Age of Mammals: The Oligocene (Pt 1)

It's been over a decade since I started including bimonthly looks at specific slices of Earth's past in this blog. In that time, I have covered three epochs: the Pleistocene, Pliocene, and Miocene. Together with the current Holocene epoch, these comprise what we currently consider to be two "periods": the Quaternary and Neogene. Both of these are dominated, more or less, by mammals of the sort we'd generally recognise today, even if the details are different. All of the earlier chunks of time since the extinction of the non-avian dinosaurs, however, constitute a different period, the Paleogene, where this was much less true.

When Charles Lyell devised the current system of dividing the "Age of Mammals" into epochs in 1833, he originally defined four. A few years later, he revised this to five, but even then, the entirety of what we'd now call the Paleogene was placed into a single epoch, the Eocene. In 1854, however, German palaeontologist Heinrich Beyrich, split off the later part of the Eocene into a new epoch, which he saw as a distinct period of transition in the development of fossil seashells. He called this the Oligocene, and it proved useful beyond his original mollusc-based definition, and so has remained in use to this day. (Beyrich's wife, incidentally, was a children's author, and made the unusual step of favourably commenting on the work of Charles Darwin in a novel for young girls at a time when it was still controversial).

The Oligocene, as currently defined, lasted from 34 to 23 million years ago. This makes it the first epoch that I've looked at where the end was significantly closer in time to its beginning than it was to the present day. It's also notable for being shorter than the epochs on either side of it, reflecting its original definition as a time of relatively rapid transition between early and modern forms. If we squeeze the history of Earth into a single year, the Oligocene would last from about 7 a.m. on the 29th of December to 4 a.m. the following morning - roughly a single day but no longer one that's crammed right up against the New Year.

Still, eleven million years is a long time, being twice the length of the Pliocene and Pleistocene combined, and it should not be surprising to discover that the world changed a fair bit as it went on. Nonetheless, one of the key things we would notice if we could look down on the Oligocene world from space was that, assuming we count Eurasia as a single entity, all of the continents were still separate. It would, in other words, still have been possible to circumnavigate the world by sea at relatively low latitudes, sailing between North and South America and then between Eurasia and Africa.

There is, however, a significant caveat to this, and it refers to what was going on much further south. Australia was certainly an island continent, although further south than it is today... but what was happening further west is less clear. For a long time, it was thought that the Drake Passage opened up during the Oligocene, and possibly only towards the end. Over the last couple of decades, this date has been pushed back and it now seems likely that South America and Antarctica were indeed, already island continents at the dawn of the Oligocene.

Nonetheless, it does seem to be around the dawn of the Oligocene that the seas between the two continents became deep enough for a deepwater current to flow through them. That's significant, because, with the seas south of Australia also recently having developed their own deepwater channel, the Antarctic Circumpolar Current was able to form, creating the Antarctic Sea as it is defined today. And the reason that matters is because it's this current that cuts off warm air from the southern continent, isolating it and keeping it frozen. 

Exactly what the sequence of events was here is not entirely clear, but what we do know is that the Antarctic ice sheets surged out at the dawn of the Oligocene, reaching the coast for the first time and creating the first of the great ice shelves that surround it. They would retreat again during the worldwide warmth of the Miocene, but nonetheless, this was a herald of colder times, and may even have had knock-on effects on the water circulation on the opposite side of the world, in the North Atlantic.

Elsewhere, mountain ranges were rising, with the formations of both the Rockies and the Andes likely at their peak around this time. As Africa moved northward, the Alps also continued to rise along with other associated mountain ranges from the Pyrenees to the Carpathians and the Zagros Mountains. Further east, the Himalayas became high enough that the Tibetan Plateau reached something similar to its modern arid climate. It's also possible that it was during the Oligocene that the last water passage between India and the rest of Asia closed up, turning the former island into the subcontinental peninsula it is today.

Our knowledge of the Oligocene climate is not as thorough as that of the Miocene, possibly because the pattern varied in different parts of the world, creating a more confusing overall picture. It was, for example, certainly colder than the epochs on either side of it, but by how much is harder to say. On the other hand, it is clear that there were three key climatic events over the course of the epoch.

The first of these is the sudden and dramatic cooling that began almost immediately after the official start of the epoch. Whether this "Oi-1 glaciation" was caused by the creation of the Antarctic Circumpolar Current or was just coincidentally amplified by it is apparently still a matter of some debate, but that it happened seems undeniable. Most of the evidence for it comes from the chemical balance in sea sediments, which show a dramatic drop in carbon dioxide levels in the atmosphere, but we know that the same thing was happening over land as well. While this apparently wasn't sufficient to create much in the way of ice caps in the Arctic, so much ice became locked up over the south pole that worldwide ocean levels fell by as much as 50 metres (165 feet), pushing coastlines outwards.

The Oi-1 event lasted for hundreds of thousands of years but, on a geological scale, the world rapidly reverted to more typical conditions afterwards. Then, around 28 million years ago, a second, and perhaps even larger, glaciation event hit. Known, appropriately enough, as the Oi-2 glaciation, this may have seen sea levels fall by about 75 metres (250 feet) and lasted for half a million years. Little is known about what may have caused this, but it's significant enough to be considered as the boundary between the "Early" and "Late" stages of the Oligocene, known more formally as the Rupellian and Chattian.

Finally, starting around 25 million years ago, the end of the Oligocene is marked by an apparent warming event, with temperatures rising above what they are today. The evidence for this is more mixed than that for the two glaciations, perhaps because it wasn't truly worldwide and Antarctica, in particular, seems to have remained frozen. But it does seem to have been noticeable in the north, and may be linked to the establishment of the first monsoons over the Indian Ocean.

The word "Oligocene" means "few recent" and while it originally referred to fossil seashells, it seems equally appropriate for mammals, among other life forms. Many of the mammal species living at the time were holdovers from even earlier times, members of groups that we would not recognise today, because they either died out during the Oligocene or early in the following, Miocene, epoch. Where animals belonging to modern families did exist, they were often early forms that were different from their descendants and were not yet numerous or particularly diverse.

When exactly a particular animal family came into existence can be a matter of definition, of the dividing line between what we'd recognise and something that's not yet quite the same thing. There's also the issue of the completeness of the fossil record, especially this far back, where we may not have found some of the earliest examples yet. But, allowing for those caveats, we can say that the Oligocene was the time of the first cats, deer, pigs, gophers, and kangaroos, and the first members of the modern families of monkeys and toothed whales. The cooling climate saw an expansion of temperate forests in places that had previously been subtropical and promoted the rise of grasses, although the latter didn't really form open steppe lands and prairie until the following epoch. 

The isolation of the continents meant that there was relatively little mixing of biota during the Oligocene but that isn't to say that there wasn't any, with one particularly significant event occurring right at the beginning. The official demarcation of the start of the Oligocene depends on a change in fossil plankton and slightly precedes the arguably more significant Oi-1 glaciation. However, and probably not by coincidence, this earliest part of the Oligocene is also marked by a truly dramatic event affecting the world's wildlife.

This was the Grande Coupure, first identified, as its name might suggest, in Europe. Next time, I will kick off my detailed examination of the mammals of the Oligocene world by looking at that event and what it meant for the continent...

[Painting by Roger Witter, in the public domain.]