Saturday 27 September 2014

Age of Mammals: The Pliocene (Pt 1)

I suspect that when most members of the public think of prehistoric animals after the time of the dinosaurs, they think of the Pleistocene, the time of the Ice Ages. This was a time of bitter cold, the time of cave men, mammoths, and Smilodon cats. Even among scientists, it's easily the most researched of the various bygone epochs that make up the Age of Mammals, not least because it's the one closest in time to our own, and therefore the easiest to study.

But there are five other epochs that precede the Pleistocene within the Age of Mammals, and, compared with most of them, it isn't even very long. Heck, it isn't even 5% of the total. As it happens, though, the epoch that immediately preceded the Ice Ages, the Pliocene, isn't much longer. If we imagine, as we're often invited to, the entire history of the Earth as a single year, the Pliocene is, very roughly, the period between 2 and 7 p.m. on the evening of the 31st December. That's not exactly a large chunk.

On the other hand, on a human scale, the Pliocene is vast; the long autumn that leads from the summer of the Miocene into the freezing cold of the great ice sheets that follow. When I first discussed the Pleistocene, I used the example of a TV documentary that whizzes through the whole of history. In fact, it takes one minute to cover each decade of time. So the entire history of the world since the outbreak of World War I is covered in just the final ten minutes. Your life so far is, I can assume with some confidence, covered in even less time than that.

So it seems like it's a bit of a whistle-stop tour. But the documentary does the same for every decade, or, if you prefer, takes ten minutes per century. It turns out, that if it begins with the dawn of recorded history (that is, the invention of writing), the program lasts over eight hours. Better go to the toilet before it starts.

Extend it out to cover even just the last bits of mammalian evolution, though, and we're on an entirely different scale. If we're going to start at the beginning of the Pliocene, the program will last a little over a year, with the Pleistocene occupying most of the second half of that. That's over six months of a program that can't even be bothered to spend ten minutes doing your entire life.

Such is the scale of geological time. But, of course, to work out those sorts of numbers, we do have to know when the Pliocene starts and ends. Ever since Pleistocene researchers were allowed to extend their epoch back by a extra few hundred thousand years in 2009, the Pliocene has been officially defined as lasting from 5.3 to 2.6 million years ago. Which is pretty short, as epochs go, but why those two dates?

Officially, we define the start and end of the epochs by magnetostratigraphic events. Or, to put it in English, times that the Earth's magnetic poles flipped over, and compasses started pointing in the opposite direction. These are, by definition, worldwide events, and, moreover, ones that leave their signature in the rocks, so it's something we can all agree on when we're trying to date some newly found stratum somewhere. In all, the poles flipped thirteen times over the course of the Pliocene, and the first and last of those flips mark the beginning and end of the epoch. (For that matter, flip #8, around 3.6 million years ago, marks the formal boundary between the "Early" and "Late" Pliocene).

Well, okay, but why those two? The short answer is that they happened to line up with roughly where we'd already put the boundaries before magnetostratigraphy got invented. And those boundaries were there because we thought we could see some changes in animal life at about those times - even before, in fact, we knew quite how far back in time we were talking about. The animal life that Charles Lyell had been looking at when the coined the term "Pliocene" in the 1820s happened to be fossilised sea shells from the south of France, so they didn't necessarily reflect anything on a worldwide scale. But, as it turns out, the changes did reflect some events that, if not affecting the entire world, were nonetheless pretty major.

The end of the Pliocene, therefore, lines up fairly well with the point that the Arctic Ocean froze over. It's possible that, at some point during the warm interglacials between the Ice Ages, it melted and refroze again, so the Pliocene is not necessarily the last time that we had an ice-free Arctic in summer, but it's likely the last time it was true for any really lengthy period. It's also not long after the Isthmus of Panama formed, joining North and South America. That may not be a coincidence, since the sudden end to mixing between the Pacific and Atlantic Oceans in the tropics probably contributed to worldwide changes in the climate.

The beginning of the Pliocene, on the other hand, is marked, more or less, by the Zanclean Flood and the end of the Messinian Salinity Crisis - both of which I'm going to turn to in a later post, so stay tuned.

If we could look down on the Pliocene world from space, what would we see? The most obvious difference would probably be the lack of city lights on the night side, but, of course, that would have been equally true of the Pleistocene. Leaving that aside, then, there would still have been a few interesting things to spot.

Most of the continents would already be in roughly the places and shapes that you'd expect them to be, perhaps only a hundred miles or so away from their present locations. On the other hand, major glacial features, such as the fjords of Norway, or, for that matter, the Great Lakes in North America, didn't exist yet. Neither, as I said above, did the Isthmus of Panama - Central America is, at this point, just a chain of islands, a western counterpart to today's Caribbean. Similarly, while the Arctic likely had floating ice in winter, and Antarctica probably looked much the same as it does now, the North Pole was clear blue sea during the summer.

The Early Pliocene was, indeed, a particularly warm time, with the world quite a bit hotter than it is today. That ended about half way through, with a million-year long slide into autumnal temperatures that would end with the dawn of the Ice Ages. We can't examine the changes with quite the same precision that we can for the Pleistocene, so likely there was a bit of up-and-down in there that we can't see, but the broader pattern seems clear enough.

This general cooling trend - slow at first, but then picking up in the Late Pliocene - is one of the defining features of the epoch. Jungles retreated, while tundra and pine forests extended, if not so much as they would later. The world also seems to have become generally drier. This latter change led to the expansion of not just deserts, but also steppe grasslands in places like the Great Plains and Central Asia. Which, among other things, drove the evolution of grazing animals, such as fast-running horses.

Many of the animals alive in the Pliocene would have looked relatively familiar, but there were a great number that did not, and that would die out when the ice arrived. South America, still an island continent, had a fauna almost as strange as that of Australia, at least until the land bridge formed, bringing with it the ancestors of jaguars and llamas, along with deer, foxes, and rodents. Elsewhere, elephants and rhinos were more varied than today, while dogs expanded out of North America to reach Asia, Europe, and Africa.

Our own ancestors split away from chimpanzees either at the beginning of this epoch, or the end of the previous one, and evolved into our current genus, Homo, roughly where it ends. If there's a time where you could say there were "ape-men", then, this is it - a time when apes first began to walk upright, even if they weren't fully human yet.

Over the following posts of this series, then, I'm going to look at many of those mammalian species that lived in the long autumn before the Ice Ages. Looking at the familiar, yes, but perhaps more at the strange, at creatures that vanished when the ice sheets came. And I'm going to start, as I did when I looked at the Pleistocene, with what was happening in Europe.

[Picture taken from a mural by Jay Matternes at the Smithsonian Institution, in the public domain.]

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