Sunday, 11 June 2017

Age of Mammals: The Miocene (Pt 1)

Five years ago, I started a series of posts in which I looked at the world, and its mammalian fauna, during the time of the Ice Ages. My plans as to how I was going to do that changed quite rapidly, and the earlier posts aren't really in the same format that I later settled in to. Nonetheless, since that time I have covered not only the Pleistocene epoch of the Ice Ages, but also the Pliocene, which immediately preceded it. Yet, even taken together, these two epochs represent only a relatively short slice of the Age of Mammals.

We currently divide the Age of Mammals - the time since the extinction of the non-avian dinosaurs - into three broad periods: the Paleogene, Neogene, and Quaternary. The last of those includes only the Pleistocene and the brief, human-dominated, time since it ended. The Neogene, however, is also dominated by more-or-less modern kinds of animal, and it is further divided into two epochs: the later Pliocene, which I have already covered, and the earlier Miocene, which I haven't.

Perhaps the first thing to grasp about the Miocene is that, compared with the epochs that followed, it is remarkably long. It lasted, as currently defined, from about 23 million to 5 million years ago. That makes it over three times as long as the Pliocene and Pleistocene put together. As you might expect, the world changed far more over this timespan than it did during the subsequent epochs; we're not just talking a couple of million years here, but it a much more substantial chunk of time. It's only because it's so much further back that it makes sense to do this - we just don't have the same sort of fine detail available, since so much of it has been erased in the time since it all happened.

Even so, scientists working on this particular epoch do divide it into smaller spans for convenience. Exactly what those are often depends on the specific interests of the scientists concerned, so that the range of possible terminologies can get really quite complicated. However, a general scheme of "Early", "Middle", and "Late" Miocene is widely used - although it's worth noting that even these are each about the length of the Pliocene and Pleistocene epochs combined, which explains the need for the other, finer, gradations for particular purposes.

Another way of looking at it is to use the traditional analogy of "the entire history of the Earth as a single year". In this scheme, the Pliocene is the time between around 2 and 7 p.m. on the afternoon of the 31st December. The Pleistocene, of course, is almost the entirety of the five remaining hours, with the Last Ice Age ending somewhere around 23:59, and the entirety of recorded history being squeezed into the last few seconds before the chiming of Big Ben (or other indicator of your choice).

The Miocene, however, starts at around 4 a.m. on the morning of December 30th, ending at two in the afternoon the following day. Sure, that's kind of late in the year, but it's suddenly something you could mark on a calendar, not just an hourly schedule. We've rolled the geological clock back quite a bit further.

Bearing all that in mind, what would we see if we could look down on the Miocene world from space? It's not so far back that we'd have any difficulty identifying the continents and oceans, but their positions were different, and the epoch is long enough that they moved significantly during its course. Most notably, at the dawn of the Miocene, 23 million years ago, all of the continents were separate from one another - at least, assuming you count Eurasia as a single landmass.

North and South America had yet to join up, although they were creeping towards one another, and as the epoch passed we'd see the Mexican peninsula stretching its way southwards, creating Central America bit by bit, until a chain of volcanic islands formed the boundary between the Caribbean and the Pacific. Elsewhere, to the east, Africa was separated from Eurasia by an increasingly narrow body of water known as the Tethys Seaway. Around 14 million years ago, during the mid Miocene, the continents finally collided in what is now the Middle East, and the last remnants of the Tethys became split between the Mediterranean to the west, and the Persian Gulf to the east.

Later still, around 6 million years ago, Africa's continued movement northward resulted in Morocco hitting Spain, closing off the newly formed Mediterranean as an inland sea that gradually evaporated, eventually becoming no more than a series of distinct bodies of water separated by swelteringly hot salt flats far below the world's general sea level. The end of this remarkable time, when the Straits of Gibraltar re-opened in the titanic Zanclean Flood, is one marker for the end of the Miocene as a whole.

All of this was accompanied by some significant mountain building, as a number of pre-existing ranges became significantly higher. Most notably, this included the Rockies and the Cascade Mountains, the Andes, and the Himalayas, but Africa's northward push was also continuing to raise the Alps and the Zagros and Elburz mountain ranges of Iran.

Another major difference is that the Miocene was, on the whole, much warmer than today. The phrase "on the whole", though, does disguise quite a bit of variation over the epoch's vast time scale. The Miocene began with a sudden, but brief, cold snap (that's "brief" as in "it only lasted 400,000 years") but the temperature rapidly recovered, so that much of the Early Miocene was distinctly warm, with little, if any, ice in the Arctic. One estimate, for instance, has Early Miocene Vermont having an average annual temperature of around what Alabama and Georgia have now, and we know that there were forests of oak and walnut near present-day Anchorage, and of pine in the most northerly of the Canadian islands.

While temperatures may have cooled a little as the Early Miocene progressed, they rose again during the Mid Miocene, leading to the two-to-three million year long Middle Miocene Climactic Optimum, with temperatures in southern Europe estimated to have reached a decidedly subtropical 22°C (72°F), accompanied by a heavy annual rainfall. Indeed, with so little water locked up in ice sheets, sea levels were much higher, so that western Europe was an archipelago of large islands and shallow seas. Similar evidence for unusually high temperatures comes from across the world.

And then it all went horribly wrong: around 14 million years ago, there was a sudden crash in global temperatures. The Antarctic ice sheets surged outward from what was likely a small refuge in the high continental interior, and have essentially never retreated since. Exactly what happened to cause this isn't clear. Although it may have had something to do with changing patterns of sea circulation due to the shuffling around of the continents, more modern theories tend to favour changes in the Earth's orbit as the main culprit.

Whatever the cause, temperatures never fully recovered. The second half of the Miocene is instead marked by a general cooling trend, and, while the current one is much younger, it's possible that Greenland first developed an ice sheet around this time. Sea levels dropped, Europe once again became a single landmass - just in time for the closing off of the Mediterranean - and, by the time that the Miocene came to an end, temperatures worldwide were not far off what they are now. This latter half of the epoch was also drier than the earlier part, resulting in a great expansion of grasslands, especially in the continental interiors.

The word "Miocene" roughly translates from the Greek as "less recent", and was coined by Charles Lyell in the 1830s because rocks of this age contained fewer modern-looking fossils than the those of the subsequent Pliocene... but not that much fewer. He was specifically thinking of seashells from the south of France, where he had conducted studies on rock strata, but the general principle holds on a broader scale.

The majority of mammal species alive in the Miocene are at least broadly assignable to modern groups. Many of the mammal (and bird) "families" that we currently recognise already existed, so that we can identify dogs, horses, and so on from the fossil record of the time. However, this was also a time of change, with many older forms of mammal still surviving, only to die out before the epoch's end. Where modern families of animals did exist, the species may have been significantly different; horses, for example, still had three toes on each foot.

Furthermore, while animals such as cats, bears, and deer can all trace their origin back rather further, a number of entirely new kinds of mammal first appeared during the Miocene, and many of them are still with us. It is the time of the first antelopes, the first mice, the first giraffes, the first seals, raccoons, and weasels. Elephants didn't exist yet, since they first appeared in the Pliocene, but the closely related mastodons, which looked pretty similar, were another group that arose in the Early Miocene.

As to our own ancestors, we first diverged from proto-chimpanzees around 7 million years ago, towards the end of the Miocene. At the opposite end, however, we see the origin of the apes as a whole, with the great apes splitting from the gibbons during the Early Miocene, and the very first apes diverging from the monkeys at somewhere round about, or just before, the dawn of the epoch.

So we're looking at a time when the world was warmer and more comfortable than it is now, able to support a profusion of life forms, both new and old, scattered across continents that had yet to join up and mingle their wildlife. There are animals both strange and vaguely familiar, changing over the course of what is, in fact, the second longest of all the epochs of the Age of Mammals. Welcome to the Miocene; over the coming months I'm going to be looking at aspects of it in rather more detail.

[Painting by Jay Maternes, in the public domain.]

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