Sunday 26 August 2012

Pleistocene (Pt 3): Ice Ages and Interglacials

Life-size reconstruction of a steppe mammoth
(compared with a 3-year old human)

When the Pleistocene began, Europe's climate was much the same as it is today, and the general shape of the continent would also have been instantly recognisable from space. The animals however, were different, many of them being ones we would now associate with Africa - rhinos, elephants, hyenas, and cheetahs, among others. In part 2, I described how that began to change 1.8 million years ago (which, incidentally, was once defined as the beginning of the Pleistocene - see part 1 for why that changed).

This was a time of cooler weather, as the Ice Ages began to dawn. Forests retreated in the face of advancing tundra, and musk oxen, bison, and (strangely) European hippos began to make their appearance. The cold snap was prolonged, and, so far as we can tell, the fauna of Europe remained relatively stable for the next 600,000 years. That's still a very long time - if we go back to my analogy where we get just one minute to watch the events of a decade, with the whole of written history thereby spread out into a nine hour spectacular, this phase of European history would last a full six weeks.

1.2 million years ago, half way through the Pleistocene, the climate changed again, and mammals (and other animals) were forced to adapt. However, the change wasn't towards yet colder weather, but back towards a warmer, more pleasant climate. The forests grew back, with all their dense undergrowth in attendance, and the harsh steppe-lands retreated into the north. As had been the case at the dawn of the Pleistocene, European weather would have been much as it is now.

However, it wasn't quite that simple. Our ability to trace changes in climate on much shorter time-scales of just tens of millennia stretches about this far back, and, armed with that information, we can see that that the climate wasn't entirely stable during this time. There may have been anything up to four ice ages interrupting the otherwise moderate climate of the day. Compared with those that came later and (so far as we can tell) those that came before, they were quite mild, and fairly short into the bargain. Looked at on a fast enough scale, we would have seen the forests and steppes locked in a dance, advancing and retreating as ice ages alternated with interglacials.

Short, mild ice ages, and long, warm, interglacials, to be sure. But still.

With that in mind, though, we can broadly say that this was a time of lush forests, and that had dramatic effects on the local herbivores. Many of these animals would presumably have headed south during the colder times, and some of the earlier species - such as Mammuthus meridionalis, the plains-dwelling "southern mammoth" - continued to prosper. But, nonetheless, new animals did appear, and they include a number that are still very familiar today.

There were already fallow deer on Europe, but now they were joined by roe deer, red deer, and primitive moose, as well by wild boars and wild sheep. Many of these were indistinguishable from the modern forms; different subspecies at best. There were, however, some other species that have since departed. For example, the largest European deer of the preceding period, Megaloceros obscurus, was replaced by the even larger M. verticornis. A moose-sized animal, this had long, branching antlers, and was the largest European deer of its day.

In other cases, species of mammal began to diversify, with a single, older, species being replaced by two or more adapted to different aspects of the continent, and which, in many cases, may have prospered and declined in alternation as the climate changed. For example, the dominant horse of the day had been Equus stenonis, an animal that looked much like a modern horse (or possibly a zebra), and stood around 15 hands high, just above the modern boundary that divides "ponies" from larger breeds. Towards the end of this warmer stage of the Pleistocene, it diversified into at least two species: one, E. sussenbornensis, that stood 18 hands high, and was even more horse-like than its ancestor, and another, E. altidens, which was much smaller, and that may well have been an ancestor, or at least a close relative, of modern donkeys.

As had been the case earlier in the Pleistocene, the change among carnivores was initially less extreme. Where the herbivores had different plants to feed on, it seems to have made rather less difference to carnivorous mammals exactly what they were hunting. Wolves were already important predators, and the immediate ancestor of the living wolf, Canis mosbachensis, appears at around this time, although it wasn't greatly different from its predecessors.

European bears changed rather more. Further east, in Asia, the modern brown bear was just beginning to appear, but in Europe, the earlier bears were replaced by the larger, more herbivorous, Ursus deningeri. At the same time, other carnivores moved north from Africa, again, presumably taking advantage of the relatively pleasant climate. These included both leopards and spotted hyenas, apparently identical to those alive today, and accompanied - at least in Greece and Moldova - by woodland antelope.

This mild phase lasted for around 300,000 years. Then, about 0.9 million years ago, the climate took another dramatic turn for the worse. As before, ice ages alternated with warmer interglacials, but now the cold times were becoming harsher, and the warm ones shorter. In general, the individual ice ages lasted about four times as long as the gaps between them, and when they did hit, Scandinavia, and many other parts of northern Europe vanished under utterly inhospitable sheets of ice. At their height, icebergs would have drifted off the coast of the Algarve.

Short though the interglacials may have been, they lasted long enough to allow forests to re-colonise the land, albeit only before being swept away again a few thousand years later. Still, that seems to have been enough to allow many of the mammals of the warmer times to survive, and even for new forest-dwelling species to arise.

An example of this can be seen with the rhinos, which still lived in Europe at the time. The earlier Etruscan rhinos gave way to two new species. Judging from the shape of its teeth, the "narrow-nosed rhino", Stephanorhinus hemitoechus, probably preferred to eat grasses and other tough vegetation, making it well-suited to the colder times. While it may well have been hairier, it otherwise looked much like today's two-horned black rhinos in both size and general appearance. In contrast, the much larger S. kirchbergensis,  almost the size of an Asian elephant, had teeth suited for browsing on softer, forest vegetation, and would have been more common in the interglacials.

The biggest change at this time was, unusually, among the carnivores. For, as the weather got colder, sabre-tooth cats suddenly went extinct in Europe, never to return. It's not immediately clear why this happened, since they don't seem to have had any great problem elsewhere. The very species that died out in Europe continued to survive in Asia, and seems to have done fairly well for itself, and the sabre-tooths certainly had a longer history in North America, as well. Nonetheless, for whatever reason, they vanished from Europe, and lions, leopards, and newly arrived lynx took their place as the great predatory cats there.

As had happened during the earlier cold snap, goats, steppe bison, and musk oxen prospered, and the advance of horses towards their modern form also continued. At the same time, the warm spells were sufficient to allow the continued survival of wild boar, hyenas, and even Barbary macaques (still found today on Gibraltar) across many parts of Europe. Steppe bison, similar to the modern European species, but with much longer horns, were joined by two new species of bovine: the auroch, which is the wild form of today's domesticated cattle, and also the now-extinct European water buffalo.

With the sabre-tooths gone, what of the other icon of the Ice Ages, the mammoths? They continued to survive, but whenever the ice retreated, they headed north, and were replaced by a new elephant, Elephas antiquus. These "straight-tusked elephants" evidently managed to struggle on through the colder periods, presumably by moving to warmer climes, and came back, time and time again, to mainland Europe whenever the weather, and vegetation, were suitable. Unlike the mammoths, they fed on forest browse, and they clearly preferred a pleasant, temperate climate. Although they were very close relatives of today's Indian elephants, they stood as tall as the very largest African elephants, and had long, straight tusks quite unlike the shorter ones of their modern relatives.

The older species, the southern mammoth Mammuthus meridionalis, had by now died out. It liked open forest or grassland, but with the straight-tusked elephants taking over the denser forests, it found itself forced to more marginal - and increasingly cold - habitats. It was soon replaced by an immigrant from Asia, the steppe mammoth Mammuthus trogontherii, far better adapted to harsh weather and tough vegetation.

This was the first elephant to be truly adapted to cold climates, and it was probably hairier than any of its predecessors. It was also much bigger, at least a foot taller at the withers than even the largest African bull elephant of today. Unlike living elephants, it had the same curving shape to the tusks as other mammoths did, and in some cases, these reached a whopping five metres (fifteen feet) in length.

It's an interesting question to consider just why some of these animals were so huge. It might be an application of Allen's rule, where colder climates mean larger animals. But it's also been argued that it's just an illusion. True, some of these animals were bigger than anything similar around today, but we can say much the same of many warmer epochs during the Age of Mammals, and perhaps we today are just unfortunate. Or perhaps our ancestors simply ate everything that looked really impressive...

In fact, while many Pleistocene mammals were indeed immense, there were exceptions. Of course, there were mice, and weasels, and hedgehogs, and various other animals that we'd expect to be small, but that isn't all. During the Ice Ages, much of the oceans became locked up in the vast ice sheets, and so the sea level dropped. This meant that many islands suddenly became reachable by land bridges. The English Channel, for example, dried up several times, repeatedly turning Britain into a peninsula.

The same happened to some of the smaller Mediterranean islands. Animals, naturally enough, wandered into these new lands, only to find themselves cut off when the seas rose again. With relatively little to eat, many of them had to become smaller to survive, a process known as insular dwarfism. It seems to be a general principle, but some of the most dramatic examples of insular dwarfism date from just this time.

When straight-tusked elephants found themselves trapped on Malta and Sicily, for example, they evolved into the pygmy elephant, Elephas falconeri. Their ancestors had been the size of a modern African elephant, but the pygmy species was no larger than a big dog, around three feet or so at the shoulders. In every other respect, they looked just like typical elephants, but the size difference is truly dramatic.

They weren't alone; elephants were common in Europe at the time, and many others also seem to have found themselves isolated on islands. As a result, similar dwarf species are known from both Crete and Cyprus. These tiny elephants are probably the most famous example of insular dwarfism, doubtless because of our clear expectations of what size an elephant should be. But there were also dwarf hippos, descended from the elephant-sized Hippopotamus major of the European mainland. Many of these weren't actually that small, except by comparison with their gigantic ancestor, but a Maltese species, Hippopotamus melitensis, was no larger than a pig.

The same process didn't generally apply to carnivores, because, faced with insufficient food on the islands, they just seem to have died out. This, however, led to a seemingly paradoxical situation: some of the mammals that were already small grew larger. Without natural predators, they no longer had any need to hide, and so we find that island dwarfism among large mammals is often accompanied by island gigantism among the small ones. A recently discovered example is the giant Minorcan bunny, Nuralagus rex, which weighed something like twenty six pounds, but we also know of giant dormice, about twice the size of their continental kin.

While Synapsida generally doesn't cover human evolution, I should also add that Europe was home to two successive species of our own genus during the timespan covered by this post. Homo antecessor and H. heidelbergensis lived, respectively, during the milder and harsher climate phases that I have described. They had moderately sophisticated stone tools, and surely must have hunted many of the animals around them. Yet, at this stage, they were simply not numerous enough to have any dramatic impact, although the latter species had, for the first time in Europe, a brain size similar to our own.

[Photo by "Titus 322", from Wikimedia Commons]

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