Miocene (Pt 6): The Coming of the Mice

Palaeotragus

The Early and Mid parts of the Miocene epoch were, for the most part, times when the world was much warmer than it is today. It wasn't a steady pattern, however, and I've already described how the fluctuations in climate, over the course of many millions of years, affected the rodents of Europe. It was a time when the most common small mammals in Europe were not mice and voles, but dormice, accompanied by early hamsters, squirrels, and the gliding eomyids.

By 10 million years ago, however, the colder, drier climate had become locked in for the long term. We know that the forests of Europe changed dramatically at this time, the old subtropical trees, such as figs and palms, being replaced by oak, alder, and elm. Likely as a result of this change in the available food supply, most of the dormice died out, leaving only a few close relatives of the relatively small number of species we have today.

Hamsters survived by switching to a drier diet, and it's at about this time that they really start resembling the ones we have in pet shops today. Replacing them across much of Europe, and taking over the dominant position formerly held by the dormice, true mice and rats entered the continent from the east, where they had first evolved not long before, probably in the tropics of southern Asia.

The first mice probably rather resembled the hamsters of the day, and were likely suited to eating soft leaves and the like. By the time they reached Europe they had become much more adaptable, and prior to the arrival of the first voles during the Pliocene, held an essentially unchallenged leading position among the small-mammal fauna.

Or, at least they did on the continent. On islands off the coast, some of the older groups were able to hang on. With sea levels still higher than they are today, some of these places are no longer islands, but were, at the time, isolated enough from the mainland to develop their own kinds of animal. Freed from the need to hide from predators, some of the smaller mammals of these islands became much larger, and we know of giant dormice from both Tuscany and Gargano (the "spur" of Italy's boot), with the largest, Stertomys laticrestatus, weighing, by some estimates, over 1 kg (2 lb 3 oz.).

Gargano, at the time probably a chain of small islands, was also home to the gigantic "hedgehog" Deinogalerix, the largest individuals of which measured around 60 cm (2 feet) long, plus tail. They probably didn't have spines, but they did have an unusually long head, with large teeth that would have been suited to eating animals such as mice, and hard-shelled crabs.

Around 6 million years ago, however, the Mediterranean became entirely cut off from the world's wider oceans, and began to evaporate. Not only did the islands disappear, but so did much of the sea itself, being reduced to a series of basins separated by vast and sweltering salt flats. This brought an end to the strange island animals, but it had less effect on those already on the continent, high above the now-exposed seabed. Probably the biggest change among the small mammals of the day would have the migration of gerbils from Morocco to Spain, presumably inhabiting the same sort of semi-desert habitat that their relatives prefer today.

It wasn't just rodents that changed in Europe at the time, so did their close relatives, the lagomorphs. For most of the Miocene, the most common lagomorph on the continent was Prolagus, a relative of today's pikas. These survived relatively unchanged for many millions of years, and seem, at least initially, to have been adapted to living in subtropical marshland (much as some rabbits in the southeastern US are today). As the climate changed, populations became separated from one another, creating many new species, and they, too, included some giant forms on the Italian islands.

More significantly, though, the Late Miocene saw the arrival of true rabbits, animals not previously seen on the continent. The oldest known rabbit in Europe is Alilepus, which first appeared in North America around 13 million years ago. Around 8 million years ago, they reached China, and then rapidly expanded across the new continent, reaching western Europe via Ukraine, where they presumably benefited from the growing grasslands.

It wasn't just the small animals that were affected by these changes in the vegetation, of course. Earlier in the Miocene, pigs such as Listriodon and Conohyus had shifted towards a more thoroughly herbivorous diet than their modern kin, making use of the subtropical plants of the day. Now, they went into rapid decline, although they would survive in warmer places, such as India, for a while to come.

More modern, omnivorous, pigs did already exist in Europe, however, and now they became more common. Perhaps most significant among the new species of pig that arose to fill the gap was Microstonyx. The fact that its snout possesses the flange of bone that, in all living species, anchors the muscle that operates the flat nose-disc indicates that this would not only have looked pig-like to modern eyes, but must have had a similar lifestyle, snuffling through loose soil or undergrowth for food. The main difference, though, was that the largest forms stood 110 cm (3' 7") at the shoulder and probably weighed around 300 kg (660 lbs), making them three times the size of a wild boar.

Musk deer and tragulids survived for longer than we might expect, but their preference for dense, damp forests eventually saw them leave Europe for good. The true deer had less of a problem, evidently finding enough suitable habitat in the forests that remained even after the Messinian Crisis dried up the Mediterranean. At first they resembled their earlier ancestors, but they slowly changed towards a more modern appearance as the Late Miocene wore on.

An early example, for instance, is Amphiprox. This was about the size of a roe deer, but, like the deer of the Mid Miocene, had antlers that simply branched in two at the tips. However, it did have longer legs than they did, suggesting it could run quickly - likely an adaptation to more open terrain. By the end of the epoch, it had been replaced by animals such as Croizetoceros, which were not only larger (it was about the size of a fallow deer), but had antlers that had finally taken on the modern form, with three to five branches.

Where the deer declined, antelope often stepped in. They were already common in Europe, but the expansion of the grasslands favoured them still further, and there is a sudden burst of new species appearing at around this time. Tragoportax was an early example, related to the nilgai-like animals of the Mid Miocene, but a long-legged bushbuck-sized species that grazed in the open woodland and had heavy backward-curving horns - known to be larger in males than in females.

Rather smaller were a profusion of gazelle-like antelopes, which the open ground would obviously have suited. Many were literally gazelles, close relatives of the living forms, but others were more related to today's spiral-horned antelopes. Examples here included Nisidorcas, with a gazelle-like diet but short legs suggesting a scrubland habitat, and Samotragus, which looked like a plains-dwelling gazelle, but had teeth more suited to eating leaves than grass.

It's also around this time that the first goats appeared, along with a very early musk ox, Plesiaddax. Unlike the modern species, this was still long-limbed, and looked rather like a smaller version of a wildebeest, but it was probably already feeding on tough herbs and grass. Much later, just before the Miocene came to a close, it was joined by Parabos, possibly the first bovine to actually be cow-shaped.

The short-necked giraffes of the day also prospered, and we know of several different species in Late Miocene Europe. The best known examples are probably Palaeotragus and Samotherium, both of which have two bony horn-like protrusions on their heads, and were already something like 3 metres (10 feet) in height. The former managed to survive right the way through to the Ice Ages, while the latter, despite having an unusually long neck for the time, seems to have fed on grasses, rather than treetop leaves, as one might expect.

Other giraffes of the day were more obviously short-necked, including such heavily-built animals as Helladotherium, which had four stubby "horns" rather than the more usual two. Others evolved in the exact opposite direction; Bohlinia of Greece and North Africa is known to have had long, slender, legs, and may be the distant ancestor of modern giraffes. Interestingly, in both cases, there is evidence that males may have been larger than females.

Arriving from much further afield, Paracamelus crossed over a land bridge into Asia from its original home in North America, making it as far as Spain, and, eventually, Africa. It's the only species of camel known to have lived in Europe, and is probably the ancestor of both Bactrians and dromedaries. Hippopotamuses, in the form of Hexaprotodon, remained in the continent for far longer, but can also date their first arrival in Europe to the end of the Miocene.

A relatively small continent like Europe being simultaneously home to camels and hippos goes some way to show the enormous diversity of animals we know of from that time. And I haven't even started on the mastodons and rhinos yet...

[Photo by "Bleached Rice", from Wikimedia Commons.]