|Indian rhino (Rhinoceros unicornis)|
How many different kinds there were depends, in part, on what you consider to be a rhinoceros. Today, there isn't much else that's like a rhino. Their closest living relatives are the tapirs, but, while there are a number of anatomical similarities (a similar digestive system, placing most of their weight on their middle toe, and so on), you'd never confuse one for the other. Indeed, tapirs and rhinos are not particularly close, probably having last had a common ancestor in the early Eocene, almost at the beginning of the Age of Mammals.
Since that time, the evolutionary line that led to modern rhinos has developed on its own, and into a much wider range of forms than the modern species might suggest. It's generally agreed now that there were at least three different families of rhinoceros-like animal; one of them long-legged and relatively swift, one semi-aquatic with a vague resemblance to hippos, and one that contains the "true", living, rhinos.
Even if we consider only the rhinoceros family itself, there have still been a considerable number of species over the millions of years of their existence. All would have been recognisable to modern eyes as being rhinos, but there was still greater variety than we see among those alive today. The woolly rhinoceros of Ice Age Siberia, for example, had a thick coat of fur, while most (but not all) of the living species, being large tropical animals, are essentially hairless.
Another remarkable rhino from around the same time, and probably also quite hairy, was Elasmotherium. Even by the standards of rhinos, Elasmotherium was big - almost the size of an elephant. It's horn was enormous, perhaps six feet in length, and with an incredibly broad base. Unlike the horn of living rhinos, it rose from the animal's forehead, not its snout. Debate as to whether Elasmotherium was different enough from living species to constitute a different subfamily, or only some narrower group, continues, but, either way, it was certainly not alone - there were other, lesser known, "elasmotheres".
One of these elasmotherine rhinos is Hispanotherium. As the name implies, the first fossil of this animal was discovered in Spain, although we now know that it also lived across Asia, with fossils known from Turkey, Pakistan, and northern China. Most of the fossils date from the middle Miocene, which makes it much older than Elasmotherium itself. When it was first discovered, back in 1864, it was thought to be a close relative of the Indian rhino, but those remains consisted only of a few teeth, and, when it became clear that it was something different, it was given its current name in 1947. We know of at least three different species, with H. matritense being the most common.
Hispanotherium Elasmotherium, Diceratherium,
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The group shown here as "Elasmotherium, etc." includes various forms, largely living later than Hispanotherium. Diceratherium, on the other hand, lived earlier, as far back as the Oligocene, and is one of a number of early elasmotheres - almost certainly with more branches and a more complex history than shown here.
Numerous fossils of H. matritense have been discovered from Spain, indicating that it must once have been a common animal in that part of Europe. Most, however, are not very complete, and that limits our knowledge of what the animal was actually like. In 2007, a particularly rich fossil bed, estimated to be fifteen million years old, was discovered just outside Madrid during the construction of a new transport interchange. Among the fossils was one that included, for the first time, a complete skull. That skull, among others, has now been described by Oscar Sanisidro and co-workers, of the National Museum of Natural Sciences in Madrid.
Sinotherium, had enormous forehead horns, earlier elasmotheres did not. Diceratherium, for example, had two small horns on its nose. Earlier descriptions of Hispanotherium indicated that it, too, had a nasal horn, albeit a single one, much like that of, for example, an Indian rhino.
However, rhino horns are not, strictly speaking, horns at all. A "true" horn, as found on cattle, goats, and so on, has a bony core surrounded by a sheath of, well... horn. The horny material doesn't tend to survive fossilisation, so it can be difficult to tell the true size of a horn from, say, an extinct antelope, but the bony core does, and that usually at least gives us a good clue. A rhinoceros horn, on the other hand, is entirely composed of a compacted hair-like material, and includes no bone at all. As a result, the "horn" just rots away when the animal dies, and is not preserved in fossils.
Fortunately, however, the bone underneath the horn in living rhinos is thick, rugged, and irregular. It's by looking for that irregularity that we know that Elasmotherium, for example, had the horn that it did. Obviously, you have to be careful, because fossils weather just like any rocks do, and they aren't always that well preserved in the first place. But, if you have a skull in good condition, it should be possible to figure out where the horn was and, at least, how broad the base of it was.
When the researchers looked for the signs of a horn on their new specimen they found - nothing. The fossil is well enough preserved to confirm that both the forehead and the snout of the animal had smooth bone, and that, however much it may have looked like a rhino in other ways, their specimen of H. matritense was completely hornless. Indeed, two other partial skulls with intact snouts had no signs of any horn, either. Neither does the only previous fossil of this species to preserve the relevant area.
So why did we ever think it did? That's in part due to another species of Hispanotherium, H. tungurense, from northern China. That specimen had clear signs of a horn, and it seemed reasonable to suppose that its closest relatives had one, too. A species unearthed in France, H. beonense, seems to have some faint signs of a horn on some specimens... but not on all of them. That raises a third possibility: perhaps the males had horns, but the females didn't? (Or the other way round, of course, although examination of various kinds of living mammal would suggest that the former is much more likely).
It seems unlikely that, when you only have four fossils showing the nose of an animal, that they'd all just happen to female. Not impossible, by any means, but the odds are against it, which suggests that the Spanish species was genuinely hornless. The authors suggest that the European forms separated from their Asian ancestors during a time of global cooling, and consequent disappearance of subtropical forests from the region, and partly or completely lost their horns in the process. If so, they presumably headed back east again later, since there are fossils of H. matritense from Inner Mongolia. But, either way, it seems that, regardless of their importance to living species, even in relatively recent geological times, rhinos haven't always needed horns.
[Picture by Krish Dulal from Wikimedia Commons. Cladogram adapted from Sanisidro et al, 2012]