The most famous of these sites are those connected with the evolution of our own species, but there are a number of sites that have yielded, for example, significant dinosaur fossils. When it comes to mammals other than humans (and whatever else was living alongside them), Kenya and South Africa have proved particularly rich sources - although, of course, there are others.
The West Coast Fossil Park in South Africa was established in 1998 near the town of Langebaanweg about 150 km (95 miles) north of Cape Town. This was a former phosphate mining town, and before the mines were closed in 1993, the operations had revealed a number of significant fossils of both mammals and birds. Indeed, by some estimates, it's the richest source in the world of fossils of its particular age.
That age is estimated to be 5.2 million years, based on the magnetic signatures in the rocks, among other evidence. This is significant because it's less than 200,000 years after the official dividing line that separates the Miocene epoch from the following Pliocene. As a result, there is something of a mixture of older, Miocene, and more "modern-looking" Pliocene animals among the fossils excavated there, showing a key period of transition.
At the time the deposits were made, the area would have had a warm temperate to subtropical climate and, because the sea level was higher, would have been even closer to the coast than it is now. It seems to have been largely woodland, with some of the key fossil beds being formed from river channels running down to the sea, washing up and depositing the remains of dead animals as they did so.
Among the many fossil mammals found at the site are a number of carnivores, with a range of different diets. These include animals such as bears, hyenas, and (on account of it being the coast) even seals. There are also mustelids.
Mustelids is the technical term for members of the weasel family, of which there quite a few, most of which aren't really "weasels" in everyday parlance. Even so, most modern mustelids are relatively small animals, and none of the African species are any larger than a badger. Some of the examples from Langebaanweg, however, are giants of their kind.
Well, okay, one could argue as to what really constitutes a "giant" anything, but two of the mustelid species recovered from the site are estimated to have weighed over twice as much as the largest mustelids living today. Which is as good a definition as any.
The largest mustelid alive today is a type of otter found in South America, so it's perhaps unsurprising that many of the truly large mustelids of the past were also otters. The South African example has had a somewhat spotted taxonomic history; it's often referred to as a species of Sivaonyx, with close relatives in Kenya, Uganda, and Pakistan, although others argue that it's really just a species of the previously known genus Enhydriodon.
Whichever is the most appropriate name, this is an exceptionally large otter, noted for having particularly heavy and solid teeth that suggest it fed by crunching up something solid. Quite what is hard to say; shellfish and armoured catfishes have been suggested, but it could equally well be the bones of something land-based. Beyond this, it's been difficult to say much about the animal, due to the rarity of finding much of the skeleton other than the jaws. Nonetheless, a recent review of the leg bones of the South African species, S. hendeyi, concludes that it was probably rather more land-based than we might expect for an otter.
In fact, the authors of the study compare it to the modern African clawless otter (Aonyx capensis). This is undeniably a semi-aquatic animal, feeding mainly on crabs and lobsters, but it also spends a lot of time on land and doesn't venture into particularly deep water. Given the limited supply of sufficiently large food in all but the widest of rivers, similar habits might make sense for otters as large as Sivaonyx. Indeed, the closely related species S. beyi, from Chad, has been interpreted as living almost entirely on land, although it could probably swim if it really had to.
The same study also looked at the other "giant" mustelid found at the formation. This is the even larger Plesiogulo, which the authors describe as "leopard-sized". Also known from both Eurasia and North America, the scientific name of this animal translates as something like "almost-wolverine" and, apart from the much larger size, it had a number of features in common with modern wolverines.
Quite what that means for its relationships is less clear. For much of the 20th century it was thought that Plesiogulo might, in fact, be the direct ancestor of modern wolverines, suggesting that they actually became smaller through history. More modern analyses suggest that this isn't the case and that Plesiogulo therefore has no modern descendants, although it's certainly plausible that it was at least a fairly close relative of today's wolverines.
All the fossils of this almost-wolverine, whatever it may actually be, are incomplete and those from South Africa are no exception. One of the individuals was identified primarily from a jaw bone. This showed that it belonged to a juvenile, still in possession of some of its milk teeth; adults had a tooth pattern that looks suitable for cracking bone, if not to the same extent as modern hyenas.
Another included the ulna, one of the bones of what would be the forearm in a human. It's apparently notable for being extremely thick for its length. Taken together with what we know from other fossils, this implies that in life Plesiogulo would have been a muscular, squat animal with considerable body and arm strength. The shape of the bone suggests that it wouldn't have been great at digging, but killing prey would be a different matter. Indeed, the authors argue that the only comparable animal with arms that powerful is the sabretooth cat Smilodon and that Plesiogulo may have therefore had a similar hunting strategy, hiding in wait for its prey and then rapidly overpowering them.
Plesiogulo did not long survive the end of the Miocene, perhaps finding the cooling of the climate reduced its preferred habitat or changed the type of animals it could feed on. Sivaonyx, however did better, and survived for a few more million years, dying out not long before the Ice Ages. But, for a time, both were successful animals, originating further north before they entered Africa, but happily living alongside one another along the same stretch of South African coast.
[Photo by Susanne Nilsson, from Wikimedia Commons.]