Sunday, 23 November 2014

Pliocene (Pt 2): Survivors of the Zanclean Flood

For verily, I shall inherit the continent!
As I noted last time, officially, the dawn of the Pliocene - the autumnal epoch immediately prior to the Ice Ages of the Pleistocene - is marked by one of many changes in Earth's magnetic polarity. However, that particular event wasn't picked at random; there really were visible changes at the time. And, in Europe, none were more significant than the Zanclean Flood.

To understand what this is, though, we have to turn to the latter part of the preceding epoch, the Miocene, and take a look at the Messinian Salinity Crisis. The Miocene was much longer than the two epochs that followed, long enough that, over the course of it, the continents moved about a fair bit. Towards the end of the epoch, then, moving northwards, Africa hit Europe.

Due to the shape of the respective continents, however, this didn't result in the sort of massive mountain building that we see in present day Tibet (or, at least, it hasn't yet - the continents are still moving). But it did have a dramatic effect nonetheless. Crucially, the continents didn't just nudge up against one another in the east, creating what is now the Sinai, but also in the west, creating a land bridge between modern Spain and Morocco.

The Mediterranean Sea became land-locked. The Mediterranean climate of the day was even hotter and drier than it is now, and, free from any connection to the Atlantic and Indian Oceans, the sea began to evaporate. Over the course of hundreds of thousands of years, the sea level dropped. Not just a little bit, but by as much as three miles.

Much of the Mediterranean, of course, isn't that deep, so what actually resulted was a chain of different isolated seas in the deepest basins, which, being disconnected from each other, weren't necessarily all at the same height. The Black Sea would, of course, have been one of these, but the Tyrrhenian Sea would have been another, bracketed by land bridges through Sicily in the east and Sardinia in the west.

What happens when a sea starts to evaporate? There are at least two good examples today: the Great Salt Lake in America, and the Dead Sea in Israel. All the salt in the sea has to go somewhere. Some of it would have become vast salt flats, like those in Utah, and it's from the gypsum deposits created by those salt flats in places like Messina ("Zanclea" in Ancient Greek) that we know that this happened in the Mediterranean at all. The rest, however, remained in the shrinking seas, which became ever more salty.

And that is the Messinian Salinity Crisis.

True, the major seas that remained would not have become as salty as the Dead Sea, so they didn't wipe out all sea life, or anything like that - although it's entirely possible that isolated brine lakes here and there did exactly that. But they would have been saline enough to have dramatic effects on the local wildlife, forcing evolutionary change onto anything it didn't just drive extinct. It's this signal in the fossil record that first led to the identification of the Pliocene as a distinct epoch.

Or, rather, it's the end of that signal that marks the start of the Pliocene. 5.3 million years ago, as that epoch dawned, the continents shuffled around a little... and the Straits of Gibraltar opened.

By which point, the surface of the Atlantic was two to three miles above that of the Mediterranean. This must have been the mother of all cataracts, a rush of water a thousand times greater than the Amazon River flooding down over a hundred miles into the western Mediterranean, and then, once that had started to fill, into the central and eastern basins.

The Messinian Salinity Crisis at its peak
The sheer scale of the Zanclean Flood is difficult to imagine. Quite how dramatic it actually was, though, is open to dispute. It may have taken a thousand years to fill the Mediterranean and Black Seas until they were level with the Atlantic. Or, then again, it may well have been less than two years. Which is... oh, round about 30 feet of sea level rise per day. Blimey.

So what effect did all this have on the wildlife? The most dramatic changes would inevitably have been on marine life, in and around the Mediterranean itself. Having adapted over the last 600,000 years or so to increasingly salty waters, they found that normality had suddenly returned, and the populations shifted back towards different species - not to mention anything new that had started swimming through the new straits.

The sea bed that had just been reclaimed had, prior to the flood, probably been fairly inhospitable - although, to be fair not much more so than present day Utah, so it could have been a lot worse. Although, two miles or more below sea level, it's worth noting that the atmosphere would have been noticeably thicker than anywhere we can experience today, with all the added warmth that that brings.

Despite that, a number of animals had wandered across from Africa during the Crisis, many of them through southern Spain, where the dry land was consistently above even the normal sea level. Living as they did in Europe proper, it's unclear how much, if at all, the Flood affected them directly, but, either way, a number of them failed to survive into the Pliocene. In the aftermath of the Zanclean Flood, hippos and camels died out across Europe, as did the hornless rhinos that had previously joined them in their trek from Africa. Also heading to oblivion were a number of horses, hyenas, and big cats, although none of those groups were entirely wiped out, so I will be looking at them in a later post.

What animals would we have seen on a tour of early Pliocene Europe? Most numerous, as today, were rodents, most of them either mice, rats, or animals more or less indistinguishable from them. Gerbils did well for a little while, at least in Spain, and there were squirrels in the trees. It also seems to be around this time that beavers first became common in Europe, although it wasn't quite their first appearance.

A genuine newcomer, however, were the voles. They first appeared in Europe around 4 million years ago, at a time when the climate was undergoing a temporary cooler phase. They had probably originated only shortly before, in the steppelands of northern Asia, and took advantage of the colder weather to head south. With teeth better able to grind up tough grasses, they soon out-competed many of the older rodents on the continent, and rapidly became surprisingly successful - today, there are 25 species of vole in Europe, compared with just 15 species of mouse.

We might not have paid much attention to the voles, but we could hardly have failed to notice the large reptiles along the recently re-defined southern coasts. There were crocodiles in southern Spain, and six-foot turtles swam in the swamps near present-day Perpignan. After all, the climate was still warmer and wetter than today, and perhaps not unlike modern Florida. That gave these "cold blooded" creatures a last gasp at European life before things turned cold in the mid to late Pliocene.

Turning to the mammals, however, we would see other creatures that would also remind us of today's warmer climes. Antelopes were common, albeit alongside the deer, cattle, and pigs that we would be more familiar with. Not just antelopes, but monkeys, too. Today, apart from ourselves, the only primates native to Europe are the Barbary "apes" of Gibraltar, a small colony on the very edge of the continent. But, in the Pliocene, not only were there macaques - relatives of the surviving Barbary species - but also leaf-eating colobus monkeys.

We'd have more to fear, of course, from the carnivores. There may have been less variety among hyenas and big cats than before, but, if anything, there were more bears, some of them really rather large. Less worrying to us, however, would be the martens, weasels, and so forth that fed on smaller prey.

Starting next year, I'm going to look at some of these animals in more detail, before eventually moving on to look at what was happening on the world's other continents.

[Photo by Peter G Trimming, painting by "Paubahi", from Wikimedia Commons]

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