Horns of the Thunder Beasts

Megacerops, one of the largest brontotheres

Today, the group of odd-toed ungulates - those large mammalian herbivores that support most of their weight on their middle toes - is represented by just sixteen species in three families: the horses, rhinos, and tapirs. But they were once much more extensive, with whole families of animals that didn't make it through to the present day. One of these was the brontotheres, a family whose name literally means "thunder beasts".

These were big, heavily built animals, some of them standing over six feet tall at the shoulder. In many respects, they would have looked quite like rhinos, having a rather similar body-shape, and crucially, large blunt horns on their snout. However, the horns weren't really so rhino-like as they appear, since the horns of rhinos are actually made of compacted hair, while those of brontotheres were made entirely of bone. The surface of the horns are rugged, suggesting that they were probably covered in skin during life, which perhaps makes the closest living parallel the projections on the heads of modern giraffes.

In the grand scheme of things, the brontotheres didn't last very long, dying out around 34 million years ago, but they must have been very impressive animals when they were alive. Despite their appearance, they seem to have been more related to horses than to rhinos. This is more apparent when you look at the very earliest brontotheres, which were relatively small and hornless, and do look rather more like the early multi-toed ancestors of the horses.

In January, palaeontologist Matthew Mihlbachler described a new species of brontothere, Diplacodon gigan.  We already knew about Diplacodon, since it was first described in the 19th century; it was a relatively large brontothere distinguished from its relatives by the upturned curves on the edges of its horns. This new species is known from a single skull discovered in Wyoming, and, judging from the state of its teeth, it was probably quite old when it died. Perhaps the most obvious way to tell this new one apart from the previously known species is that it's a lot bigger - the skull was 72 cm (28 inches) long, which is about 20% larger than the largest known skull from the genus so far. If the rest of the animal scaled up accordingly, it might well have been seven feet or so high at the shoulder, making it far larger than any living rhino.

Incidentally, the gigan part of the scientific name of the new fossil does, as you might expect, reflects its size... but it's not some Latin word for 'gigantic'. Instead, the author named it after this. Which certainly does look quite large.

I've mentioned the size of fossil species before, and how two fossils of the same species might be quite different sizes because, for example, one happens to be younger than the other. But its not just the size of fossils that can change within the same species, it can also be the shape. Consider the red deer. Not only are stags much larger than does, they also have magnificent antlers, making them look quite different. We see such variation in many mammal species, and they are often particularly dramatic in the big herbivores, where its frequently only the males that have horns, antlers, or other highly visible adornments.

So how do we tell that what look like two different species aren't really just males and females of the same species? Ideally, you want to be able to say that almost all the features of the two fossils are the same, except for one, which just happens to always have one of two distinct shapes. Its not always going to be that easy, and you do need quite a few fossils to be able to make your case, but the principle is generally a good one.

In the case of brontotheres, we already have pretty good evidence that the males and females looked different. Indeed, we already have a number of apparent males and females for the known species of Diplacodon, so the status of this new specimen as something new is probably safe on that front. There's no absolute way to know which is which, but comparison with related living species suggests that the larger, more heavily built animals with the powerful horns were males, and the smaller, more slender ones, were female. If this is right then, as I mentioned recently with guinea pigs, it probably means that it was the males that dominated sexually. Since other evidence suggests that brontotheres lived in herds, we can imagine that large alpha males had harems, driving away their younger rivals.

That we have relatively large numbers of male and female brontothere skeletons makes it possible to tell other things about how they lived, as well. Its not just the horns that vary between the two sexes, but also the canine teeth, which are much larger in the presumed 'male' specimens. While the earliest brontotheres had few distinguishing features, and little apparent difference between the sexes, it seems that larger canine teeth in the 'males' evolved before the nose-horns did.

Male and female musk deer

There's an obvious parallel here with deer. We're familiar of course, with medium to large deer, where the males all have antlers, and the females don't (reindeer are unusual, in that the females also have antlers). But smaller deer, such as muntjacs, have only tiny antlers, or none at all, and antlers are also absent in musk deer and chevrotains, both of which are thought to be more like the ancestors that deer originally evolved from. In all of these species, the males have relatively large, sabre-like canines that are used in competition with rivals. In deer, therefore, it looks likely that such sharp teeth appeared before the antlers did, and exactly the same thing seems to have happened with the brontotheres and their nose-horns.

In deer, the variation in the size of the teeth went away as antlers evolved, and some deer don't even have upper canines any more, perhaps finding them unnecessary for their vegetarian diet. Brontotheres were also vegetarian, browsing on leaves and shrubs, and while most of them seemed to have kept their canines, they are greatly reduced in genera such as Embolotherium, which has amongst the most spectacular horns of its family. The others perhaps used both horns and teeth in contests - something that rhinos still do today.

There is, however, undoubtedly more to be learned about this fascinating group of long-dead mammals.

[Pictures from Wikimedia Commons]