How diverse that was depends on how broadly you interpret the word "rhino" when fitting it to formal scientific classifications. Even if we take the narrowest definition, considering only animals descended from the last common ancestor of the living animals, you can still add over two dozen species to the tally, although obviously, they weren't all alive at the same time. Adding in all the "rhinoceratoids" - anything more closely related to a rhino than to any other living animal - obviously gets you a great many more, although many of them didn't look much like the creatures we'd recognise.
A more middle-of-the-road view would consider just the animals that scientists, based on an admittedly arbitrary dividing line, place in the official "rhino family", the Rhinocerotidae. This lets us include such creatures as the giant elasmotheres, often assumed to have had a truly gigantic horn. But it also includes the aceratheres, a group whose name literally means "hornless beasts" and which therefore, for the sake of simplicity, are often referred to as hornless rhinos.
Rhino horns, not being made of bone, don't normally fossilise, so there's inevitably some degree of interpretation here - and it's why I said "often assumed to..." when I mentioned the elasmotheres, since we don't actually know how long their horns really were. But what we can say is that the hornless rhinos lack the features on the snout that are associated with the base of the horn in living species, so it seems a safe bet in their case. The group was clearly successful, lasting for around 30 million years, with the last known examples dying out at the end of the Pliocene, not long before the first of the Ice Ages began.
Rhinos of all kinds, the hornless sort included, were particularly diverse during the Late Miocene. In many places, we find several species apparently living alongside one another at the same time. This is certainly plausible, since black and white rhinos both live in South Africa today, but how true it is in every instance depends a lot on how good we are at identifying species.
Individual species can be difficult to distinguish from fossils alone, and the majority of sources outside the depths of the technical literature resort to naming fossil animals only by their genus - Triceratops, Smilodon, Archaeopteryx, and so on. For living species, this would be odd, since we'd be describing (say) lions, tigers, leopards, and jaguars as if they were all one thing - Panthera. For many fossil species, however, this can be perfectly reasonable, since only one species has ever been named. But where that isn't so, since the species will be, by definition, extremely similar, there can be disputes as to exactly how many we're talking about.
A particularly rich Late Miocene fossil site lies on the Greek island of Samos in the Aegean, just 1.5 km (one mile) off the coast of the Turkish mainland. Among many other animals, the site has been the source of no fewer than four named fossil species of horned rhino. But one could legitimately ask just how distinct those really are. After all, none of them come from exceptionally complete fossils and one could always argue that, for example, apparent differences might be due to some other factor such as the age or sex of the animal, or the exact nature of any damage to the skeleton over the millions of years since it died.
The first description of mammalian fossils from Samos was published by Charles Forsyth-Major in 1884, when the island was still part of the Ottoman Empire. In 1905 M. Weber described the first hornless rhinos from the site, naming two new species of Aceratherium, a genus previously described for fossils in Germany. In 1921, however, by which time the island had been handed over to Greece, Julius Andrée named two more from the same site, bringing the total to four.
It has later become apparent that Aceratherium was just a dumping ground for animals that weren't necessarily all that similar beyond being large, hornless, rhinos and in 1975, all four of the Samos species were shifted to Chilotherium, a genus known to also have species native to China, India, Kazakhstan, and Ukraine. But four species seems rather a lot to all have existed in one relatively small locality, so are they all really distinct?
When a new fossil species is named you naturally have to point to a particular fossil specimen that you think is distinct from previously known species and describe it in detail. If anyone later questions whether it really is different from something else, all they have to do is look at the fossil you used in your description and compare it to the original fossil of whatever other animal they are thinking of. All very simple in theory, but there can be complications.
The main complication in this specific instance is that all four fossils used to name the Samos hornless rhinos were stored in the Bavarian Palaeontology Museum in Munich. Which was hit by a bomb in World War II, destroying them all.
So, strictly speaking, we'll never know. Theoretically, each of the fossils could belong to some animal that has never been seen again and all four species could be valid, or they could all be examples of the same thing... or somewhere in between. There is, however, a recognised way to at least reduce the scope of this problem; we take a look at other specimens since assigned to the original species, designate one of those as our new defining sample (technically a "neotype") and use that as a point of comparison instead.
This has now been done, in a study that reduced the original four species to two but, at the same time, determined that one of them was distinct enough to be given its own genus, Eochilotherium. Some of the differences between the two include a wider head and a different curve to the forehead, but the most visible are perhaps the teeth at the front of the lower jaw. In Chilotherium proper these are huge tusks projecting forwards, possibly larger in the males, but they are smaller, flatter and less conspicuous in the newly named genus. This is a primitive feature, linking Eochilotherium with hornless rhinos from China, while the other Samos species seems very closely related to one in Ukraine.
We know that the Samos deposits span a time scale of at least 9 to 5 million years ago, but it's not clear exactly where within the rock strata the relevant fossils were found. So it's possible that the two species we are now left with did not, in fact, live at the same time; they could have been at least a couple of million years apart. The more primitive one could have wandered across from China while a second population headed through Kazakhstan and/or India (we have fossils from both countries) to reach Ukraine, and there developed into a more "advanced" form with larger tusks and more complex grinding teeth suitable for heavy grazing. That could have then reached Samos, giving rise to the local species and supplanting the older one that had got there first.
[Photo by "Iaikayiu" from Wikimedia Commons.]
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