Sunday, 16 January 2011

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]

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