Sunday, 21 May 2017
Secret Origins of the First Hippos
Still, while whales and dolphins may be their closest living relatives, the latter have been around for a very long time, and it follows that the hippo lineage must have been around equally long. So, especially given that they aren't exactly small and easy to overlook, it's reasonable to expect that there should be a number of fossil species that are a good deal closer to living hippos than anything we have today.
And, indeed, there are.
Some of these, of course, are extremely close, and relatively recent, relatives of the two living species. There's the stalk-eyed Hippopotamus gorgops and the narrow-snouted Archaeopotamus from the Pliocene, and a number of Ice Age animals from the tiny Maltese hippo to the huge Hippopotamus major. Going slightly further back, there's Kenyapotamus, which lived in Africa during the mid to late Miocene. Fossils of Kenyapotamus are, however, incomplete, consisting of little more than a few bits of jaw, and, moreover, they only go back to around 16 million years ago... whereas the history of whales goes back over three times that far.
So where did the hippos come from, and what were the ancestors like during the long gap between the oldest hippos we know of, and the much earlier point in time in which they must actually have originated? Presumably, they weren't so much like living hippos as to be unambiguously identified as such, so we have to cast around looking for animals that are kind of similar and that lived around the right time in the right sort of place.
One candidate has been suggested to be Morotochoerus, a pig-like animal living in Africa during the early Miocene. This is not uncontroversial, but even if it is true, it still only gets us back to 21 million years ago, which isn't really that much further. It seems plausible therefore, that hippos, as we now understand them, may have originated around this time, but it doesn't answer the question of where they came from before that.
Some researchers have argued that we really don't have any fossils from this particular missing link, and that the mystery is, so far, unanswered. However, a more widely held view relates the hippos to a group of animals called the anthracotheres. The name for these animals literally means "coal-beast", because of the nature of the rock deposits in which their fossils were first found (and they are not to be confused with the anthracobunids, whose name means broadly the same thing, and for exactly the same reason). Physically, they would have looked rather like small skinny hippos, although, since that also means they would have looked quite a lot like pigs, of itself it isn't really proof of anything.
However, at least one recent discovery does support the theory that hippos evolved from anthracotheres, and even suggests where their closest relatives might have fit within that group. This, it should be noted, means that the anthracotheres are not a natural evolutionary group, because some of them would have to be more closely related to hippos than they are to others of their own kind. In the technical jargon, the anthracotheres are therefore "paraphyletic", arbitrarily excluding some of their descendants. (Or, alternatively, hippos are anthracotheres, but nobody seems willing to go that far).
How far back can we take this? While the anthracotheres didn't finally die out until around 3 million years ago, their lineage extends much further back than that of true hippos, and bridges much of the gap between them and the whales. It doesn't quite go back all the way, to be sure, since even the first anthracotheres we know of didn't live as far back as the earliest known whales did, but it's still quite substantial. And we do know of several different anthracothere species - rather more than we know of for fossil hippos, in fact.
The most primitive of all anthracotheres is probably an animal called Siamotherium, which lived in Southeast Asia during the Eocene epoch. So primitive is it, in fact, that it has been claimed that it's not really an anthracothere at all, but rather a sort of primitive pig. We know of two species, one described in 1988 from Thailand ("Siam"), and another from Myanmar, first described in 2000.
The latter species, S. pondaungensis, is the older of the two, and, while it's presumably not a direct ancestor of hippos, it does take their lineage back to 41 million years ago - about three quarters of the way to the oldest known whales. Given its age, and the number of more recent anthracotheres that lived in the same general area, it suggests that that these almost-hippos originated in southern Asia long before some of them headed over to Africa. Unfortunately, all we have of it is some small bits of jaw and a couple of teeth, which is precisely why its identity as an anthracothere has been questioned - there really isn't a lot to go on.
Until recently, that is. Because, in March this year, the first description of a nearly complete skull belonging to the animal was published. The teeth of this new fossil are essentially identical to those previously described for the species, and we now know enough about the head they were attached to say that, yes, it does seem to be an anthracothere, and not a pig. We can also say that, whether or not it's literally an ancestor of the later forms (and it probably isn't, what with the fossil record being as incomplete as it is) it is, indeed, the most primitive such animal we know of. Primitive enough, in fact, that it can't be placed within any of the known anthracothere subfamilies, presumably having evolved before they arose.
Furthermore, now that we have a better idea of what it looked like, we can say a little more about what sort of animal this early almost-hippo was, and how it lived. The skull shows attachments and space for powerful jaw muscles, suggesting that it was capable of chewing up tough food. The generally rounded shape of the teeth (in particular, those beyond the two we already knew the shape of) implies that it was at least capable of eating hard, crunchy, meals, while the shape of the skull, with it's eyes being relatively far forward, also doesn't suggest something that fed solely on plant matter. The best bet, according to the authors of the new study is that it was at least partially omnivorous, eating some invertebrates and perhaps scavenging on carcasses, as well as eating a mixture of leaves, nuts, and softer fruit.
The rest of the body remains a mystery, since we only have the skull. However, we do have further bones from its apparently very close (if slightly more recent) relative from Thailand, and, if it was anything like that, there's no reason at all to suppose it would have been at all amphibious. What we know of the climate and terrain of Southeast Asia 40-odd million years ago suggests that the habitat in which the animals lived was open woodland, with marshy floodplains, heavy monsoons, and a long dry season.
So this, perhaps the earliest known relative of today's lumbering river-loving hippos, was a woodland omnivore not much bigger than a domestic cat, that might not have minded marshland, but would really rather have stayed on dry land. It's a long way from it's few living kin... or from whales, come to that.
[Photo by Nevit Dilmen, from Wikimedia Commons.]