While there was nothing similar in the south, there had been plenty of cat-like animals on the northern continent before. True, they had not literally been cats, but rather nimravids, a group of remarkably cat-like animals that lived long before actual cats existed. The climatic changes of the Early Miocene seem not have suited them, however, and almost died out as the new epoch dawned. It's possible that one, Dinictis, a bobcat-sized sabretooth, did manage to struggle on for a few million years, but even that is debatable. But, otherwise, while their relatives, the barbourofelids, continued on in Europe, to eventually be replaced by actual cats, in North America... there was nothing.
This was the North American "Cat Gap", a time when there was nothing particularly cat-like on the continent at all, and it may have lasted for over seven million years - a substantial chunk of time indeed.
Of course, there were plenty of other mammalian carnivores to take up the slack. Some were relatively small. Oligobunis and Zodiolestes, for instance, belonged to an extinct branch of the weasel family, the Mustelidae, and were no larger than modern badgers at best. The latter, known from Nebraska and Florida, was evidently reasonably skilled at digging, since one was found half-way down a fossilised beaver burrow, evidently having died while searching for a meal.
While the weasel family has a long history in North America, two other kinds of small carnivore that are now unique to the Americas first appeared in Europe, and only reached the west during the Middle Miocene, around 16 million years ago - after which, they eventually died out back in their place of origin. One of these groups were the skunks, whose fossil record remains largely obscure, but the other is somewhat better known.
The first members of the raccoon family to reach North America included Probassariscus, a primitive small-bodied animal that somewhat resembled the tree-climbing cacomistles of modern Central America. It lived from at least Nevada to Texas, but whether it is genuinely closer to the modern cacomistles than to true raccoons is still unclear. Other early members of the family to appear around this time include Arctonasua, which somewhat resembled the living coatis, but probably has no close living relatives.
But perhaps the group of carnivores that benefited most from the absence of cats were the dogs. Dogs had originated in North America, and, in fact, wouldn't even begin to leave the continent until much later. Up until the Miocene, the majority of dogs had been relatively large, strongly carnivorous species, and few of these seem to have been much affected by the climatic changes of the time, allowing them to survive for millions of years, right through the Cat Gap. The last of these early dogs, Osbornodon, was particularly large, with teeth strong enough to crack the bones of its victims, and only died out around 15 million years ago.
While these large dogs seem to have been the dominant North American predators of their day, others were forced into the seemingly "lesser" role of small, omnivorous animals, feeding on a mixture of plants and small mammals in the shadow of their larger kin. Cynarctoides is one of the more extreme examples of these kinds of dog, being about the size of a chihuahua, and likely having a diet closer to that of a modern raccoon than any kind of living dog
At the beginning of the Miocene, this was joined by Desmocyon, a noticeably larger species (about the size of a Norfolk terrier) that lived across much of the western and southern United States. While this was likely still omnivorous, including a fair amount of plant matter in its diet, it, and some of its close relatives, had adaptations to its elbows that suggest that when it did hunt animal prey, it did so by pouncing on them - more like a modern fox than a fast-running wolf.
The absence of cats, which are themselves pouncing ambush predators, may have helped spur this particular direction in evolution, there being no competition from anything that was already better at it. Around the middle of the Cat Gap, Desmocyon, or something closely related to it, gave rise to a whole host of new dog species that spread across the continent.
Some of these were similar to their forebears, retaining their largely omnivorous diet. Examples here include Cynarctus, another terrier-sized animal, with fossils known from California across to Maryland. This makes it one of only a relatively small number of fossil dogs known from the eastern seaboard, and imply that it was better adapted to forest living than its contemporaries. Its teeth were remarkably weak by the standards of dogs, with the flesh-tearing carnassials smaller and wider than usual - likely reflecting a far less meaty diet than living species possess.
Others, however, shifted back to a more predatory lifestyle. These more numerous species also began to become somewhat larger, although the largest, Tephrocyon, was still only around fox-sized. While it was likely at least partially omnivorous, some of its relatives, such as Psalidocyon, seem to have had a more purely meat-based diet. While it's difficult to know, it does, however, seem unlikely that they hunted in packs, since they lacked the adaptations that modern wolves have for cooperative chasing of prey.
The Cat Gap came to an end around 16 million years ago with the arrival in North America of Pseudaelurus, a species that had first appeared in Europe. At around the same time, however, as the world, North America included, reached the height of Miocene warm spell, the dogs underwent another burst of evolution. Among the first examples of this new kind of dog was Tomarctus, which at first, wasn't really all that different from the moderately carnivorous forms that had preceded it.
But it didn't last long, being rapidly replaced by what's likely its own descendant, Aelurodon. This was a much larger animal, at least coyote-sized, with large, sharp teeth that had clearly evolved for tearing meat. While its relatively short legs suggest that it had not evolved the chasing habits of modern wolves, and may well not have been a pack hunter (although this has been questioned) it would certainly have been an effective predator, taking down relatively large, living, prey, rather than merely scavenging. Later species in the genus became even larger, with strong jaw muscles and a heavy head somewhat reminiscent of that of a hyena, or a more muscular version of the highly predatory African wild dogs.
Remarkably, we even have what may be the penis-bone from one of these dogs. Since this isn't the sort of bone that we'd expect to be attached to the rest of a skeleton anyway, this is perhaps hard to prove... but, given its shape, it doesn't seem to belong to any other known group of animals around at the time, and, at 10cm long, if it belonged to any contemporary dog other than Aelurodon, that dog was remarkably well-endowed.
At any rate, it is likely the success of this animal - six different species are currently recognised - that spelled the end for Osbornodon not long after. At this point, the long lineage that had begun with the likes of Desmodon and Cynarctoides so long before had finally become the dominant group, shifting from omnivory to a generalist predatory lifestyle, and growing to the point that they eventually supplanted what had once been the top carnivores of the continent.
But, for all the importance of the dogs to the history of mammalian carnivores in Miocene North America, they were never alone, even if we pass over the smaller weasels and skunks. Bears, too, have a long history on the continent, and it is to them, and their relatives, that I will turn next...
[Photo by "Ghedoghedo", from Wikimedia Commons.]