Oligocene (Pt 8): The First Tapir and the Last Hoofless Horse

Miohippus

A close look at the evolutionary history of horses reveals that it's more complicated than sometimes presented, with numerous side branches forming a bushy tree of different species, many of which ultimately died out without descendants. This applies both to the origins of the group in its early days and its period of great diversification through the Miocene and Pliocene epochs. In the Oligocene, however, the picture, at least so far as we can tell, was rather simpler.

We know of two genera of horse that made it into the Oligocene from the preceding epoch. Mesohippus had been around for a while, Miohippus was a relative newcomer, appearing towards the very end of the Eocene. The two are rather similar, to the point that it has been argued they should be treated as different species of the same genus (which would be Miohippus, as that was named first) and they are both found in fossil beds across the United States and southern Canada, with Miohippus being known from Washington state to Florida and Mesohippus primarily in the west. The similarity also led to proposals early on that the one directly evolved into the other, but it's now clear that they lived alongside one another for millions of years, which scuppers that idea.

Rise of the One-Toed Horses

The horse family contains, depending on your definition, just seven or eight living species of wild animal. If you count them separately, you can add the two domesticated species to those (that is, the horse and the donkey) but that's it. Moreover, all of these species are so closely related to one another that they can interbreed, albeit usually to produce sterile offspring, and so are traditionally placed into a single genus: Equus.

The genus is noted for its members having just one toe on each foot. The story of how this happened, and the number of toes became reduced, is one of the most frequently repeated in mammalian evolution, although the detail may be more complex than is sometimes presented. The story of how the genus evolved since that point, however, is much less so.

Prehistoric Mammal Discoveries 2021

Lesmesodon, a weasel-sized hyenadont.
A new species from Early Eocene France was
 described this year

And so another year approaches its conclusion and the pandemic seemingly isn't done with us yet. I had to self-isolate after a positive test myself for a couple of weeks (no symptoms, though) but if there's one thing that doesn't interrupt, it's blogging, so everything went smoothly here. But now it's time for what's becoming a traditional look at the paleontological discoveries of the past year that didn't get covered here but are worthy of at least a brief mention.

Large Herbivores

When we think of vertebrate fossils, the first thing that pops to mind is almost certainly going to be a skeleton of some kind, or perhaps just part of one. But there are also such things as ichnofossils - fossilised remains of how an animal affected its environment that no longer include any physical part of the animal itself. Perhaps the most obvious of these are fossil trackways - footprints of long-gone animals preserved in mud or other soft material that has since turned to stone. A study published this year examined the tracks left by two species of fossil horse. One of them, a one-toed close relative of the living species inhabiting southern Canada during the Ice Ages, turned out to have been galloping at around 34 kph (21 mph), which is quite fast for its small size (perhaps it was running from something). More significantly, however, the three-toed Miocene horse Scaphohippus was using a relatively unusual gait called the "rack" typically only seen in specialised domestic breeds today. 

Miocene (Pt 24): Of True Elephants and Three-toed Horses

Stegotetrabelodon

The first horses entered Africa towards the end of the Middle Miocene, about 10 million years ago. These have commonly been assigned to the same genera, Hipparion and Hippotherium, as were found in Eurasia at the time, although the fine details of the exact relationships are unclear. Although the latter in particular seems to have been reasonably successful on the continent, a more significant immigration from an African perspective took place later on, around 8 million years ago as the drier climate heralded the start of the Late Miocene.

Miocene (Pt 16): Giant Camels and High Llamas

One group of animals that you might expect to have done reasonably well as the climate dried out in the later stages of the Miocene epoch were the camels. Camels, of course, are not found wild in North America these days, but, back in the Miocene, it was the only place that they were found, having first evolved on that continent millions of years before.

Indeed, camels had been diverse in the Early and Middle Miocene, inhabiting a number of habitats that we would not associate with the animals today. Once those habitats changed with the shift into the Late Miocene, however, many of them failed to adapt, and a number of earlier forms died out. These included the stenomyline "gazelle-camels", the short-legged miolabines, and the floridatragulines, which had been adapted to the subtropical forests of the southern coasts.

The Last of the Chinese Zebra-Donkeys

Asian wild asses

With the exception of our own species, few mammals have been the subject of quite so much interest in their evolutionary history as the horse. The number of named species of fossil horse far outweighs the number of species that are alive today. A great many of these are, of course, the older three-toed horses, with all living horses being placed in the single genus Equus.

Today that genus consists of just seven widely recognised living species. But, even among just this genus, and ignoring all the older, extinct ones, there were once many more species than there are today. But just how many is that? That's a matter of considerable confusion and debate.

In a way, that's hardly surprising, especially when you consider the focus of attention that there has been on horse evolution. Even just looking at the living species, not everyone agrees that 'seven' is the appropriate number, with some authorities arguing that particular subspecies are distinct enough that they should really be species in their own right.

Miocene (Pt 11): Horses on the Grasslands

Daeodon

The lush greenery of Early Miocene North America was a good place for large mammalian herbivores. Many of these, such as musk deer, pronghorns and camels, were, in one fashion or another, cud-chewing animals, able to extract maximum nutrition from a grassy or leafy diet. But many, of course, were not, either finding different ways to get the most out of their food, or else going for plants that were generally easier to digest.

Some of these were, like the ruminants, cloven-hoofed animals. Today, the main group of non-ruminant cloven-hoofed animals are the pigs, but they have never truly lived wild in the Americas, with feral 'razorbacks' only having arrived with the white man. Instead, America has peccaries, also known as javelinas, animals that look very much like pigs, but have a number of crucial differences.

Miocene (Pt 7): Hornless Rhinos, Long-Tusked Elephants, and Three-toed Horses

Anancus arvernensis

As the climate cooled around 11 million years ago, the forests of Europe began to thin out once more, something that favoured fast-running animals such as horses. Until this time, the only kind of horse in Europe, however, was the small, three-toed, Anchitherium, which was likely adapted to dense woodland and not so suited to the new climate. Its own ancestors had reached the continent from the east, having crossed over during one of the periodic rises of the Bering Land Bridge, but now, not coincidentally, given the colder climate, the Land Bridge rose again, and a second kind of horse followed it out of the Americas.

Are Horses Self-Aware?

I'd imagine that the first response from anyone who regularly deals with horses to the above would be "well, of course they are!" Your horse shows not just awareness and recognition, and is clearly a fairly intelligent animal, but there seems to be something going on behind those eyes. Horses seem, for example, to be aware of the emotional state of their handlers, and respond appropriately. There is surely more to their actions than simple, pre-programmed instinct.

And, if that is your response, let's face it, you're not wrong.

But then, awareness isn't a simple "all or nothing" phenomenon. All living things respond to their environment in some way; it's part of the definition of being alive. But even once we exclude say, tomato plants, there's still a massive gulf between jellyfish and humans. Once we get specifically to mammals, there is clearly more going in their mental and emotional states than is the case for, say, starfish or parasitic worms. But even then, there is no sharp line between the full consciousness of an adult human and the awareness of every other sort of mammal.

Pliocene (Pt 10): Before There Were Zebras

At the dawn of the Pliocene, Africa, like Europe, was a much wetter place than it is today. As a result, it was also much greener, a place of lusher vegetation, and the animals that fed on it. While that likely made little difference to the heart of the Congo jungle and to the more tropical reaches of West Africa, which are about as green as they're going to get, elsewhere the changes would have be obvious to any putative time traveller.

The biggest difference was likely in the north, where the even the very heart of what is now the Sahara Desert was likely covered in arid scrubland - hardly hospitable, but a significant improvement over baking hot dune-fields. By one estimate, moist savannah and open woodland stretched as far north as 21°, covering what are now countries like Chad, Sudan, and Mauritania. Further east, Somalia would also have been covered by woodland, rather than its current dry grasslands, and, at the opposite end of the continent, there may have been small forests in what are now the Namib Desert and the Kalahari.

It didn't last, of course. Around 3 million years ago, as the world fell irrevocably into the long autumn of the late Pliocene, Africa became not only cooler, but drier. And, if the generally cooler climate did not make too much difference to a continent sitting on the equator, the loss of rain certainly did. It's at this time that the Sahara, and the other deserts we are familiar with today, began to form, and the wildlife had to either adapt to that fact, or die. What was good news for voles in Europe, promoting the tougher grasses on which they thrive, was bad news further south, where the grass gave way to open sand.

Pliocene (Pt 7): Home, Home on the Steppe

Mylohyus

The beginning of the Pliocene epoch in North America was not marked by any great cataclysmic event, such as happened in Europe at that time. Nonetheless, while we're not far enough back that North America is unrecognisable, or in a different place, or anything like that, there would have been clear differences if we could see it from space.

Perhaps the most obvious difference would be that the Great Lakes didn't exist yet, since they were carved out by the advancing glaciers of the Ice Ages - which have yet to happen. With sea levels higher, Florida (then, as now, not a place known for its mountain ranges) is largely underwater, and there were probably many other changes around the coast, too.

Arguably the most important difference, however, is further south. Mexico is not so different, at least in its general outline, but beyond that things start to change. Depending on the exact point within the Pliocene we're talking about, you could perhaps have walked as far as Nicaragua without getting your feet wet. Beyond that, however, the Central American peninsula tapers to a point, and where Costa Rica and Panama should be, there is nothing but a chain of tropical islands, a southern counterpoint to the much larger chain of the Caribbean further north.

Pliocene (Pt 3): Of Gazelles and Three-toed Horses

Hipparion

The Zanclean Flood may have been a catastrophe of epic proportions, but, so long as you were above where the flood waters eventually stopped, Europe at the dawn of the Pliocene was a fairly pleasant place. The weather was warmer than today, and, apparently wetter too, which might not be what time-travelling tourists would be looking for, but was certainly good news for the plants that were actually there. Where places like Spain, Italy, and Greece are today dominated by... well... "Mediterranean" scrubland, back then they would have been considerably greener. And what's good for plants is good for herbivores.

Especially when it comes to cloven-hoofed animals, many of these would have been animals that would have been, at least in general terms, familiar to us. Not necessarily familiar to us from Europe, though, since, in addition to pigs, bovines, and deer, there were also a number of antelopes. These were mostly members of the gazelle subfamily, although there were others, including some, for example, related to the modern sable antelope. The gazelles included Hispanodorcas, a small and slender antelope with slightly twisting horns, with fossils found in southern Spain. However, some were even closer to the gazelles of today, to the point that, if, like most people, you'd be pressed to tell the difference between a Dorcas gazelle and a Speke's gazelle today (or at least, to know which one was which), you'd probably not have identified these as anything different, either - although at least some of them were smaller than any living species, which might have helped.

Before the Dawn Horse

Eohippus

Horses are surely among the most familiar large herbivorous mammals. Yet, compared with the deer and cattle families, the horse family is remarkably small. It contains only seven living species: the horse itself, three kinds of ass, and three kinds of zebra. Yet there used to be many more, and, if we include all the extinct species, the family starts to look quite large.

Indeed, the story of horse evolution, with its steady progress towards the modern animals, with only one toe on each foot, is one that's particularly familiar. With the possible exception of the line of ever-more-upright apes leading towards modern humans, the steady line of horse evolution may be one of the most well-known, and oft-repeated, images in mammalian evolution. Of course, it's not really a line, more of a bush, with multiple different kinds of horses living alongside one another, but the general pattern is accurate.

The horses either evolved from, or are very closely related to, another family of animals called the palaeotheres. These were more or less dog-sized animals, and probably had three or four toes on each foot, just as the earlies horses did. Their exact classification, and quite how they relate to horses, isn't entirely clear. That's partly because, for most of them, we don't have very complete skeletons, and there are different opinions among scientists as to which features of those skeletons are important, and which aren't.

Pleistocene (Pt 3): Ice Ages and Interglacials

Life-size reconstruction of a steppe mammoth
(compared with a 3-year old human)

When the Pleistocene began, Europe's climate was much the same as it is today, and the general shape of the continent would also have been instantly recognisable from space. The animals however, were different, many of them being ones we would now associate with Africa - rhinos, elephants, hyenas, and cheetahs, among others. In part 2, I described how that began to change 1.8 million years ago (which, incidentally, was once defined as the beginning of the Pleistocene - see part 1 for why that changed).

This was a time of cooler weather, as the Ice Ages began to dawn. Forests retreated in the face of advancing tundra, and musk oxen, bison, and (strangely) European hippos began to make their appearance. The cold snap was prolonged, and, so far as we can tell, the fauna of Europe remained relatively stable for the next 600,000 years. That's still a very long time - if we go back to my analogy where we get just one minute to watch the events of a decade, with the whole of written history thereby spread out into a nine hour spectacular, this phase of European history would last a full six weeks.

1.2 million years ago, half way through the Pleistocene, the climate changed again, and mammals (and other animals) were forced to adapt. However, the change wasn't towards yet colder weather, but back towards a warmer, more pleasant climate. The forests grew back, with all their dense undergrowth in attendance, and the harsh steppe-lands retreated into the north. As had been the case at the dawn of the Pleistocene, European weather would have been much as it is now.

How the horse began to run

Hyracotherium, a close relative of Arenahippus

One of the advantages of studying fossil mammals, compared with dinosaurs, is that there are many close parallels still alive today. There isn't anything remotely like a Tyrannosaurus stalking the plains of present-day Africa, but comparing sabre-tooth cats to animals such as tigers and leopards can tell us quite a lot, with rather less need for guesswork. Also, while complete mammal fossils are still quite rare, they are, nonetheless, more common than those of animals from the more distant past.

Still, when it comes to early mammals, complete skeletons are rare enough that finding one can provide a significant opportunity to learn more about them. Species are often described on the basis of their skulls alone, since skulls tend to be the most distinctive parts of the skeleton, and you can tell a lot just from that, but having the rest of the skeleton stll attached to the skull is obviously pretty useful.

A recent report in the Journal of Mammalian Evolution described a remarkably complete skeleton of the early horse Arenahippus, with the tail being almost the only part missing.

As a side note, exactly what this animal should be called is a matter of some controversy. When specimens of the species were first found, they were thought to belong to the genus Hyracotherium, which may (or may not) be an alternative name for Eohippus, the "dawn horse" that appears at the beginning of so many charts of the evolutionary history of horses you see in museums and the like. But its probably neither, so we'll stick with the name it was given in 2002, even though there are counter-arguments to that one, too.

At any rate, whatever its called, Arenahippus is one of the most primitive members of the horse family known. It lived in the early Eocene epoch, just ten million years after the extinction of the dinosaurs, when many of the modern groups of mammals were just getting started. We don't know that later horses evolved from it, because there were lots of species of early horse living alongside each other, and while one of them must have evolved into the later ones, there's no way to tell which it was - if its even we've found yet. Those step-wise evolutionary charts you see of horse evolution don't really show exactly what evolved from what, just general pictures of what horses at a particular point in time looked like.

In reality, like all evolutionary stories, that of horses is a branching tree, although its interesting to note that Arenahippus appears to branch off even before the more famous Eohippus did, putting it even closer to the origin of the horse family:

True Equines    Mesohippus     Eohippus
     ^              |             |
     |              |             |      Arenahippus
     |              |             |           |
     ----------------             |           |
            |                     |           |
         (3 toes)                 |           |
            |                     |           |
            -----------------------           |     Palaeotheres
                       |                      |          ^
                       |                      |          |
                       ------------------------          |
                                   |                     |
                            (First horses)               |
                                   |                     |
                                   -----------------------
                                              |
                                              |

In fact, even the tree above is greatly simplified - there are a great many other fossils branching off in between the steps shown above. Nonetheless, we can see that Arenahippus diverged at a point when horses still had four toes on their front feet (although, like Eohippus, they only had three on the hind feet). In terms of its size, and to some extent, its shape, it looked more like a dog than like a modern horse.

So what can this new skeleton tell us about the life of these earliest horses? Perhaps the most obvious place to look is the legs, since the one-toed foot of modern horses is one of their most distinctive features. The tops of the thigh and upper foreleg bones are clearly rounded, with flexible hip and shoulder joints. This is quite different from modern horses, where the shape of the joints means that the limbs can only move forward and back, with very little flexibility in any other direction. The authors suggest that this would have helped in an environment more cluttered with bushes and other obstacles, rather than the open grassland that favours the gallop of modern horses. Since other evidence suggests that the area of Wyoming where the fossil was found was woodland with dense undergrowth, this makes sense.

Furthermore, the shape of the bones of the hind limb show the presence of powerful muscles, especially the calf muscle. Taking into account the shape of the knee and ankle joints, this indicates that the hind limbs would have been bent as the animal pushed itself forward and began to run - something you see in dogs, but not in horses, whose hind limbs are fairly stiff.

However, its not just the shape of the limbs that show us how the animal would have moved, but also the backbone. Reconstructions of early horses tend to show a straight backbone, as can be seen in the photograph at the top. This is how the backbone of modern horses look, and the way that the individual vertebrae lock together makes the whole structure quite rigid, a pattern also seen in other fast-running hooved animals, such as antelopes. But there haven't been many good fossils with intact backbones before, and, looking at this one, it seems the pattern isn't quite so simple.

Back flexed, legs pushing towards the midline

The vertebrae at the far end of the back, just before it joins the pelvis, were, indeed, rigid, with processes that would have locked them tightly together. But just before this was a more flexible region where the bones would have prevented the back from twisting, or from bending upwards, but would not have prevented it from bending downwards. Thus, unlike modern horses, Arenahippus could have arched its back, and most likely did so just as it began to run.

All in all, Arenahippus seems to have been a more flexible animal than a modern horse, or even than its more horse-like later relatives, such as Mesohippus. That may be partly because the later animals were bigger, and a more stable body would have made them more energy efficient while running. Arenahippus's movable knees, strong calf muscles, and flexible hips would have enabled it to push off the ground with some force, while the arching back ensured its centre of mass stayed in line. Still, it does seem to have been more rigid than, say, a modern dog and was, perhaps, just beginning on a path that would lead its later relatives (if not, necessarily, its literal descendants) to their fast-running lifestyle.

That leaves aside the question of why later horses became larger at all, requiring the change to the more familiar shape and posture we see today. That may be due to the changing climate of the time, and the spread of grasslands. Arenahippus, like other very early horses, mainly ate herbs, and perhaps fruit, browsing on low-lying vegetation, while later horses grazed on grasses. Grass is harder to digest than herbs, so that a longer digestive tract is needed if you're going to eat it. One way to increase the length of the digestive system is to increase the size of the animal its inside, and its at least possible that this was a major reason for the change. The more open environment of grasslands may also have meant that the longer stride that the size and body shape of later horses promotes would have been more useful for them than for something living among dense undergrowth.

[Pictures from Wikimedia Commons, cladogram adapted from Mikko's Phylogeny Archive]