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| Anoplotherium |
Most, however, were nothing we would recognise today. There were as yet no cattle, antelopes, deer, or pigs, and many of the creatures that did live that far back left, so far as we can tell, no living descendants.
One group that may be an exception are the anthracotheres. These migrated to Europe from Asia and have been proposed as possibly including the ancestor of hippopotamuses (although this has been questioned). This is in part due to a physical resemblance between the two. For example, they did not have hooves and, while they had the basic anatomical arrangement seen in cloven-footed animals, they walked on all of their toes. There was a full set of five of these on their front feet, and four on the back, albeit with those that would form the hoof in later animals taking most of the weight.
Anthracotherium itself, first discovered in France, but known to have lived across Asia as well as Europe, was perhaps the most hippo-like of all the Eocene anthracotheres. It was about the size of a modern pygmy hippo, roughly two metres (6 feet) in length with a heavy build and short thick legs. However, the head was long and narrow, perhaps more horse-like than anything else despite the short neck. The teeth were strong with sharp cusps, suggesting a mixed diet of leafy browse, fruit, and grazing. Elomeryx was another common anthracothere in Europe, smaller than its relative and possibly more agile. Its teeth were more adapted to eating tough plants, and like its larger relative, it was probably semi-aquatic, reinforcing the similarity to hippos.
The choeropotamids have had a complex taxonomic history, sometimes being grouped together as relatives of the anthracotheres, and sometimes split into different groups or given different names. As currently defined, it probably isn't a real evolutionary unit, but a collection of similar forms, with much of the debate being about where the dividing lines should go.
Choeropotamus itself, which by definition must belong to the family, was a pig-like creature in both size and general shape, and, like the anthracotheres, had a long narrow head. Distinguishing features include the lower canine teeth taking the form of incisors, and the tooth immediately behind forming a tusk that almost resembles a regular canine. Taking this and other features of the teeth into consideration, it was probably omnivorous, perhaps with a diet similar to modern pigs. The name translates as "river pig", although whether it was genuinely semi-aquatic, as was thought when it was first described in the 19th century, is highly debatable.
We only have the skull and ankles of Choeropotamus, limiting our knowledge of its overall form. We have rather more of Amphirhagitherium, known from England, France, and Germany. It was smaller than Choeropotamus, and had a relatively slender build, accentuated by the fact that, unlike many artiodactyls today, it still had a long tail. It is likely to have had a similarly omnivorous diet to its larger relative, but with more of a focus on herbs than leafy bushes. It has the same tusks as well, which might indicate at least some meat in its diet, but which could, as in Choeropotamus, also have been used in fights for mates or for defence against predators.
Depending on the taxonomic definition used, the choeropotamids may be unique to Europe. They are not alone in this, with two other significant families being found only on the continent. The anoplotheres (literally "unarmed beasts") have an unusual mix of advanced and primitive features. The shape of their teeth resembles that of ruminants, which would enable them to eat relatively tough plants. However, all ruminants (and, indeed, pigs and choeropotamids) have a diastema, a gap between the front and back teeth that allows the cheeks to manipulate vegetation as it is being eaten. Anoplotheres did not, and still had relatively normal canine teeth as well.
Diplobune is one of the smaller members of the family, although still weighing in at around 20 kg (44 lbs). It had three toes on each foot, still relatively elongated, and shaped such that it probably still partly walked on its soles, rather than tip-toe, as hoofed mammals do. This unusual shape was formerly interpreted as indicating that the animal was semi-aquatic, using its feet to paddle, or at least to support itself on soft muddy river banks. However, the internal structure of its ears is thought to rule out any possibility of underwater hearing, as well as indicating that it was rather slow-moving. A flexible index finger is an unusual feature; one interpretation suggests that this might have been used as a thumb, implying that the animal might have been able to climb trees to get at the leaves there. If true (which it may well not be), this would certainly be unusual for an artiodactyl.
Anoplotherium is the largest known anoplothere, as well as the first to be named, all the way back in 1804. It was at least the size of a sheep, and probably quite a bit larger, with most estimates suggesting that it weighed at least 230 kg (510 lbs). It has been suggested that the shape of the snout indicates the presence of a long tongue, similar to that of giraffes, and presumably used to pull at small branches and leaves in the same way.
It had just two toes on each foot (and was formerly interpreted as being related to camels on these grounds) and retained a long muscular tail, perhaps reminiscent of that of a kangaroo. The shape of muscular attachments elsewhere on the skeleton and of some of the joints suggests that it may have been able to stand on its hind legs, allowing it to browse from the lower branches of trees. The limbs are too short for it to have been able to do this very effectively, so it likely braced itself with its forelimbs while doing so but even this would have enabled it to reach food that no similar European animal of the time could have done.
The xiphodontids were another group unique to Europe. Appearing around 40 million years ago in the Late Eocene, they belonged to a group regarded as more advanced than the earlier forms. Xiphodon was the size of a small gazelle, perhaps standing around 80 cm (2'7") at the shoulder. It was, however, one of the largest, with others, such as Haplomeryx, perhaps only reaching half that height.
Its placement as a relatively advanced form is due to the shape of the teeth; the name means "sword-tooth" and refers to the sharp crescent-shaped cutting blades on the molars that would have been ideal for cutting up leaves. Like the anoplotheres, they lacked a diastema, and they had distinctly cloven feet, holding the side toes off the ground. Xiphodon, the only genus for which we actually have limb bones, had long legs and a slender build that would have made it a fast runner, perhaps resembling miniature llamas, although they were not close relatives.
The closely related amphimerycids were, perhaps, even more changed from their ancestral form. They had similar teeth, and walked on only two toes of each foot, but they had also developed the cannon bone seen in living ruminants, further adapting the limb for running. Only two genera are known, both unique to Europe. Both were tiny animals, at least by the standards of artiodactyls. Amphimeryx, which was the larger of the two, is thought to have weighed around 1.5 kg (3 lbs 6 oz.), about the size of a rabbit, while Pseudamphimeryx was even smaller. They had large eyes and a narrow snout, somewhat resembling the modern chevrotains.
Despite the similarity of the limb bones, it's unlikely that the amphimerycids were related to, let alone ancestral to, the modern ruminants; if anything, they may be closer to the camels. The similarities are probably just a case of parallel evolution. Nonetheless, it is likely that the first ruminants did appear towards the end of the Eocene although, since the defining feature is the four-chambered stomach, which obviously wouldn't fossilise, we don't know for sure.
The best candidate for an early ruminant from Europe is Gelocus, a small hornless animal which had limbs resembling those of musk deer and could, conceivably, be related to their ancestor. Bachitherium is another possibility. It was larger, standing around 60 cm (2 feet) tall at the shoulder and, while otherwise similar to Gelocus, had tusk-like upper canine teeth, but no incisors in the upper jaw - also a point of similarity with musk deer. Other differences rule out a direct relationship, and it was probably a side-branch in evolution, the only known member of its family, and may or may not have been an actual ruminant.
The earliest known artiodactyl is Diacodexis. Although some of the earliest fossils are known from Portugal, some of similar age are also known from North America, as well as Asia. It seems unlikely that a single genus could spread so rapidly early on, so it's entirely possible that the primitive nature of the animals has made it difficult to distinguish different genera on either side of the Atlantic. Either way, however, we cannot, at present, say on which continent the artiodactyls originally appeared, since the identity of their immediate ancestors is not clear, either.
On the other hand, it seems likely that the first artiodactyls resembled Diacodexis, since whether it is one genus or two (or more) there is no doubt that it lived very early on - almost at the beginning of the Eocene - and is distinctly primitive. It did already have the trend towards putting its weight on the third and fourth toes, as cloven-footed animals do, despite still having five toes on the fore-feet and four on the hind ones. It was small, about the size of a rabbit, with a short but slender snout ideal for rooting around in leaf litter, while the teeth suggest that it still had a relatively omnivorous diet. The hind-limbs were noticeably longer than the front ones, increasing the resemblance to a rabbit and likely indicating that it was capable of jumping. Analysis of the ears implies that it was best at hearing high-frequency sounds.
There were, however, many other herbivores in the island forests of Eocene Europe, and next time I will take a look at some of them.
[Photo by "ghedoghedo" from Wikimedia Commons.]
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