Oligocene (Pt 3): From Musk Deer to Hell-Pigs

Entelodon

The Grande Coupure that marked the beginning of the Oligocene in Europe did not have as dramatic an effect on the cloven-hoofed animals as it did on some other mammalian groups. But the change was nonetheless noticeable, with several primitive forms that had once inhabited the continent suddenly dying out as it merged with Asia. Others, however, survived through the break, and the absence of their earlier competitors may even have helped them prosper.

These include the gelocids, which first appeared close to the end of the previous epoch. Few of the known fossils of these animals are in good condition, and there is some debate as to whether they are a true group of animals at all, or just a vague term used to collect similar-looking creatures that we can't place elsewhere. That aside, we can at least say that they physically resembled (but were probably not related to) musk deer. That is, they were relatively small, hornless animals with long legs suited for running fast, but lacking the large fang/tusks that mark true musk deer. They did well enough that some, such as Pseudogelocus, are known not only from France and Germany, but also Mongolia, suggesting that they crossed over in the opposite direction to most other mammal groups.

What's the Closest Living Relative of Whales?

(For the TLDR crowd; just read the last paragraph).

It's been known for a long time that, at the highest level, mammals can be divided into three evolutionary groups based on their method of giving birth: marsupials, the egg-laying monotremes, and the placental mammals. That last group contains around 95% of all living mammal species, including everything from wolves to dolphins and moose to monkeys.

Given that wide variety, it's less obvious how we should divide up the placental mammals into smaller groups that genuinely reflect their evolutionary relationships. It's obvious that, say, cheetahs are members of the cat family and, at a slightly higher level, it's unsurprising that, for example, the dog and bear families are reasonably close relatives of one another. But once you get much higher than that, it becomes less so.

Miocene (Pt 2): Before There Were Mice

Heteroprox

After a brief cold snap at the very dawn of the epoch, the world of the Miocene warmed rapidly. Europe became, if not truly tropical, at least subtropical, with the interior covered by great forests of oak, laurel, and cinnamon, with magnolia and figs joining pine trees in the highlands. Along the coasts, the hot, damp, climate encouraged the growth of mangrove swamps and palm trees as warm sea currents flowed in from the Indian Ocean - still connected to the Mediterranean at this time. With no ice caps at the North Pole, and relatively few at the South, sea levels were much higher, and parts of continental Europe may, in those days, have still been islands. Certainly, Aqutaine in south-western France and the lower Rhône valley in the south-east were shallow bays stretching some way inland, as was what is now the Tagus valley in Spain and Portugal.

This rich and verdant landscape was home to a wide range of animals, many of them survivors of even earlier times. Many of these, such as the tapirs, didn't survive long in Europe, but a great many did, with musk deer, pigs, and rhinos dominating the herbivorous fauna, and animals less familiar to modern eyes taking the lead among the large carnivores.

But then, as now, the great majority of mammal species were small. While the sight of Diaceratherium rhinos wallowing in the lush swamps of the Swiss shoreline is the sort of thing that would draw the immediate attention of a time-travelling tourist, there was also plenty going on underfoot. Yet the two most common groups of small non-flying mammals that we have in Europe today - the mice and the voles - did not yet exist. So what was there?

Grazers and Browsers - and how to tell them apart without watching

Wildebeest are grazers - note the squarish muzzle

One advantage of studying the fossils of prehistoric mammals, as opposed to dinosaurs, is that mammals are still around today, while non-avian dinosaurs aren't. That gives us the ability to compare fossil species with living ones, and be fairly confident that our comparisons make sense. That's not to say, of course, that we can't infer quite a lot from the shape and structure of dinosaur bones, and work out details of their lifestyle and habits. But there's nothing much like non-avian dinosaurs around today, so there will inevitably be some guesswork involved when we do - educated guesswork, to be sure, but guesswork none the less.

Although the same can be said of fossil mammals - especially the stranger ones - in many cases, we can be more confident that our educated guesses are likely to be accurate. For example, sabre-tooth cats were, well... cats. So we can look at, for example, the proportions of their limbs, compare them with living cats, and deduce whether they were more like, say, jaguars, than they were like  leopards. Because leopards, jaguars, and sabre-tooths are, in many respects, quite similar, it's pretty likely that inferences drawn from the first two will apply to the third, unless there's some good reason to suppose otherwise. We know what cats are like, and sabre-tooths were cats, so that tells us a lot.

And what about herbivores? Herbivory includes a range of different diets, such as animals that feed mainly on seeds, or fruit. But large mammalian herbivores tend to have two possible feeding strategies: grazing and browsing. The best way to tell the two apart would be by examining their dung, and, failing that, the structure of their digestive systems could well be helpful. Neither, of course, are possible, if all you have is a fossil skeleton, but, fortunately, there are other clues we can examine.

Why Cows Have Four Stomachs

(If all you want to know is what animals do, and what animals don't, have a four-chambered stomach, the shorter answer is here).

The stomach is an organ found in almost all vertebrates. Although there is considerable variation among the different groups, and there are some fish that don't have one at all, in general it has two functions. Firstly, it helps to store food for later digestion so that you don't constantly have to be eating, and secondly, it begins the digestion of food both by physically grinding it up and by mixing it with acid and digestive enzymes. Sometimes these functions are separated to some extent - for example, birds have a large crop for food storage, and a smaller true stomach below it that digests the food, and includes a muscular, grinding gizzard (a useful thing when you have no teeth). But the most complex stomachs of all are found in mammals.

A cow does, indeed, have four stomachs. Or, at least, it has a stomach divided into four separate chambers, which amounts to the same thing. Nor, of course, are cows alone in this. It's a feature found throughout the cattle family, which is a fairly large group consisting of over a hundred species - most of them antelopes, although it also includes the sheep and goats. The cattle were not the first family to evolve the feature, and we know that because its also found in all their close relatives, including such animals as deer and giraffes. In fact, the only truly cloven-hoofed animals that don't have four stomachs are the pigs and peccaries - which is why they aren't kosher.

In fact, we can group all the mammals that have this four-chambered stomach together. This group are called the "pecorans". Why not just "ruminants"? We'll get onto that later.

The Origin of the Cloven Hoof

Foot of a giraffe

The shape of the feet in mammals falls into one of three categories (ignoring those that don't really have feet, such as dolphins). Plantigrade animals, such as humans, walk on the soles of the feet, with both the heel and the toes touching the ground. This might seem the obvious way of doing things, but its actually fairly unusual among large mammals. More common is the digitigrade stance, which we can see in, for example, cats and dogs. Here, the animal effectively walks on tip-toe, with the ankle held well clear of the ground. The part of the foot in between the ankle and the balls of the feet is generally elongated and narrow, so that the ankle looks rather like a backward-pointing knee.

This pattern is thought to allow the animals to move more quickly, making the limbs longer and more flexible, with an extra joint that can bend in the direction of motion. This is usually more important in the hindlimbs than the forelimbs, because they are the ones that push against the ground to propel the animal forwards, while the forelimbs are more important for braking. As a result, there are a few animals, such as raccoons, that have evolved digitigrade hindlimbs, but never got around to doing the same with the forelimbs.

The third possible stance is called unguligrade. These animals go even further than the digitigrade ones, standing only on the very tips of their toes, like a balancing ballerina. Such animals are usually hoofed, and an obvious example is the horse. Yet, while the evolution of horses and their single hooves has been described in many places, the origin of the cloven hoof is perhaps less well known. This, despite the fact that are far more animals with cloven hooves than there are species of modern horse. But then, for the same reason, the story is also rather more compliacted.

The group that these animals belong to (the "even-toed ungulates") includes ten families, although, admittedly, not all of them actually have hooves. These families can be placed into five broader groups, as follows:

  Other cloven    Chevrotains     Hippos
 hoofed animals                   etc.
      ^               ^            ^         Pigs
      |               |            |         etc.
      2               |            |          ^       Camels
      -----------------            |          |        etc.
              |                    |          |         ^
              4                    |          |         |
              ----------------------          |         |
                         |                    4         2
                         |                    |         |
                         ----------------------         |
                                    |                   4
                                    |                   |
                                    ---------------------
                                              |
                                              |

When it comes to fossil animals, it's often difficult to determine exactly how they walked, since skeletons are rarely found in a perfectly articulated form. When they are mounted for museums, a fair degree of guesswork has to go into how the bones all fit together, and sometimes those guesses are going to be wrong. Fortunately, with mammals, unlike older and arguably stranger, fossils, like those of dinosaurs, its possible to compare the animals with living forms that look fairly similar. Comparing such things as the proportions and shapes of the limbs can reveal a lot of information about how mammals long vanished from the world moved and lived when they were still alive. An article recently published in the Journal of Vertebrate Palaeontology used this approach to examine some of the details of how the cloven hoof evolved.

Like the ancestors of horses, the ancestors of cloven-hoofed animals originally had five toes. Among their living relatives today, this is only true of the two species of hippopotamus, an animal whose large size makes it very different from its distant ancestors. Indeed, the living animals whose feet most resemble those of these early creatures appear to be the dogs.

For the most part, their feet were fairly typical of other early mammals, although the thumb/big toe was already quite small, and some of the bones do have a similar shape to those found in dogs (and, to a lesser extent, cats). It seems likely that, just as dogs are fast running animals because they want to catch their dinner, these early ancestors of the cloven hoofed animals would have been fast running because they wanted to avoid becoming dinner.

If this is right, then the first animals of this group would have lacked many of the distinctive features that the group has today. Their toes probably weren't strong enough to support their body weight without some assistance from the balls of the feet; in other words, they would have been digitigrade rather than resembling modern hoofed animals. They may even have had pads on the soles of their feet, as dogs do, although that's the sort of thing that can be difficult to know for certain.

These animals, then, would already have been quite good at running. The next step was the loss of the thumb/big toe. This was clearly a useful adaptation, because it appears to have happened more than once. We can tell this, because hippos, which still have all five toes, are not the most primitive members of the group. This means that the evolution of a four-toed pattern must have happened at all of the points marked '4' on the diagram above - and this ignores some extinct groups that don't appear to be the direct ancestors of anything around today.

In addition to the disappearance of the thumb, the index finger/2nd toe and little finger/5th toe also become much shorter. At the same time, the other two toes become stronger, able to take more of the weight of the animal, and the muscles that, in most mammals, move the toes apart are replaced by a tough ligament. There are also changes in the structure of the wrist or ankle to accommodate the changing arrangement of muscles.

The most obvious modern example here is the pig, which has a true cloven hoof, but also has two additional toes on each foot that do not reach the ground. So it should be at this stage that fossil mammals evolved from a more typical digitigrade stance to something at least much closer to the unguligrade one. At least one group of extinct mammals, one thought to be related to the ancestors of camels, does quite closely fit this description.

Some other extinct four-toed animals in the group, however, are a little different. Their feet look more like those of their primitive ancestors, with only some of the changes found in their living four-toed counterparts. Their joints are not quite the hinge-like shape found in pigs, although they are certainly more so than in dogs, and some of the other changes also seem less extreme. It would seem that this represents the actual change from digitigrade to an unguligrade stance, a stage reached with the three-toed Mesohippus in the evolution of horses. It seems likely - although far from certain - that these animals still had a foot pad, and not hooves, and that (unlike pigs) they used all four toes to walk.

Its obviously tempting to assume that these animals were the transition between the 5-toed and "more evolved" animals, such as pigs. There's probably some truth in that, but it can't be the full story, because at least some of these groups never evolved into anything else. Perhaps the best known such group, the Protoceratids, survived with more or less this arrangement for around 40 million years - about two thirds of the total timespan since the extinction of the dinosaurs. So its obviously a pretty effective way of moving about, something that works well in its own right, without being just a necessary transition to anything 'better'.

Nonetheless, in three groups, the outer two toes became more radically reduced, leaving only two true toes - the middle finger/3rd toe and the ring finger/4th toe - on each foot. Two of these groups are shown in the tree above, marked with a '2', but the other is extinct, and exactly where they belong really isn't clear.

Quite what happened to the other two toes varies quite a bit between different groups, but in all cases at least some of the bones disappear altogether. For example, in goats, only the very last bone in each toe survives, as a sort of small dewclaw at the back of the foot, while in camels, the toes vanish entirely. At the same time, the limbs became longer, especially between the ankle ("hock") and the foot, allowing a further increase in running speed. The shape of the joints also changed, to allow for the newly vertical posture of the foot, and to create hinge joints more suitable for fast running.

Again, the pattern isn't entirely neat. While there is a lot of similarity between the way that the cloven hoof formed in the three groups, there are also some differences, reflecting the fact that this happened more than once, instead of being a simple linear progression. In the case of camels, for example, the animals lost the unguligrade stance of their ancestors, and went back to being digitigrade, with a foot pad instead of a hoof. Flat splayed feet are, after all, more use in a sandy desert than sharp hooves.


[Picture from Wikimedia Commons]