Eocene (Pt 4): Ancient Beasts of the European Archipelago

Propalaeotherium

The story of the evolution of the horse is one of the most commonly cited examples of evolutionary trends, often illustrated by a series of increasingly horse-like animals with an ever-reducing number of toes. The animal typically shown at the start of that series is, depending partly on the age of the picture, either Hyracotherium or Eohippus.

Hyracotherium was long regarded as the earliest known member of the horse family. In recent decades, it has become apparent that it wasn't really a horse, in the sense that modern horses don't descend from it or its relatives, and today we call the family it belonged to the palaeotheres. The North American Eohippus, on the other hand, despite long being thought to be identical to Hyracotherium, probably is a horse. The confusion between the two means that it's often difficult to tell which is being referred to in older sources. 

Fortunately, there are other palaeotheres that, while less famous, have been much better studied. Hyracotherium is one of the earliest forms, somewhere between the size of a cat and a fox, and was originally discovered in southern England. Its contemporaries include Eurohippus from Germany (for which we even have the fossil of a pregnant mother) and Propaleotherium, which was more widely distributed across what was, at the time, the European archipelago. The latter was one of the larger palaeotheres of its day, standing as much as 60 cm (2 feet) tall at the shoulder. Remarkably, we have fossilised gut contents from one specimen, which indicate that it was a fairly indiscriminate eater, feeding on a mix of leaves, fruit, and seeds.

The palaeotheres may not have been horses even in a general taxonomic sense, but they were more closely related to horses than to anything else alive today. Although we do know of some from Asia that appear to have been less successful in the long run, the great majority lived only in Europe. There, isolated from the outside world, they continued to evolve separately from their equid kin in North America. While some reached as far as Greece, they were, by this point, mostly restricted to western Europe, with a particularly large number of unique species found on the island that would later become Spain and Portugal.

These later palaeotheres became much larger than the early forms. While the smallest species of Plagiolophus were no larger than Hyracotherium, the largest may have reached up to 150 kg and stood about a metre (3 feet) tall at the shoulder. Palaeotherium, for which the family is named, was larger still, with one estimate for the largest species putting it at 700 kg (1,500 lbs), which would make it larger than most modern horses.

While the earlier species had four toes on the front feet and three on the hind ones, these later animals showed a similar reduction to their equid kin, so that Palaeotherium, for example, had three toes on each foot, which, together with other features of the limb bones, suggests that it was becoming better suited to running. The diet varied significantly, with some preferring to eat relatively soft foods as their ancestors had done, presumably to extract maximum nutrition, while others were also able to eat tougher food, such as twigs or hard fruit and nuts. The ability to eat abrasive leaves may have helped Plagiolophus to become the only palaeothere to survive the sudden cooling of the climate at the end of the Eocene and survive halfway into the following epoch.

Palaeotherium was, incidentally, the first unambiguous prehistoric mammal to be discovered, all the way back in 1798. This explains the scientific name, which simply means "ancient beast". When George Cuvier formally described it in 1804, he caused something of a stir since he insisted that its existence proved that extinction had to be possible; it had previously been believed that God would never allow such a thing to happen. (Since some people were still unconvinced, this would lead, in the early 19th century, to naturalists making a serious effort to find where all the mastodons were hiding in western North America).

Lophiodon is, like the palaeotheres, a perissodactyl, a member of a group of medium to large herbivores that walk by putting most of their weight on their third toe. Unlike the palaeotheres, however, it is not closely related to any of the modern perissodactyls. It is the best-known member of its family, which, so far as we know, never expanded beyond the islands of western Europe, dying out long before they united with Asia at the end of the Eocene. 

In life, it would have been a fairly strange-looking animal, with a comparatively narrow snout and a domed forehead. They were relatively heavily built, with muscular limbs and, like early palaeotheres, four toes on the forefeet and three on the hind ones. They were long considered to be somehow related to tapirs, which at least have a similar body form, but more recent analyses show that they were early relatives of the chalicotheres, large knuckle-walking horse-headed clawed herbivores that would be more prominent in later epochs.

A great many species of Lophiodon are known, with the smallest about the size of a fox, but some were much larger. The biggest, L. lautricense, has been estimated to have weighed possibly as much as two tons, similar to a rhinoceros. Assuming this is true, and the remains are incomplete, so it's difficult to know for sure, it would likely have been the largest land mammal in Europe at the time. Its heavy build makes it unlikely to have been especially fast or graceful, with some analyses suggesting that it may have been partially aquatic, like a hippo. It seems to have browsed on leaves, like the palaeotheres adapting to a tougher diet as time went on. Nonetheless, it and its relatives were apparently unable to adapt fully to the changing climate of the Late Eocene, dying out well before the cold snap that marked its end.

Crivadiatherium is less easily placed. A large-bodied rhinocerous-like animal with a small horn on the snout, it is not a rhino, or even a perissodactyl, but an embrithopod, being related to species that are otherwise mostly known from North Africa and the Middle East. With fossils known only from Romania, it may have crossed over from Turkey to the easternmost islands of the European archipelago and never reached further. It seems to have lived in lakeside environments and may have been a distant relative of elephants.

The subtropical jungles of Eocene Europe were also home to primates. At this early time, none of them belonged to any living families but we can at least say that they were closer to lemurs than to monkeys, although they likely represented one (or more) early side-branches from the line that led to the former. 

Europolemur, from Germany, is a typical example. In many respects, including its body size, it resembled a living lemur and had, for example, an opposable thumb and a grooming claw on the hind feet. On the other hand, it lacked the toothcomb that is a primary distinguishing feature of living lemurs and lorises, although the general pattern of the teeth is the same as that of many living primates. Darwinius is another German species and likely a close relative. At the time of its discovery, it was hailed as a possible missing link between the lemur-like and simian primates, but this has been disputed since, and no longer seems to be a common opinion.  Whatever it is, an analysis of its gut contents showed that it ate mostly fruit, along with plenty of leaves, but no insects, suggesting a diet similar to that of living lemurs.

The omomyids were much smaller primates, with one of the larger forms, Necrolemur, still weighing only around 200g (7 oz.) They had large eyes, suggesting a nocturnal lifestyle, short faces and long tails and are thought to be related to tarsiers, rather than lemurs. The shape of its hips and ankles suggests that it would have been well adapted for leaping from tree to tree, while its teeth suggest a diet heavy on fruit, although probably supplemented by at least some insects. 

But of course, the island jungles of Eocene Europe were not just home to herbivores and insectivores. Many mammals were carnivorous, and they too, belonged to families that are no longer with us today. Next time, I will look at some of them.

[Photo by Frank Vincentz, from Wikimedia Commons.]