of the ancient palaeomerycid family common in Europe and Asia. However, some recent studies hint that the latter may be more closely related to giraffes than to deer, leaving the American animals as something else entirely, a side-branch in the evolution of more broadly deer-like animals that left no modern descendants. The debate is currently far from settled.
Whatever they were, they rapidly filled the role in Miocene North America that true deer do today, with a number of different forms appearing. They were smaller than most modern deer, but their teeth suggest that they initially had a very similar diet, browsing on forest leaves, and they had a generally similar form. The most significant difference, however, was they had horns, not antlers - that is to say, permanent bony structures that were not shed annually. Unlike the horns of antelope or goats, however, these were probably covered with skin, which is one of the reasons that they have been compared with giraffes. An even more radical difference from modern deer was that many of them, such as Cranioceras, had three horns, not two, with the 'extra' one projecting from the back of skull, behind the more conventionally placed pair.
North America was not, of course, devoid of herbivores before these deer-like animals arrived. Among the animals the musk deer and their kin would have encountered when they first reached America were camels, three-toed horses, and various pig-like creatures. The camels of the day were nothing like today's Bactrians or dromedaries, looking, perhaps, more like modern llamas (which are also members of the camel family, but did not come along until later). They also still had hooves, which would have been less effective for walking across soft sand than the foot-pads of the modern species.
There were at least three different kinds of camel in the early Miocene. Some, such as Oxydactylus, were tall and slender, with long necks. Another type, represented by Floridatragulus, was slightly larger than a modern alpaca, and distinguished by a remarkably long and narrow snout that it might have used to snip particularly tasty-looking leaves from bushes. They lived, as their name suggests, only along the southern coast of the continent, from the peninsula that would later become Central America to those parts of northern Florida that remained above water.
The third sort, however, was, perhaps the strangest, and, like the long-necked form, one that continued to survive for some way through the Miocene. These were the "gazelle-camels", such as Stenomylus, which had become much smaller than their ancestors, standing no more than about 60 cm (two feet) high at the shoulders. In addition to their slender build, presumably adapted for fast running, these miniature camels were also notable for their teeth, which had deep roots and strong grinding surfaces suggesting a tough and abrasive diet.
Another group of cloven-hooved animals surviving from the previous epoch were the protoceratids. There is still debate as to what these actually were; in many respects, their skeletons resemble those of camels, but some features, particularly around the ears, suggest a closer relationship to ruminants such as deer and antelopes. Clearly, they were neither camels nor any living sort of ruminant, but something unique that never left their North America home.
Fairly innocuous to begin with, these animals became more flamboyant as the epoch wore on. The best known example from the Early Miocene is probably Syndyoceras, a deer-sized animal, which, like Cranioceras, had three horns - but in quite a different arrangement. The horns on the forehead curved upwards and inwards in a near-semicircle, while the third one was located near the tip of the snout, and almost immediately branched into two, forming a clear V-shape. Not content with this armament, Syndyoceras also had sharp tusks, although it more likely used them for biting at food than for combat.
Odd though it may have looked to modern eyes, Syndyoceras was clearly successful, because it survived for a long time. Somewhat less fortunate in the long run, because it seems to have left no descendants was its contemporary Paratoceras. A much smaller animal, this had its third horn near the back of the skull, flaring up and out in a short, flattened, V.
All these animals lived alongside one another for millions of years. Sometime around 19 million years ago, however, yet another group of cloven-hooved animal made their first appearance. These were the ancestors of today's pronghorn 'antelope', a fast-running creature commonly seen on the American plains. While they may look like antelopes, however, they are actually more closely related to giraffes (and, in this case, we have the genetic data to prove it), and, given the general absence of giraffes in America, they presumably evolved from something that crossed the land bridge not long before.
The earliest pronghorn we know of from anything more than a bit of jaw and three teeth is Paracosoryx, of the western US and Canada. Although it hadn't yet developed the fast-running trait of its living descendant, in other respects it wasn't so different in appearance. Modern pronghorns differ from true antelopes in that (among other things) their horns are anatomically like antelope horns, but are shed every year, like deer antlers. Also like antlers, they branch, which antelope horns never do, albeit only to form a single, very short prong about half way up. Paracosoryx, however, had long, vertical horns that split rather more visibly near the tip.
Different, certainly, but not really all that weird. In the rather better known Merycodus, however, the horns were shorter, but the branch much larger, giving them a Y-shape that certainly isn't like anything alive today. Ramoceros had multiple branches, so that their horns would have looked just like antlers (albeit likely covered in horny material, rather than simply being bare bone). In Merriamoceros, they were broad and flattened, vaguely resembling the antlers of a moose, but sticking straight upwards, rather than out.
It's plausible that these were used for sexual signalling, with the wide variety of forms dictated by the need to distinguish one species from another. But we don't really know, and at least some of them could also have been used in defence when the need arose. We do know that, like the modern species, they filled a niche similar to that of true antelopes elsewhere in the world, but that even the very first forms already had a significant amount of grass in their diet - something that would stand them in good stead once the climate dried and the prairies appeared.
But that's later on in the Miocene, and I'm not done with the herbivores of the first half yet. Next time, I'll take a look at some pig-like animals, as well as seeing what the horses were up to...
[Photo by James St. John, from Wikimedia Commons.]