|The largest predator in Africa today|
For one thing, the cats, hyenas, and so forth, were not the only carnivorous mammals to have made the crossing from Asia. The animals known as bear-dogs, or more technically, amphicyonids, are far better known from Europe, Asia, and North America, but they did reach Africa, too.
I've mentioned bear-dogs a number of times before; they are, in essence, the "lost" family of dog-like carnivores, resembling both bears and dogs, yet distinct from both. Quite how different the bear-dogs of Africa were from their better-known kin elsewhere in the world is arguable. It is clear that they represented uniquely native species, but these days, they are not always assigned to their own genera. Of course, this could still mean that they differed from their northern kin just as much as, say, lions do from tigers, or brown bears from black bears. Indeed, while the differences may look subtle to us when all we have to go on are fragmentary fossils, if we could see the animals in life, it might have been rather easier to distinguish them.
Be that as it may, the most widespread and common bear-dog in Early Miocene Africa appears to have been Cynelos (some of the African forms are often regarded as distinct enough to be placed in their own genus, Hecubides, but this is far from universal). This would have looked quite like a bear, but at 50-60 cm (20 to 24 inches) at the shoulder was noticeably smaller than any living species of those animals. It is known from places as far apart as Kenya, Egypt, and Namibia, and so plausibly lived in many other parts of the continent too, where fossil digs are less common.
The various species of Cynelos would have been, so far as we can tell, particularly predatory although their build suggests that they were more likely ambush hunters than animals that relied on a prolonged chase. The same can be said for Amphicyon, a type of bear-dog that seems to have been relatively common elsewhere in the world; there is some suggestion that the African forms may have been more suited to scavenging, although clearly not to the same extent as a modern hyena.
Afrocyon is rather more obscure, and would qualify as a uniquely African genus if it isn't simply rolled in with one of the more widespread ones (as it often is). Like Cynelos, it seems to have been a pure predator, and it was rather larger and so presumably quite fearsome.
This diversity seems to have died away in the Middle Miocene, after around 16 million years ago, leaving only two genera: Agnotherium and Myacyon. The latter is often included in the former, since the known fossils are little too fragmentary for us to be sure how much difference between the two there really was; if it was distinct, Myacyon would have been purely African and lived from at least Algeria to Kenya. Agnotherium, however, was common in western Europe, with at least one species probably also reaching into Morocco. They were about as deadly as the bear-dogs ever became; being highly carnivorous and weighing something like 275 kg (600 lbs), which is basically the same as a modern grizzly bear.
This has to have been one of the top predators of Middle Miocene Africa, its sheer size dwarfing a modern lion. But, in the Early Miocene, the bear-dogs did not yet have the same advantage. That's because, when the bear-dogs - along with the early cats, mongooses, and so on - entered the continent they found other predators already there, and some of them did not give up their dominant position for a long time.
Perhaps unsurprisingly, yes, there were crocodiles, which had been on the continent for some considerable time. But among the mammals, there were the creodonts.
Creodonts were survivors from an even earlier time. They were not carnivorans, close relatives of the cats, beat-dogs and all the rest that survive today, but represent a separate, older lineage of carnivorous mammal that was eventually replaced by those we are familiar with. It's not even clear that the creodonts as a whole were a single evolutionary group, rather than being a collection of early predators that happen to share some similarities, with different studies coming to different opinions on the matter.
Whatever the case for creodonts more widely, those in Africa were native to the continent, having lived there since very early times. By the dawning of the Miocene they were already a diverse group, despite the fact that, by this point, they had entirely died out in Europe and North America and had almost completely vanished from Asia. Examples are known ranging in size from a weasel to... well, larger than a lion.
Not so much is known about the smaller African creodonts of this time, in part because their fossils tend not to be so well preserved. There certainly seem to have been a number of them, and they were presumably related to the larger forms in some way, although quite how closely is unclear. They also seem to have been highly adapted carnivores, perhaps similar to today's polecats in terms of their diet.
The larger forms are better known and, while they don't seem to have been terribly fast running (and neither was anything they might have hunted) they certainly aren't the sort of creature you would have wanted to get into a close-up fight with. Probably the most widespread and numerous was Hyainailouros, while the largest was the very closely related Megistotherium. The former of these was successful enough not only to survive through the Miocene, but also to cross over into Europe, where fossils are known from Switzerland and Portugal.
The skulls of these animals are immense, perhaps twice the size of that of a modern lion. From what we know, however, Hyanailouros would not have been much larger than a lion in general terms, having a proportionately large head with a long jaw full of powerful teeth. In the case of Megistotherium, it's much harder to say, because the remains are incomplete, but its teeth were enormous and some estimates have put its overall body weight at a staggering 880 kg (roughly 1 US ton), over double that of even a large male tiger. Others, however, have argued that the size estimates were misleading and that animals identified as Megistotherium are merely unusually large (or large-toothed) versions of Hyanailouros.
But even it may not have been the largest representative of its kind. That honour may go to Simbakubwa, whose fossils were recovered from Kenyan deposits dating back to the very beginning of the Miocene, around 25 million years ago, and which were described just last year. As I noted when I covered that discovery, it's difficult to know quite how large the animal really was (again, the skull seems to have been disproportionately large compared with the rest of it) but estimates of over a ton are far from unreasonable.
This despite the fact that it seems to have been a fairly pure carnivore (unlike, say, omnivorous bears, which still only reach about 0.7 tons) and, moreover, one that actively killed its victims rather than scavenging on carcasses - although surely it would have done the latter when it could, since it ought to have scared away just about anything that might have tried to object. Indeed, Hyainailorous has been discovered close to the remains of elephant-like gomphotheres that it appears to have killed, so we can imagine that these creatures went after some particularly large prey, and smaller carnivores are unlikely to have frightened it.
Simbakubwa, so far as we know, only lived very early in the Miocene, and Hyainalouros and Megistotherium did not long survive into the Late part of the epoch. The last of them died out around 13 million years ago, and left no descendants. All of the smaller creodonts died out even earlier, leaving these great predators as the last of their kind in Africa. They had, by this point, taken the opportunity to travel north into Eurasia - just as there was nothing like cats or dogs in Africa prior to this time, there wasn't anything quite like the giant creodonts in Asia, either.
It's possible that these Asian descendants slightly outlasted their African ancestors, but the gap between the last known fossils in each continent isn't great. Either way, once they died there were no more creodonts of any kind on Earth. The modern carnivorans, whether through direct competition or simple luck, had won.
Once they had gone, though, what did Late Miocene Africa look like? That is where I will turn next...
[Photo by Winfried Bruenken, from Wikimedia Commons.]