Eocene (Pt 2): In the Jungles of Europe

Heterohyus

During the Eocene epoch, Europe was very different from the way it is today. For one thing, it was still separated from Asia, with the Turgai Strait running between the two, roughly from what would now be the Black Sea to the Arctic Ocean. But, were you to look at a map without any modern day context, you probably wouldn't call it a continent, because it wasn't a single landmass, but a chain of large islands.

The biggest of these lay to the north, encompassing Scandinavia and the lands to the east. Just south of that, the second-largest was the one that would later become Britain, France, Germany, and some of their smaller neighbours. A smaller, but still sizable, Iberian island lay to the southwest, and a collection of low-lying ones occupied the south and east, with the more mountainous parts of that region having yet to form.

But, even if you didn't know the changes in the geography, if you could simply travel back in time to the Early Eocene and look around you, it wouldn't feel much like Europe. The continent was closer to the equator than it was now - northern Germany was about where Milan is today - but, even ignoring that, the world as a whole was much hotter. This is part of the reason for the islands, since there were no ice caps back then, but it means that our hypothetical time traveller would be, in almost any part of the landmass, standing in a jungle.

The temperatures went up and down as the lengthy epoch wore on, so the vegetation wasn't the same throughout it but, for the most part, there would be a lot of dense woodland, plenty of rich plantlife, and a lot of rain. Fossilised plants from the Middle Eocene of Germany, for example, resemble those now common in Southeast Asia. The wildlife that lived in these forests, however, would often have been unfamiliar.

Even early on, however, there were a few exceptions. For instance, the oldest known rodent fossils in Europe date to the very beginning of the Eocene. They belonged to a group called the ischryiomyids, and they appear to have entered the continent from North America, using a temporary land bridge running from Greenland to Scotland. Examples include the mouse-sized Microparamys and the marmot-sized Pseudoparamys, with the former proposed as a possible ancestor of the first dormice. Masillamys, from France, was quite common during the Middle Eocene, and has some limb adaptations that suggest it may have been good at digging, in much the way that modern voles are.

Ailuravus is also usually classified as an ischriomyid. It was native to Germany, although its own ancestors are also thought to be North American, and is known from some exceptionally well-preserved fossils, including impressions of the fur. From these, we know that it was about 40 cm (16 inches) in length, not counting the long bushy tail. It had sharp claws and teeth suitable only for soft foods, which, taken together with some fossilised stomach contents, imply that it climbed trees and fed on their leaves.

A different group, the theridomorphs, became common in the Late Eocene, diversifying into numerous species. Roughly squirrel-like in form, and presumably also arboreal, most, such as Treposciurus from Spain, were still only able eat soft food such as fruit. A few later forms, however, such as Elfomys from France and Switzerland, had begun to develop more resistant, grinding teeth that would have allowed them to eat tougher and more fibrous food.

In addition to plenty of fruit and leaves, jungles are also rich in insects, so it should be no surprise that many of the other small mammals living among the branches and in the undergrowth were insectivores of one kind or another. These included the amphilemurids, currently thought to be an early side-branch in the evolutionary line that would later lead to shrews and hedgehogs. 

Macrocranion is perhaps the best-known example, also being known from North America, probably having emigrated in the opposite direction to the rodents. It was about the size of a small squirrel, but with a more rat-like shape, with small eyes and large ears. The mobile snout has invited comparisons to elephant shrews, although this is almost certainly a case of parallel evolution. It most likely lived on the forest floor, with insects forming the bulk of its diet, although some larger species may have had a more varied predatory diet.

Its close relative Pholidocercus was, however, perhaps rather more peculiar. It was similar in size to a hedgehog, and had similarly spiny fur, although otherwise, the shape was more rat-like. Significantly, it had a horny plate on its head like an armadillo, which it may have used to help it burrow into the ground. The tail was also encased in small bony scales, a feature seen in some modern rodents from Africa, but not in anything else from its branch of the mammalian family tree.

Other small mammals of the day are more difficult to place. Apatemys and its close relatives Heterohyus and Carcinella belong to a group that survived the arrival of the rodents and other competitors at the dawn of the Eocene, having lived in Europe since the previous epoch, and that would go on to reach the next one before dying out. They were peculiar animals, about the size of a long-tailed rat, but living in trees and possessing unusually long front teeth and elongated second and third fingers. 

It is assumed that they had a similar lifestyle to the modern aye-aye, gnawing into wood to extract grubs and pull them out with their fingers. Analysis of the skull of Carcinella suggested that it had a brain that was, about as large as we would expect for a modern mammal of its size... which is to say, larger than that of most of its contemporaries. They don't appear to closely related to anything alive today, with one study suggesting that they may have branched off from the common ancestor of rodents and primates... so quite a long way back, then.

The pantolestans are similarly difficult to place, perhaps being related to carnivoran mammals, but, again, having diverged very early on. Buxolestes is a typical example, a small semi-aquatic animal known from Germany, France, and England. It had strong claws that may have helped it dig burrows, a muscular otter-like tail, and limbs that appear adapted for swimming. The teeth were large and somewhat flattened, suggesting that it may have fed on clams and snails, crushing their shells, although the one fossil well enough preserved to still have food in its mouth had been feeding on fish.

The tree-climbing herbivorous Merialus may be related, although how closely is unclear. It had unusually large and sharp incisor teeth, possibly for defence against predators, but possibly for clipping fruit from trees (or both, of course). 

Leptictidium may not even be a placental mammal at all, in the sense of being descended from the last common ancestor of living placentals, although it was certainly closely related - and some studies have placed it either with the primate/rodent branch or with the early African mammals. Either way, it was another odd-looking animal. It had a long, mobile, snout and a long tail, but what really would have stood out was that its hind legs were much longer than its forelegs. 

Normally, we would assume this meant that it moved by jumping, like a kangaroo or certain rodents. However, its ankles weren't structured for absorbing impacts on landing, which has led to debates as to how often it could really have done this. Instead, the assumption is that it must have been bipedal, using its long legs to run rather than leap, although it probably did a bit of both. Living mammals just don't do this (arguably, humans come closest), making it more like certain kinds of dinosaur than anything else. Analysis of the bones around the inner ear suggests that it had a highly developed sense of balance, and would have been very agile. It was likely insectivorous, although, at around 21 cm (8 inches) in height, it could easily have eaten small lizards or other mammals as well.

During the Eocene, however, Europe still had native marsupials. Or, at least, metatherians, members of the branch that today consists entirely of marsupials, since the term can only strictly be applied to descendants of the last common ancestor of all the living forms, and the ancestors of the European animals probably split off just before that common ancestor lived. Examples include Peratherium and Amphiperatherium, which were both around the size and shape of an opossum and likely had a similar, ground-dwelling, omnivorous lifestyle.

But, if we're talking about small, insect-eating mammals, there was another group present in Europe at the time that remains significant today. Bats first appeared very early in the Eocene, with some of their oldest known fossils coming from Europe. This isn't necessarily where they originated, however, since there are equally old fossils way across in Wyoming, and ones not much younger in Africa, Asia, South America and Australia, perhaps reflecting the benefits of flight when it comes to rapidly spreading across the world.

Even the earliest known bats would be instantly identifiable as such today, their form having changed relatively little since and leaving the question of what exactly they evolved from an ongoing mystery. Nonetheless, very early examples such as Archaeonycteris do have some differences from the modern sort, most notably in that they still had claws on their index fingers, rather than keeping them only on their thumbs. It lived across Europe, with some of the oldest fossils being from Portugal, and also reached as far east as India. The last known example comes from Dorset in England, living at the end of the Middle Eocene.

It had a wingspan of around 35 cm (14 inches) and so was fairly large for a bat, as well as having a long tail. The shape of the wings suggests it flew in open spaces, rather than navigating between densely packed tree trunks, a habit that resembles that of most modern European species. It did not, however, belong to any living family, and the best bet is that it was a member of an early branch that has since died out, and possibly one that predates the last common ancestor of the living animals.

Palaeochiropteryx may have been more closely related to modern bats. It was smaller, with a wingspan of perhaps 25 cm (10 inches), but, more significantly, had a wing shape that suggests it was capable of slow, agile flight. This would allow it fly close to the ground, dodging trees and bushes in the dense undergrowth of the semitropical jungles in which it lived. Being preserved in the same high-quality deposits, it is another animal for which we have fossilised stomach contents so we know that, like most modern bats, it ate insects - one had eaten a diet of moths, while another had consumed caddisflies.

Eating moths would suggest that the animals were already nocturnal, while the shape of their inner ear indicates that they could also echolocate, albeit perhaps not quite as effectively as modern species. Vielasia, from southern France, shows another key adaptation familiar from living forms: it is the oldest known bat known to have lived in caves.

All of these animals are relatively small, and while a time traveller would surely note some of them running about on the tree branches, their eye would likely be more quickly drawn to the larger animals of the day. Next time, I will take a look at some of herbivores that lived in the jungles of Eocene Europe...

[Photo by Chris Woodrich, from Wikimedia Commons.]