|Wild Brazilian guinea pig (Cavia aperea)|
The guinea pig family (Caviidae) as a whole doesn't include many more species, and most of them, such as the rock and mountain cavies, are physically rather similar to their better known kin; exceptions include the large capybaras and the long-legged and oddly rabbit-like maras. In the distant past, many other species existed, some of them even larger than capybaras, and positively gigantic for a rodent. When it comes to the guinea pigs proper, however, we don't really have much of a fossil history, and none of the living species can be definitively traced back any further than the tail end of the Ice Ages.
We do, however, know of a few fossil species of guinea pig. Two of these were discovered in a cave complex in Brazil, and they, too, date from to the last Ice Age, and likely lived alongside some of the first humans to reach the area. The one exception is Cavia galileoi, first described in 2005. This is much older; it lived in central Argentina, and has been dated to around 2.5 million years ago, at the end of the Pliocene. The climate was already beginning to get noticeably cooler and drier at this time as the first of the Ice Ages approached, and this probably had something to do with the arrival of the animal, which, like the modern species, had complex teeth well suited to feeding on tough, grassy, vegetation.
This all fits with what we know of guinea pig evolution from other sources of information, but it can't be the whole story. This is because estimates derived from genetic information, and based on known rates of evolution within the group suggest that guinea pigs must have first appeared at least four million years ago, and probably closer to six. There's nothing particularly unusual in this; it's difficult enough to find fossils in the first place, and the older they are, the harder it is, especially when you're looking for something that's relatively small, rather than some whopping great dinosaur. And, of course, the earliest guinea pigs may not have been very common, so that we don't have this particular "missing link" doesn't really mean much.
Except that now we do.
This new specimen consists of one half of the lower jaw of an ancient guinea pig, discovered in the Andes of northwestern Argentina. This may not sound like much, although it is rather more than one often gets when finding these sorts of fossil. Crucially, because the teeth of guinea pigs are, as I noted above, more complex than those of related species, we do have enough unique features to say that, yes, that's what it is, and the overall shape of the jaw also matches what we'd expect.
There are, however, two crucial differences. Firstly, while the teeth do have some of the unique features used to identify guinea pigs, they don't quite have all of them. This suggests that some of the detailed features of guinea pig teeth had yet to evolve, and places this animal roughly half way between the living species and the late Miocene fossil Palaeocavia, which lived in northeastern Argentina, and was itself more closely related to living guinea pigs than to the likes of mountain cavies. This is exactly what we'd expect of an early, relatively primitive, guinea pig.
The other key difference, however, is the size of this new fossil. Since all guinea pigs are pretty much the same shape, and we have enough of the jaw that this was no different, we can use the dimensions of this fossil to get a reasonable idea of how large the animal was as a whole. When we plug the numbers into an equation that seems to work well for the living species, we discover that this animal should have weighed around 1.2 kg (2 lbs 10 oz.). This is actually around the maximum weight for a healthy domestic guinea pig of the very largest breed, and about twice the weight of the largest living wild species (C. magna). So, yeah, that's big.
The new species has been named Cavia cabrerai, in honour of Spanish zoologist and wild guinea pig expert Ángel Cabrera. But just how old is it, and does it really fill that gap in the fossil record?
As it turns out, the rock layer that the fossil was uncovered from lies between two layers of solidified lava ash, and must therefore have been deposited between the eruptions that produced them. And solidified lava is something we can date. This is because pretty much all lava will contain potassium, a known proportion of which will be in the radioactive form of potassium-40. This decays very slowly to release argon-40 which, being a gas, will escape into the atmosphere - but only so long as the lava is still liquid. So, by comparing the amount of argon-40 in the sample with the amount of regular, non-radioactive, potassium-39 (which, in turn, tells you how much potassium-40 you must have started with), you can work out when the lava set.
In practice, what you're actually doing is a bit more complex than that, requiring calibration samples and bombarding your bit of rock with neutrons. But the basic principle is the same.
At any rate, in this case, a combination of argon-argon dating the ash layers, measuring where the fossil was relative to them, and assuming a constant rate of deposition of the stuff in between, gives us a pretty narrow possible date range for the sample. C. cabrerai, it turns out, lived 4.74 million years ago, during the early Pliocene.
This, you will note, is almost double the age of the previously oldest known guinea pig. The authors who described the new species note that around this time, northern Argentina was becoming drier than it had previously been, with an increase in relatively open woodlands, rather than dense and verdant forests. This wasn't a worldwide phenomenon, unlike the later changes at the end of the Pliocene, but were more likely associated with the final phase of the rise of the local stretch of the Andes, increasing the extent of the rain shadow in that part of the world. (The Andes as a whole, of course, are much older than this, but mountain-building continued until unusually late in northern Argentina/Chile).
Thus, what we may have here is a shift to a drier habitat pushing the evolution of early caviines towards the first true guinea pigs, a process that was completed by a second push as the Ice Ages approached. Either way, this new species is both the earliest and the largest known species of guinea pig, supporting our guesses from the genetic evidence with actual, physical remains.
[Photo by Vince Smith, from Wikimedia Commons.]