Age of Mammals: The Eocene (Pt 1)

When Scottish geologist Charles Lyell first created the system of epochs we now use for dividing the Age of Mammals, he designated four of them. This was in 1833, so he did not know the true age of the Earth, let alone the timespans of the epochs he was naming - he was basing them purely on geological strata and the types of fossil seashells found within them. We now know, however, thanks to the wonders of radiometric dating techniques, that the oldest of the four epochs he defined spanned over half of the Age of Mammals, longer than the other three put together.

Two epochs have been carved out at either end since, so the Eocene is not quite as long now as it was when Lyell named it. However, it remains the longest of the seven epochs since the extinction of the non-avian dinosaurs, still occupying a third of that entire stretch. As currently defined, it runs from 56 to 34 million years ago. Compared with the entire age of the Earth, that's not very much, but from the point of view of most mammalian palaeontology, that's unusually long. 

If we imagine the history of the Earth as a single year, the Eocene would run from 5 p.m. on 27th December to 7 a.m. on the 29th. That's over a day and a half, and, at least as importantly, you've still got nearly three days to go by the time it's over. It's probably only because it's that far back in time that we are happy to keep it that long, when more recent epochs span just a couple of million years. Like the other epochs, we do subdivide it, but, here, even the subdivisions are relatively long. While there are detailed subdivisions for specific purposes, the most commonly used are the standard scheme of "Early, Middle, and Late" and a more formal division into four "stages", the latter splitting the Middle Eocene into two and giving each of the bits fancier names.

Given the length of time involved, it should not be surprising that the continents shifted about significantly during the epoch. But although we could easily identify them all if viewing the planet from space, there were also significant differences from the forms they take today.

Europe, for instance, genuinely was a separate continent, separated from Asia by the Turgai Strait on the eastern side of the Ural Mountains. The closure of this strait is one of the markers of the end of the epoch, but for much of the Eocene, an island continent stretched from the Urals to Scandinavia. South and west of that was an archipelago of large islands. At times, the Balkans and Anatolia formed a peninsula stretching westward from Asia south of the Turgai Strait (which connected, via the Caspian Sea and much of what is now eastern Europe, to the Baltic), but at others, they were at the eastern end of the archipelago. With the Alps not yet fully formed, shallow seas meant that Italy was also an island, separated from a larger one in Central Europe to the north.

On the other hand, during the Early Eocene, it may have been possible to walk from France to Canada. This is partly because the English Channel may have been dry at the time, but also because the North Atlantic had not fully finished opening. A land bridge ran from Scotland to Greenland - probably a distance of less than 500 km (300 miles) at the time - and the Davis Strait west of Greenland may have been at least partially closed. 

At the same time, India was an island, heading north towards Asia and pushing up the Himalayas in the process. It collided with the continent around 52 million years ago, during the Early Eocene, but only at the western end, around what is now Pakistan. For the remainder of the epoch, a bay stretched across the Gangetic Plain, and fluctuations in sea level may have meant that the last water passage between the two did not close off until the following, Oligocene epoch.

The Drake Passage between South America and Antarctica probably opened up, at least as a shallow strait, during the Early Eocene. Perhaps more significantly, Australia and Antarctica were still a single landmass, only separating in the Late Eocene with the creation of the Tasmanian Passage.

These differences are further amplified by the fact that the Earth was warmer then than it is now. Without ice caps, the sea levels were much higher, creating islands from what are now peninsulas. Indeed, the official marker for the start of the Eocene is a sudden and dramatic change in the isotopic composition of carbon, indicating a rapid rise in CO₂ levels.

What caused this is a matter of debate. Theories include widespread volcanic activity and a cometary impact, but one of the more popular is that a pre-existing rise in temperatures reached a tipping point. At the very beginning of the Eocene, things became just hot enough to liberate gases from the frozen methane clathrates on the seafloor. Once that happened, a vast reserve of greenhouse gases was released in one, sudden, titanic belch. 

Global warming accelerated as carbon was pumped into the atmosphere. It took 200,000 years for the Earth to recover by locking the carbon back into newly formed sediments. This time is known as the Palaeocene-Eocene Thermal Maximum, and it's the hottest the Earth has ever been since the extinction of the non-avian dinosaurs.

Worldwide temperatures were at least 5°C (9°F) higher than they are today. There were tropical jungles in Montana and England, complete with citrus trees, cashews, avocados, and lianas, and there were even denser rainforests and mangrove swamps across the American southeast. Alaska was covered in broadleaf forest, and much of Siberia was subtropical.

If anything, the changes were even more noticeable beyond the polar circles, with alligators swimming in the rivers of Ellesmere Island in the Canadian Arctic and forests of beech trees in Antarctica. This, bear in mind, is despite the permanent darkness of midwinter at these latitudes, regardless of the temperature. Indeed, we have fossilised tree rings from Greenland showing that it experienced the same seasons then that it does now, so it isn't as if the Earth had a different tilt. Evidently, so long as it was warm enough, the trees could happily go dormant for months at a time when the sun never rose.

The equatorial regions were, perhaps surprisingly, the least affected, with their temperatures increasing only marginally. This is likely because the global warming was primarily due to methane, not carbon dioxide. The former promotes the growth of stratospheric clouds over the poles that trap the heat there, but does relatively little where it is already hot. Either way, the fact that warm air can hold more moisture meant that the world's climate was much wetter than it is now, promoting rainforests far beyond their present reach.

Nonetheless, all of this did come to an end. Even once the PETM was over, temperatures initially remained well above where they are now, and the decline was slow, although not entirely steady. The cooling accelerated suddenly around 49 million years ago, at the end of the Early Eocene. One theory for why this happened is the so-called Azolla Event, when freshwater ferns in the Arctic Ocean built up to the point that they rapidly sequestered carbon as they died, bringing the CO₂ levels down to not much higher than they are now. 

After stabilising, there was a brief reversal of the cooling trend around 40 million years ago during the Eocene Climatic Optimum, when CO₂ levels rose again, possibly due to volcanism. This lasted almost a million years before temperatures resumed their downward trend, culminating in the "icehouse" of the Oligocene.

The name "Eocene" means "dawn of the recent", referring to the fact that it is the first time that the fossil seashells Lyell was studying at least begin to approach their modern forms. When we look specifically at mammals, however, it's arguably less appropriate, since recent types of mammal are comparatively rare. The Eocene is defined in large part by the fact that most mammals were of ancient kinds that no longer exist, not by the ones we are familiar with today.

Furthermore, the fact that this is both and early and a long-lasting epoch within the Age of Mammals means that there are almost no living families of mammals that stretch back beyond it. It was the time of the first shrews, moles, hedgehogs, rabbits, armadillos, rhinos, and monkeys, among many others. Indeed, the Early Eocene is marked in part by a rapid diversification of mammals, such that even the living orders - the larger groups into which we collect the families - often have their origins here. The oldest fossils of carnivorans, cloven-footed mammals, whales, and bats all date back to the Eocene. 

Moreover, these were generally in a minority at the time. Carnivorous mammals existed, but very few of them belonged to the main living group, the carnivorans, with other, vanished, animals taking their place. For mammals, this was often a time of beginnings and (to modern eyes) of oddities.

Over the next couple of years or so, I will be looking at as many of them as I can. Starting with mammals that lived on the European archipelago...

[Painting by Larry Felder, in the public domain.]