Sunday 9 December 2018

Miocene (Pt 11): Horses on the Grasslands

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.

Peccaries had long been present in North America at the start of the Miocene, although not in any great variety. That changed early on in the Miocene, with a number of new species appearing. I discussed the fossil history of peccaries just last week, so I won't go over it again now, but the two most significant kinds of peccary at this time were Floridachoerus, living on the swampy north coast of the Gulf of Mexico, and Hesperhys, further inland on the plains of Wyoming and Nebraska.

They were not, however, alone. The exact relationship between the entelodonts and the pigs and peccaries is not entirely clear; they're clearly outside the pig/peccary grouping, but whether they are closer to pigs than to (say) hippos is difficult to prove with certainty. At any rate, they did look rather like pigs, being relatively large, cloven-footed omnivorous animals with no need to chew the cud.

They had been far more common in the previous, Oligocene, epoch, but by the start of the Miocene, 23 million years ago, only one kind was left. This was Daeodon, and it was not only the last of the entelodont "hell-pigs", it was also the largest. It stood around 180 cm (5' 10") at the shoulder, similar to the height of a rhinoceros. It perhaps wasn't quite as heavy as a rhino, though, since much of that height was made up by its long legs, which ended in just two toes (that is, lacking the largely non-functional "dew claws" of pigs).

It did, however, have a large and heavy pig-like skull, about 90 cm (3 feet) in length, with huge teeth vaguely reminiscent of those of hippos. The vertebrae above the shoulders had elongated spines that would, in life, have anchored a powerful muscular hump, largely to support the weight of that head; we see something similar today in living rhinos, as well as bison.

Although they were never particularly common animals, Daeodon must have been adaptable, with a broad omnivorous diet. They lived across pretty much the whole of what is now the US, with fossils being known from, among other places, New Jersey, Texas, California, and Oregon. They died out around 19 million years ago, having made it only a short way into the Miocene, although they must surely have been impressive when they lived.

Just as the last of the entelodonts were dying out, however, horses were undergoing a brief burst of diversification, adapting to the Early Miocene climate. The horses of the day were much smaller than the modern sort, and still had three toes on each foot. Perhaps the typical example of the time was Anchitherium, although its likely ancestor, Miohippus, did, like Daeodon, survive a short way into the Miocene. This did well enough that it eventually crossed over the Bering land bridge to Asia, and has fossils known from Japan to Spain, as well as across North America, all the way from Canada to what was then the very southern tip of the continent.

It was, however, soon joined by Parahippus, a horse that appears to be further along the path to the modern species. For a while, there was even some dispute as to whether it more closely related to the browsing Anchitherium than to was to the likes of the modern animal, perhaps representing a transition from the one to the other. It's now generally thought to be something of a side-branch in horse evolution, and so, while it was shaped by the same general climatic and geographic changes that led to the modern 'equine' horses, it left no descendants and can't really be considered one of them.

Parahippus stood about one metre (three feet) tall at the withers, similar to Anchitherium, and far smaller than any modern horse. It too, seems to have been mostly a browser, but its teeth were already beginning to change towards the pattern seen in modern equines. They were higher-crowned than those of early browsing horses, and the fine structure of the tooth enamel shows the same changes seen in the true equines, and which enable them to dine on tough grasses - but, at this point, the changes weren't quite significant enough to have had more than a minimal effect. It probably lived in open, not wooded, environments, even if it couldn't have coped with true grassy prairie.

Another feature that parallels the evolution of later horses is that, for the first time, while Parahippus still had three toes, the two side ones were relatively small and held close to the central hoof. It could probably have used them to help steady itself on uneven ground, but, for the most part, it's unlikely that it used them much, with only the central toe fully functional. Analysis of tooth wear in various fossils suggests that they typically lived only three to four years, and that they either had no distinct breeding season or alternatively, that they were no more likely to die in winter than in summer. This, in turn, may have been due to an unusually stable climate where they lived.

Due to a similar shape to the feet, Archaeohippus is often thought to have been a close relative. It, however, was unusual in a number of ways, not least in that it was much smaller than other contemporary horses, with some adults perhaps weighing as little as 10 kg (22 lbs). With teeth that would only have been useful for chewing relatively soft leaves, it likely had quite a narrow diet. As one might expect, given their small size, they, like Parahippus, probably had a short lifespan; mares probably gave birth for the first time in their second year, compared with the fourth for modern horses.

These horses died out when the grasslands began to spread in the Middle Miocene, around 16 million years ago. But what was bad for them was good for the horses as a whole, as a second, even larger, burst of new species occurred at around that time. For the next ten million years or so, there was a great diversity of horses across North America, with around fifteen or sixteen genera in existence at any given point.

Some of these Middle Miocene horses, such as Hypohippus, belonged to the same general group as Parahippus and its kin, with teeth better adapted to eating leaves than grass, despite some minor modifications in that direction. The great diversity of the time, however, may have occurred in part because these animals continued to survive just as the first 'equine' horses made their appearance, and spread into the grassy plains.

Probably the first of these new horses to appear was Merychippus, although the confused history of that particular scientific name makes that less clear than one might think. At any rate, the animal currently known by that name was the largest horse to have existed up to that time, perhaps weighing as much as 100 kg (220 lbs). While its hoofed feet were not so different from those of Parahippus, its teeth were more fully adapted for grazing on tough grass, and its face was also, for the first time, truly like that of a modern horse.

This was followed by a number of other genera, that fell broadly into two groups, distinguished by subtle differences in the structure of their tooth enamel. One group included animals such as Hipparion, which crossed over into Asia and Europe, becoming very widespread indeed, while the other was at first found only in America. It was this latter group, however, that went on, in the following, Pliocene epoch, to lose the two side toes altogether, and give rise to the modern horses and zebras. For a while at least, however, a few animals besides Merychippus itself continued to share features of both of the later groups; these included Scaphohippus from California and Nebraska and the plains grazer Acritohippus, both of which were somewhat larger than their immediate predecessors.

Even so, they were small by the standards of modern horses, and certainly had nothing on Daeodon, long gone by this time. But other herbivores also lived on the plains of Early and Middle Miocene North America, and some of them were impressively large. We will look at some of those next time...

[Photo by James St. John, from Wikimedia Commons.]

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