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NOTE: This piece is under construction!
An armed man on a rearing horse – this is the very image of martial valor. And it’s been done and redone by a great many artists over the centuries. The soft, glowing colors, stately battle gear and upraised armaments do set quite a scene, but the subject in these sorts of compositions – for me at least – has always been the horse itself, with muscles strung taut, nostrils flaring and hooves ready for the charge. Although the age of cavalry charges and horse-drawn plows has long since past, the horse remains a potent cultural and historical icon – more than 40,000 books have been written on the subject, from Xenophon to Michelangelo – and it might be worth investigating how this remarkable animal came to be. Happily, fossil horses are abundant and we can answer many questions about horse prehistory with some measure of certainty.
The horse fossil record is often seized upon by writers of elementary textbooks as a classic example of how paleontology can help inform our understanding of long-term evolution. Horse evolution, in these instances, is usually portrayed as a sort of evolutionary ‘procession’, with the humble Hyracotherium (invariably described as being “about the size of a fox terrier”) shunted off to the rear and the high-shouldered and gallant Equus leading the parade. Long term readers of this blog will know that evolution is a tremendously bushy affair, and that these sorts of neat, linear progressions are simplistic. Nonetheless, it does capture certain important trends we shall discuss in the succeeding paragraphs.
How do horses fit in, taxonomically speaking, with respect to other hoofed animals?
All hoofed mammals fit into the cladistic group, Ungulata. Hooves are, essentially, the modified tips of toes – and differences in the structure of the foot can be used to divvy up the ungulata into two broad categories: even toed ungulates and odd toed ungulates. Animals in the first category sport two major weight-bearing toes – the third and fourth toes of each leg. This group includes camels, goats, cattle, deer, pigs and a number of other hoofed animals. Odd toed ungulates, on the other hand, support themselves, for the most part, on one toe per foot – the third toe. This group includes horses, tapirs and rhinos.
There is evidence to show that, among the ungulata, horses share a more recent common ancestor with tapirs and rhinos. For one, the odd-toed ungulates all have an extended caecum (an outpocketing of the large intestine that is found in greatly reduced form in humans) that helps ferment and digest the cellulose in grass. They also share striking similarities in the anatomy of the teeth and the ankle bones.
Equus is the only surviving genus of the family equidae, and it includes 3 species of asses, 3 species of zebra and the horse.
Hyracotherium to Equus
Hyracotherium is the earliest known fossil horse. This unassuming animal was about 20 centimeters high at the shoulder and was probably a browser, seeking out its fill amidst the leaf-shrubbery rather than out on a grassy plain, as modern horses do. It lived around 50 million years ago, just 15 million years shy of the extinction of the dinosaurs (the Eocene). It spread throughout the Northern Hemisphere and was, by most counts, an evolutionary success story. But it differs from the modern horse in a number of key respects:
1) The modern horse dwarfs the dog-sized Hyracotherium.
2) Hyracotherium has four toes, whereas the horse sports a single sturdy toe/hoof on each foot.
3) the low crowned teeth of Hyracotherium imply a diet of soft leaves. shoots, nuts and fruits. Modern horses, however, are adapted to high-fibre grasses
4) the brain (specifically the frontal cortex) of the modern horse is considerably larger than that of Hyracotherium.
Why did horses get bigger?
Well, for starters, the fossil record does not tell us a story of uniform progression towards larger body size. For example, some of the distant descendants of Hyracotherium, like the Pliocene horse Nannipus, were even smaller than the earliest horses. The body size of Equids remained roughly constant for several million years before large horses appeared on the scene.
So what selective pressures might have led to an increase in body-size over time? The typical explanation has to do with large body size being a line of defense against predators on the open plains. It may also have to do with the shift in diet from high-quality forage to nutrient-poor high-fibre grass (which was roughly concurrent with the spread of open grasslands throughout the world).
In terms of energy derived per unit bulk, horses cannot process food as efficiently as even-toed ungulates that chew their cud and have a modified stomach with four chambers. The fermentation process that digests cellulose (with the aid of symbiotic bacteria) in the caecum of the horse is almost exactly mirrored in the proverbial “four stomachs” of cows and goats. While they cannot match even-toed ungulates for energy efficiency per unit mass, they can push a greater amount of material through their digestive system in a given amount of time. They are also specially adapted to subsisting on low-quality grasses which their even-toed counterparts could not survive on for long.
Larger animals are able to conserve energy better (on account of their greater ability to retain heat compared to smaller animasl) and this might have been driving force towards greater size. Body size may have also contributed to an increase in running speed.
Hooves and legs
As horses became more adapted for life on seas of rolling grass, they underwent a number of crucial anatomical changes. The length of the bones of the foot increased (a trend we saw in earlier posts on cats and theropod dinosaurs) and the number of hooves decreased, with the third digit becoming more pronounced: these are both adaptations for a cursorial lifestyle. The arrangement of tendons in the lower leg and the connections between the leg bones work to store elastic energy and reapply it with each stride (the so-called “springing step”). Horses have even evolved a way to expend less energy standing up than sitting down.
Grasses are hardy plants and have evolved various means of protecting themselves from plant predators. They inflict heavy wear and damage upon the teeth of herbivores. And, as a consequence, herbivores like the horse, have evolved high crowned teeth, covered in cementum with folds of enamel (hypsodonty) to deal with the tough food. Many of premolars changed to molars. There is also a long gap between the incisors and the premolars that is absent in the very earliest horses, like Hyracotherium. This adds distance between the nose and the eyes, allowing the horse to keep an eye out for predators whilst grazing. Rather fortitiously, it also provides space for the insertion of a bit, an important part of horse riding.
The Horse brain also increased in relative size over the course of the last 50 million years, although the precise reasons for this change are uncertain. It may reflect a major increase in intelligence (a notoriously difficult concept to define in animals to begin with) or may be related to the increasing complexity of the sensory apparatus of the horse.
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