Sunday 9 August 2020

Miocene (Pt 21): Mongooses and More

A modern mongoose
When Africa, moving north, collided with Asia around 19 million years ago, a number of hoofed animals, and other large herbivores, headed south to colonise the continent. Naturally, where herbivores go, predators follow, so an influx of carnivorous animals occurred at around the same time.

Until this time, Africa had been an island continent, separated from the larger northern landmasses and lacked many of the kinds of carnivore we are familiar with today. It had, for example, no cats, dogs, or bears, or even such quintessentially "African" animals as hyenas. All of these, and others besides, belong to the group of mammals known as "carnivorans", which essentially includes all large land-dwelling mammalian carnivores today, apart from the Tasmanian devil. They had evolved in the north, and the new land bridge gave them their first chance to reach the more southerly continent.

All the evidence suggests that they did so very quickly. In at least some cases, that might be because they brought new hunting techniques that native animals in the south had not had time to evolved defences against. Cat-like animals may have been a good example, being fast, nimble, and, even at this early date in their history, highly efficient killing machines. Even if that's true, though (and many of the older African predators in their size are known from fragmentary remains that make it difficult to judge how effective they would have been) it's not clear how many of these animals really were cats as we know them today.

The earliest examples are a couple of animals known as Afrosmilus and Ginsburgsmilus. These were closely related, and their oldest fossils date from not long after the merger of the continents, suggesting that their ancestors must have been relatively recent immigrants. Unfortunately, their fossils are rather fragmentary, which makes it difficult to know too much about them. They were around the size of an ocelot, which is considerably bigger than a housecat, but nothing exceptional by feline standards. Significantly, both of them had enlarged canine teeth showing that they were evolving towards the classic sabretooth pattern.

Their other teeth, however, were primitive, similar to those seen in the early cats of Europe and Asia. Other details of their skull, however, show that they belonged to an extinct group of animals called the barbourofelids. Later examples of this group had particularly impressive sabretooth adaptations, but they represented a different line than the more familiar sabretooth cats such as Smilodon. Instead, they represent a parallel branch, now thought different enough to represent their own family, although they were more closely related to the "true" cats than to anything else alive today. 

In fact, Ginsburgsmilus is both the oldest and most primitive known member of this group, suggesting that they must have originated in Africa, later using the land bridge to spread out in the other direction. where later examples are known right across to North America. Since there isn't anything else in Africa that we know of that it could have evolved from, however, it presumably did so from some as-yet-unknown cat-like ancestor that crossed over from Eurasia not long before it lived.

The reason much of this is unclear is a dearth of high-quality fossils. While it's probably true that fossil sites in Africa have been less extensively surveyed than those elsewhere, this is by no means a problem unique to the continent. It's simply an issue of relatively small, perhaps not especially common, animals not leaving many remains that have survived intact for quite such a long period of time. We thus know relatively little about the early history of other cat-sized (or smaller) carnivores, even in Europe, never mind Africa.

For instance, it seems plausible that the ancestors of mongooses entered Africa during the Early Miocene, but we don't know quite how early. For a while, the oldest known mongoose was thought to be Herpestides, which was one of the earliest carnivores to make the crossing to Africa, and is also known from some European deposits that slightly predate the collision of the continents. It's now less clear that Herpestides really was a mongoose, although, whatever it was, it was certainly something very similar and probably quite closely related. For example, it might be an early kind of civet, living not long after the civets and mongooses had evolved into separate families.

But, if it wasn't a mongoose, there are plenty of other candidate fossils of animals that might have been; Leptoplesictis, for example, has been cited as the earliest known member of the family, and it, too is known from both Europe and North Africa, suggesting an early migration. On the other hand, there are other civet-like animals as well, with Orangictis from Namibia being a plausible early member of that group. The tree-dwelling Kanuites from Kenya has also been identified as an early palm civet.

Mustelids, members of the weasel family, have a similar problem. They may well have reached Africa relatively early on, but most of the fossils aren't complete enough to know. When it was first discovered in 1987, Kenyalutra was thought to be an otter, just the kind of animal that could cross over into Africa even before the land bridge was fully formed. More complete fossils, however, led to the conclusion that it wasn't anything distinct, and was instead an example of the previously known species Kelba.

Which didn't help all that much, because we're not really sure what Kelba was, either. It seems to have looked something like a dog-sized mongoose, although all we have is part of the skull, so the full proportions are hard to judge. Apart from otters, fossils now assigned to the genus have been identified as civets, shrews (yes, really... that would be a big shrew) and other more obscure groups. It's still a bit of a mystery today, although there's now thought to be a decent likelihood that it's not a carnivoran at all, but an odd-looking descendant of the ancient predators that had existed in Africa before the cats and their kin arrived.

You might not think of hyenas as looking much like mongooses, but at this point in their evolutionary history, they still did, and many of them probably still lived up trees. They were relatively late arrivals in Africa, perhaps arriving a good four or five million years after the landbridge formed, but that's hardly surprising, since the group itself seems to have evolved, in Europe, not that much earlier. Indeed, the earliest African forms closely resembled the first European hyenas, being not much larger than cats and being omnivorous, probably with an emphasis on insects and other small prey. While there do seem to be some uniquely African species, they are similar enough to the European forms to be placed in the same genera, with Protictitherium being a notable example.

Another early example of this kind of animal in Africa was Moghradictis, known from Early Miocene deposits in Egypt, and possibly further afield. It can be distinguished primarily on the unusual shape of one particular tooth in the upper jaw, and it's at least possible that other, fragmentary fossils that happen to be missing that tooth are actually the same thing. That cuts both ways, of course, and Moghradictis was originally thought to be another example of Herpestides.

While that doesn't seem to be the case any more, we're still left with another animal that clearly belongs somewhere in this general part of the family tree, and that, in life probably looked at least vaguely like a mongoose. (Although how much like one is hard to say, given how little of it we have). At times it has been considered to be an early insect-eating hyena, a member of the civet family, or an early example of the "false hyenas", a parallel line of bone-cracking scavengers that were more prominent in the Late Miocene. 

That far back in time, a close similarity between the ancestors of false hyenas and the true sort is quite plausible, but most analyses these days prefer to consider Moghradictis as something else again. Instead, it is placed in a family of animals that were more prominent during the earlier, Oligocene epoch, representing perhaps, some late survivor that left no descendants. But perhaps all we can say for sure is that it belongs somewhere near the base of the hyena/mongoose/civet family tree... one that's complicated enough when we just look at the modern, living species.

These small to medium, often predatory, but often omnivorous animals, were not, of course, the dominant carnivores of Africa in the Early Miocene, in the way that lions and spotted hyenas are today. But such large, and often fearsome, predators did exist. Some were newcomers from Asia, but others were survivors of the mammalian carnivores that had lived on the continent when it was still an island, and which lingered there rather longer than their counterparts had elsewhere. Next time, I'll take a look at some of those.

[Photo by "Shandris" from Wikimedia Commons.]

No comments:

Post a Comment