Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"

I remain with the prehistoric animals I reconstructed for the Bishop Museum of Science and Nature. The Bishop Museum of Science and Nature in Bradenton, Florida, is the largest natural and cultural history museum on Florida’s Gulf Coast. Its mission is to inspire the joy of discovery and wonder for all ages through excellence in stewardship and engagement.

These illustrations were done for a physical and virtual exhibition on animals found in their Nebraska Diggings of about 35-30 million years ago, around the changeover from the Eocene to the Oligocene Periods. The time was one of transition: global climates became drier, forests gave way to grasslands, and ancient groups of mammal gave way to more modern lines.

In two previous columns I showed you the huge Megacerops, of a primitive family, related to horses, now extinct, and the lightly built Hyracodon, one of the first rhinos. So now let me show you one of the first true horse species, that was also around in the same time and place. These animals are Mesohippus bairdii. The name means 'Baird's middle horse'. Mesohippus was intermediate in the sense of being quite a bit more horse-like than the very first horses, Eohippus ('Dawn Horse'), but still not quite as horse-like as its later successors and descendants. It was quite small, standing 60 cm/24" at the shoulder. The genus Mesohippus was successful, persisting for about 10 million years from the middle Eocene to the early Oligocene periods, 40-30 million years ago, throwing up several different species. They lived over much of the continent now called North America.

A Tale of Toes

A very interesting feature of the evolution of horses, is the changes that happened in the toes. For all mammals, the original 'primitive' condition is having five functional toes and/or fingers on each hand and/or foot. So we humans are actually 'primitive' in that regard! This should clue you in that 'primitive' doesn't really mean 'backward' or 'unevolved', but simply that it means similar to the situation or configuration of original members found in the fossil record. All early mammals, the ones that lived during the Mesozoic, which were the ancestors of all modern mammalian groups, had five digits per limb. Today, five digits per limb are still found in for instance most primates, bats, rodents, many carnivores, and others. But in several mammal groups, there has been a reduction in the number of digits, until at the extreme, modern horses have only a single toe per foot. Moreover, the tip of each toe is completely encased in a hard, horny hoof. But horses evolved to this condition over a very long period of time. Their earliest ancestors were the phenacodonts, a group that survived the great extinction event that killed off the non-avian dinosaurs, about 65 million years ago at the end of the Cretaceous, and diversified in the subsequent Palaeocene Period. The phenacodonts had five toes per foot. They gave rise to the first true horses in the Eocene period, about 52 million years ago, in North America and Europe (which were close to each other, periodically connected, at that time).

The very first horses, such as abovementioned Eohippus, had five toes on each foot, but the on the front foot, only four had hooves, and only the middle three toes on the hindfoot had hooves. These hooves were small and pointed. The outer, hoofless toes had been much reduced in size. The undersurfaces of the toes were padded like those of dogs and cats. The feet and limbs were quite flexible and able to move in different planes. These original horses lived in moist rainforests. Their small size enabled them to skulk about the dense vegetation of the forest understory, and their toes could splay out a bit to give them better traction on soft, moist mulch and soil. They could put out short bursts of speed and long leaps to escape predators, but weren't built for sustained running.

Mesohippus shows a further reduction in toe number and size. The two outer toes on each foot had been lost, so that all the feet were three-toed. But there was also a reduction in the size of the side toes. The middle toe of each foot bore most of the weight; the side toes just barely touched the ground but as the substrate softened, the feet would sink in more and the side toes would then splay and bear more of the bodyweight. Though the toes still had pads, the hooves were larger than in Eohippus. So Mesohippus was starting to adapt itself to running on the flat, grassy plains that were increasingly invading the scene.

From Mesohippus onward, horses kept evolving. In the later genera such as Merychippus and Pliohippus, the outer toes shrunk to the point where they were no longer functional. In modern horses, Equus (including wild asses and zebras), the outer toes are no longer there at all, leaving just the single, middle toe. This, along with the greatly lengthened legs, is the ultimate adaptation to running on hard ground. But even quite recently, there were still three-toed horses around. It's just their bad luck, so to speak, to have gone extinct, leaving the genus Equus as the sole surviving representative of what used to be a very big and diverse family.

Additional evolution

Apart from the toes, Mesohippus was also becoming closer to modern horses in its skull, teeth and body build. The limbs were quite a bit longer than in its forest-skulking forebears. The neck was longer also, and the back straighter, less arched. The skull was lengthening, and the brain enlarged and elaborated. Its eyes were set farther apart and more to the rear of the skull. Its cheek teeth became bigger and taller, able to grind coarser vegetation such as grass. Between its grinding cheek teeth and its cutting front teeth, a gap called a diastema appeared; in modern horses, this gap is where the 'bit' for the reins goes – of course it's preposterous to think that the gap evolved for this purpose! A diastema evolved in a great many different groups of mammals, mostly plant-eaters, and likely simply so that there would be no interference between the functions of snipping off vegetation with the front teeth and/or the lips, and the chewing and grinding of it with the cheek teeth.

Mesohippus, going by all the fossils we found, was a very successful animal at its time. It likely lived in small or even large herds. It is likely to have been the ancestor of Miohippus ('Small Horse') which in turn was ancestral to many other horse species and groups. The two species lived together for a while, but Miohippus took over the scene around the mid-Oligocene when Mesohippus proper went extinct. At the end of the Oligocene, the horse family diversified greatly, leading to several different 'branches' including many groups that were large, three-toed browsers living in forests. Horses around the time made use of a land bridge connection to move into Asia and from there to Europe (and Africa only somewhat later). The height of horse diversity was reached in the late Miocene persisting into the Pliocene. Around the time of the Ice Ages there was a great reduction in horse diversity, but wild horses until very recently (twelve thousand years or so ago) still lived in Europe, North and South America. Their extinction there might have been partly or wholly due to humans. Today, while domestic horses are everywhere, wild horses, asses and zebras are all to a greater or lesser degree endangered.

Willem

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