|Brontotherium skull - or should that be Brontotherium prousti?|
A genus is a group of closely related species. There are many genera that only contain one species, but most have a number of different species at any given point in time, and even more when you look over the whole of their evolutionary history. It's as if we thought of the living genus Canis as a single entity, ignoring the fact that it includes wolves, coyotes, and jackals, never mind extinct species such as the dire wolf.
There's a reason for this, of course, which is that it's really tricky to tell actual species apart from their fossils. Taking the traditional definition of a species as something that doesn't cross-breed with other species to produce fertile hybrids (and the modern definition is a bit more complicated than that), how on Earth would you know, if you're only looking at a fossil? Instead, the definition more commonly used for fossil animals is that a species is a group of specimens so similar that there's no way to consistently tell them apart, even when you have a complete skeleton. But most fossils aren't complete, or are at least damaged by several million years of being squished under tons of rock, which makes it very difficult to spot the tell-tale differences between the equivalents of coyotes and jackals.
Not that this stops scientists trying, so, yes, there are, in fact, multiple officially recognised species of (for example) Stegosaurus. But even they tend to use just the genus name when they can - it's easier to tell genera apart than it is for species, so you're on rather firmer ground. Mammals fossil are (on average) younger than those of dinosaurs, and often in rather better condition, plus we have the advantage of being more easily able to compare them with living species. But even so, it can be an issue, and a large number of species are known only from their teeth.
Metarhinus is a genus of brontothere that lived between 46 and 43 million years ago in California, Wyoming, Utah, and Colorado. This is around two-thirds of the way back to the time of the dinosaurs, and many mammals were quite different to those around today. Exactly how the brontotheres were related to living species is unclear; they certainly don't seem to have left any descendants. It is clear that they belonged to an order called the odd-toed ungulates, which includes some of the larger herbivores around today, but it's less obvious where they fit within that order; its currently thought that they are most closely related to horses.
Brontotheres Palaeotheres Horses
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| | | Rhinos, Tapirs
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Although the earliest members of the family were quite small, by the time of Metarhinus, they were big animals, looking rather like a modern rhino. Unlike the rhinos, however, they had relatively weak teeth, which implies that they mostly ate soft vegetation. This is most likely what eventually drove them to extinction; the climate changed around the time they disappeared, and this is also roughly when the rhinos themselves appeared, replacing their earlier counterparts.
A recent paper looked at multiple specimens of Metarhinus to determine just how easy it was to tell the three described species of the genus apart. About the only distinguishing feature between the three species is the shape and size of the nasal bones. In humans, these bones form the bridge of the nose and are rather small, but in most mammals, they're much larger, forming the top of the snout. But even so, and considering that brontotheres were pretty big animals, an immediate problem arises because it turns out that most Metarhinus fossils don't even have any nasal bones. So, for most specimens, you simply can't say what species they belong to, even if you wanted to.
Still, on the positive side, it proved possible to distinguish these fossils from any other sort of brontothere, and they clearly all belonged to the same sort of animal. So, as expected (although this has been disputed in the past), it seems that you can determine the genus of the fossils - it's just narrowing it down further that's the problem.
When the nasal bones were present, however, they really did seem to be different enough to distinguish the three species from one another. But just because the difference is real doesn't necessarily mean that it's anything to do with species. Consider that the species M. abbotti, with a narrow nose, is almost always found close to fossils of M. fluviatilis, with dramatically flaring nasal bones. Now, this could mean that the two species both ate similar foods, and tended to live in the same areas. After all, they really aren't very different in any other way, so it makes sense that they'd live in similar environments. It's not as if there aren't plenty of examples among living species.
On the other hand - how do you know that one isn't the female, and the other the male? Many brontothere species had skin-covered bony lumps on their noses, further increasing their resemblance to rhinos. That's something of a coincidence, since the "horns" of rhinos are composed of hair-like material, not bone, although its possible that they used them for similar purposes. Anyway, the shape of these horns often varies between specimens of what otherwise appear to be the same species, and it's plausible that this reflects male and female animals.
Metarhinus, however, doesn't have any horns, so they might have used the difference in the shape of the nose as a sexual signal instead. It also turns out that M. fluviatilis is smaller than M. abbotti, which, again, might represent a sexual difference. If so, its a somewhat unusual that it's the smaller form that has the more elaborate snout shape, since generally its the big male animals that have the ornate antlers, or whatever. There's also an issue in that, while M. abbotti is usually found with M. fluviatilis, the reverse isn't always true, and a third species, M. pater, sometimes replaces it. Now, it could be that these are also males, and that M. fluviatilis is therefore really two species, not one, but there doesn't seem to be any evidence for that. Which isn't proof, but is at least strongly suggestive.
One thing we can rule out is that the smaller specimens are just younger animals, and that they change shape as they mature - otherwise quite a likely possibility, as the recent furore with Triceratops showed. In this case, however, we can look at how ground down the teeth are by a lifetime of eating vegetation. If the smaller specimens were younger, the teeth would be less worn down, and they don't seem to be, so that's out.
The authors conclude that, in this case, the fossils really can be divided into three species on the basis of their nasal bones, although the differences in size aren't great enough to be useful. If so, then two (or more) species of very similar animal lived side by side. That's certainly possible, and there are examples today, but there must have been some difference in what they ate, or one would surely out-compete the other. Was there such a difference? We don't know.
It's because of these sorts of difficulty that, useful and important though species are, when it comes to fossil species, we tend to stick to the genus name.
[Image from Wikimedia Commons]