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.