Tuesday, September 13, 2016

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part V, Evolution Part 1


Here we are again, back to this 17-part series on the basics of equine anatomy and biomechanics for the Intermediate student. So far we've explored some terminology, tissues, and the main systems that are relevant to art. Now it's time to learn about the animal's past. While this may seem a strange subject for an artist to study, a peek at his natural history brings the animal into full focus. It’s easy to forget that this creature has biological context beyond our narrow experience with him. This genus has a vast and fascinating prehistory that long predates domestication, and understanding a bit of it places this animal into a fuller perspective. And it's context a realistic equine artist needs most of all. Why? Because it makes us responsible and compassionate artists. This gracious animal has been around a lot longer than we have, and was never designed for the uses and aesthetics we thrust upon him, yet he accepts it all the best he can, even amidst our capricious follies. His natural history reminds us that the horse is an autonomous being with his own evolutionary agenda, and his body has rather narrow biological parameters for his well-being. Awareness of his evolutionary development also deepens an appreciation for the animal beyond aesthetics or utilitarian interests while heightening an understanding of the logic underlying his biology. And through these insights, an artist can make better creative decisions that advocate for this animal we so love.

We must always remember that we have yanked this creature out of his natural habitat, thrust artificial demands and constraints on him, altered the course of his natural evolution, and manipulated his physique for our own purposes. Indeed, the domestication of the horse many thousands of years ago is usually regarded as the single most important event in the development of human civilization. It allowed us to leap beyond our physical short-comings and set the stage for the rapid advancement of our species. And in sports technology, advanced horsemanship is considered the most complex biological science ever created by man; two entirely different creatures merged into one unique entity. In many ways, we’re becoming a symbiotic species, horse and human. Nevertheless, embedded within this relationship is a responsibility to the horse, one that extends even into the studio. It must be remembered that the horse isn’t a frivolous toy, a malleable status symbol, or mindless robot. He’s an autonomous mind and body that’s deserves respect, gratitude, and appreciation. And he has his own history, his own continuing story and, ultimately, his own future, and perhaps it’s good for an artist to meditate on how to best express this ongoing journey.

Now please keep in mind that this exploration into equine history is merely a summary of some major evolutionary turning points. In reality, his development is remarkably far more complex and dynamic. Fresh discoveries and technologies will undoubtedly shed new light on the accepted thoughts today, so the information presented here should be considered “what's hypothesized now." Also, it should be understood that even the experts don’t always agree on the details of equine evolution, which adds another layer of interest. A further caveat is that the information presented here deals only with those lineages that produced Equus. In reality, his family tree was so astonishingly diverse, a full description of all the details would result in an enormous tome on this subject alone. So proactive education on the subject is highly encouraged to tease out more details and additional insights omitted here for the sake of brevity.

Most of all, study on this subject will reveal one curious truth: the horse is an unlikely, archaic animal. Created entirely by his lifestyle, this animal has survived countless events that threatened to extinguish his existence to become one of the most universally loved animals today. Indeed, it's believed by many scientists that Equus was on its way to extinction if not for domestication that brought it back from the brink. In fact, every other member of Equidae, except the Plains Zebra and the donkey, is endangered. This is an ancient, curious creature with a unique history all his own. Nothing else on the planet is built like or moves like an equine. He's wholly unique. To understand all this is to truly understand our subject, and to finally have the last piece of the puzzle for responsible art.

So let's get to it, shall we?... 

Taxonomy of Equus cabals caballus

Now to lay the groundwork…Walking on his 3rd toe, the horse is of the order Perissodactyla (perissos meaning “odd-numbered” and daktylos meaning “finger”). Called Equus caballus caballus, or often just Equus caballus for short, (which literally means “four legged nag, packhorse, or hack”), the modern domestic horse, in brief, belongs to the:
  • Order: Perissodactyla (odd-toed, non-ruminating ungulate)
  • Suborder: Hippomorpha 
  • Family: Equidae
  • Subfamily: Equine
  • Tribe: Equini
  • Genus: Equus (horses, Takh, zebras, asses, and half-asses, or hominids)
  • Subgenus: Equus (modern horses including the Takh, but excluding zebras, asses, and half-asses)
  • Species: Equus caballus (modern horse and recently extinct Tarpan)
  • Subspecies: Equus caballus caballus (domestic horse)
But now for some necessary caveats. For starters, the classification of prehistoric horses is still under debate. Teeth are the primary means for classification, but they're variable and have overlapping properties. Proportions of limb bones, the shape of the skull, and other distinctions have also been used to define the species, but these can be variable, too, particularly in a family as varied Equidae. Also, little is known of the colorations, growth patterns, soft anatomy, and behavior of ancient equines, which might have helped to better define a species. Also what was thought to be a family of vast diversification may have been a family of great plasticity. In other words, rather than an array of different species, there may only have been a few species but with variations, just like Equus caballus demonstrates with breeds today. On that note, it should also be mentioned that debate exists over the definition of a “species," clearly illustrated by the debates over the Takhi and Quagga. Furthermore, confusion sometimes exists with names assigned to ancient horses. For instance, the name Protohippus has been used intermittently for Merychippus, Mesohippus, Orohippus, and even eohippus. It certainly makes one wish for standardization!

But First, A Clean Slate about Evolution….

The adaptive history of the horse is well documented thanks to the prolific number of fossils fragments discovered so far (about a half a million specimens in North American storage alone). Consequently, the horse is often used as an example for the processes of evolution, yet there still seems to be some confusion about evolutionary mechanisms.

One of these misconceptions is the concept of orthogenesis (also referred to as anagenic, or linear evolution), or that evolution runs in a straight line. In other words, that evolutionary descendants are superior replacements. This can lead to comments about how modern Equus is better than his “primitive” ancestors, which in turn creates negative implications for the Takh and even feral horses. But evolution is packed with multiple pressures, myriad experiments, adaptive dead-ends, surprise benefits, and mind-numbing diversity, all produced by chance responses to different lifestyles. It’s only natural then that he possess many adaptive features, each legitimate for each mode of survival. Nature is certainly full of extinct animals supremely adapted to their environment, but were simply unable to cope fast enough when that habitat changed. Undeniably, as early horses began filling the new niche on the plains during the Tertiary, the evolution of Equus was hardly linear. Instead, the lineage tells of tremendous diversity, co-existence and over-lap of many related lines, back-and-forth modes of adaptation, and multitudes of experiments.

Another misconception is that adaptation is smooth and gradual, like progressive rungs of a ladder, having logical intermittent versions sandwiched between dominant new forms. But evolution produces spurts of adaptation, shaping animals in fits and starts, as species evolve chaotically to changing habitats. New traits arise at different rates, sometimes “suddenly” and sometimes changing direction or reversing with multiple co-existing variations. This interpretation of evolution is referred to as cladogenic or cladogenesis (branching evolution), of which Equus is a supreme example.

Evolution also doesn’t have a “trend," either. For example, in hindsight, the apparent trend from Hyracotherium to Equus appears to be a straightening of the spine, increase of size, reduction of toes, elongation of the face, neck, and legs, and changes to his teeth. But that’s only because all other forms of Equidae no longer exist. In reality, many horses got smaller, such as Archeohippus and Calippus while some independently evolved a one-toed structure when many still had three-toes. And some of these changes didn’t evolve together or at a consistent rate. For instance, during the Eocene, the feet didn’t change much, though the teeth started to evolve. Yet during the Miocene, both the feet and teeth started to change. So these characteristics didn’t really evolve as a “package deal," but as chance independent adaptations to meet differing ecological pressures. And sometimes one adaptation randomly lead to another. That's to say some changes happened because of a previous adaptation and not necessarily in reaction to an environmental pressure. For instance, once the grazing lines committed to grazing, it became increasingly difficult to revert back to a browser lifestyle. But we perceive a “trend” through hindsight only because Equus is the only survivor of a diverse family whose other “trends” have simply vanished. Evolution only follows the route dictated by the ecological pressures facing a species at the moment, and it occurs at different rates with different variations; it’s chaotic and random. In fact, sometimes speciation occurred rapidly (Miohippus from Mesohippus) or a species broke off and co-existed with another, along with its ancestors, too. At times there were long periods of little change (Hyracotherium during the Eocene) while sometimes a type evolved gradually until it changed enough to become a new species (Equus from Dinohippus). And sometimes, bursts of evolution explode that produce numerous co-existing species and genera, such as the Merychippine Radiation. Evolution works in many ways, but it's certainly not an ordered, simple progression. 

But one of the biggest fallacies about evolution is that it has a purpose, some goal that shapes a species according to some great plan. But it’s clear from the different rates and avenues of speciation characteristic of equine expansion that a consistent goal didn’t exist between Hyracotherium and Equus. Adaptation simply happens in response to an environmental condition, with no foresight of future consequences. Plus, evolution can only make do with what previous generations have left as genetic material. So how implausible is the horse? If anything had turned out differently, would this series be about horses as we know them today? A cascade of random coincidences produced the species and not some great biological purpose. Modern Equus isn't the goal or purpose of equine evolution. No. It's simply one of the tiny, surviving remnants of Perissodactyla and the last living genus of a previously enormous family now gone. Equus caballus has merely continued to win the evolutionary toss-of-the-dice. And evolution is an on-going process which makes some breeding practices important for an artist to consider somberly. 

The First Fossils

The first fossils were found in Paris, 1825 and was named Paleotherium. However, this animal wasn’t recognized as part of the horse lineage, but considered a relative of the modern tapir. Thirteen years later in 1838, a bunodont tooth was discovered, later finding a similar tooth with the jaw attached. Scientists from the London Geological Society thought these remains were from a primitive monkey, “proving” that England once had a prehistoric monkey population. However, in 1839, a jaw with a full set of teeth was discovered in Eocene deposits in Kent, and this specimen was named Hyracotherium. Understandably, this wasn't recognized as the fountainhead of Equus, but was interpreted as a rodent because of its rabbit-like build and skull. Indeed, Hyracotherium literally means “mole beast” or “animal like a rabbit or hyrax” so, at the time, the remains were believed to be an extinct subspecies of Hyrax. But remember that neither a paradigm for evolution existed at the time nor did other equid bones exist to make a comparative connection. The floodgates opened when, in 1859, Darwin published his revolutionary whammy, Origin of Species, providing the scientific community with a theory of adaptive change, or what we now know as “evolution."

The first fossils identified as part of the horse lineage were unearthed in Europe and it was Thomas Henry Huxley (who called himself “Darwin’s bulldog”) who was the first to see the connections between these animals and placed them in their apparent chronological order. In 1872, Huxley promoted the notion that these horses illustrated a successive line of development because they were discovered in consecutive geological layers. This inspired Vladimir Kovalevsky (a palenontologist) to identify Hyracotherium as a member of the horse family in 1873, though he failed to place the critter in the correct chronological order.

Meanwhile in America, in 1867, a nearly complete skeleton of Hyracotherium was unearthed in Wyoming with further remains found in New Mexico, Colorado, and Utah (the most complete Hyracotherium skeleton was found in 1931 in Big Horn Basin, Wyoming, and is mounted in the California Institute of Technology). And in 1871 and 1872, an early Eocene horse was discovered in North America. It was this specimen that was named eohippus, or “dawn horse” in 1873. And in 1876, eohippus was placed in the correct order in the lineage of the horse—at the beginning. A few years later it was realized that eohippus was the same animal as Hyracotherium, discovered over thirty years prior. But because eohippus was originally given the name Hyracotherium, this latter name takes precedence, becoming the animal’s true scientific name. (Therefore, eohippus shouldn’t be italicized and technically shouldn’t be capitalized, either. But the poetry of “eohippus” has allowed the name to linger in common usage.) After further study, it became clear that Equidae originated in North America and those specimens found in Europe were really extinct lineages.

The Big Picture

In overview, his “beginnings” originate in the Cenozoic era (or Cainozoic era), sometimes termed the Age of Mammals. It's the most recent time in Earth’s history, beginning about 66 mya (million years ago) with the Paleocene. The Tertiary is the first big chunk of the Cenozoic (about 95%), during which time the continents became roughly located as they are today and the climate began to cool and dry out, reducing the tropics and favoring cool forests and grasslands. Quaternary is the most recent portion of the Cenozoic, comprising the later two epochs. The simplified timeline of the Cenozoic is, as follows...

The Paleocene, about 65-54 mya, saw the first mammals burst into the niches previously left by the dinosaurs, and they were quite small, only few as big as a small bear. They all had short legs with plantigrade stance (planta meaning “sole of the foot” and gradus “to walk”, hence to walk on the soles of the hand, like a bear, rat, or human) with five toes on each foot (all tetrapods, or vertebrates with four limbs, are based on the same blueprint and one of these features is five digits on each limb, being pentadactyl, from our common ancient fish ancestor about 360 mya). Most had 44 bunodont (low crowned) teeth and nearly all had slim heads, narrow muzzles, and relatively small brains (their brain to body weight ratio was much smaller than late Cenozoic mammals). The warm climate produced wet tropical habitats and high sea levels, isolating many of the continents and allowing independent evolution of these first mammals. Among the dominant lifeforms at the time were the “primitively hooved” and mostly herbivorous Condylartha. These 75 million year old animals are believed to be the progenitor of all modern hoooved animals.

The Eocene had a similar climate and mammals continued to diversify. During this time, grasses appeared, though limited to water habitats. The ancient Condylartha became extinct and through one of their lineages, the Phenacodonta, the ungulates arose (hooved mammals; “ungulate” and “unguligrade” derive from the Latin word “ungula” which means “hoof” or “provided with hooves," also derived is “ungule phalanx," the old term for the coffin bone, which literally means “to stand on one’s ungus”). Is still one of the most successful and diverse mammal groups today. A hoof is simply a specialized claw or toe-nail as opposed to plantigrade stance and digitigrade stance (digitus meaning “finger” in which the foot is supported on fibrous pads with elevated digits, like a dog or elephant). The true ungulates, the Perissodactyla (from the Greek, perissos, meaning irregular or uneven and daktulos meaning “finger," hence “odd toed”) and Artiodactyla (from the Greek artios, meaning “even," hence “even toed” or “cloven hooved”), began to develop during this epoch. Perissodactyla include horses, tapirs, rhinos, etc. and Artiodactyla include camels, giraffes, bovines, sheep, swine, deer, antelope, etc. “Ungulate” used to refer only to these two orders but recent molecular and fossil evidence has broadened the term to include elephants, hyraxes, sea cows, and aardvarks. Recent research also implies that Cetaceans (dolphins and whales) should be included as well. This is because evidence suggests that all these animals may have originated from a common Condylarth ancestor some 90 million years ago.

The Ogliocene, about 34-24 mya, saw more volcanic activity and tectonic movement, but here began the cooling trend, seen with the first glaciers developing in Antarctica during the Cenozoic. This lowered the sea levels and the tropics began to recede, allowing cooler forests and grasslands to expand. Grass also began to venture beyond its water habitat and blanket the expanding savannah. Up until this time, the Perissodactyla reigned as the dominant ungulate while Artiodactyla remained quite rare. However, as the grasslands expanded, Artiodactyla began to gain dominance because they were first to develop ruminant digestion (first with the early camels) to use grasses and fibers as a viable food source. 

The Miocene, 25-5.3 mya, was the longest epoch of the Cenozoic era, being some 18 million years long, and saw the greatest diversity of mammals as the prehistoric world gave way to the more modern environments. For example, land bridges developed and migration between continents took place. Also, the grasslands continued to expand, coinciding with the cooling and drying trend of the climate, reducing the conifer and tropical forests. This forced adaptive changes in herbivorous mammals from browsers to grazers, and both Perissodactyla and Artiodactyla experienced an explosion of diversity during this period. 

The Pliocene, 5.3-2.5 mya, saw the beginnings of humans and the environments of the modern world. This period is considered by some as the climax of the Age of Mammals, being characterized by the evolution of many presently existing orders, family, and genera. This period also continued climatic cooling and began to form the polar ice caps, though Antarctica wasn’t yet completely frozen. Grasslands further replaced forests, favoring grazers over browsers, and migration increased over the land bridges. For instance, the North American three-toed Hipparion crossed the Bering Strait land bridge, entering Asia and Europe while North American animals were able to cross into South America, with devastating results on the previously isolated native fauna there.

The Pleistocene, the epoch of the Ice Ages, about 2.5-0.01 mya, continued the cooling trend, which raised and lowered the oceans alternately, temporarily exposing land bridges. Mammalian megafauna flourished and early humans became adept at using fire and tools.

The Holocene, 10,000 years ago to present, is a tiny blip in the history of evolution, yet typified by the profound effects of Earth’s most peculiar new mammal, homo sapien

So it was during this time of great change that Equus emerged and, without a doubt, became a product of this new landscape. But he had humble beginnings, starting as a creature barely recognizable as what we understand today as a “horse." So let's learn more about little Hyracotherium...

The Stage

His story really begins about 55 million years ago, in the Eocene, with Hyracotherium, who appeared in North American forests. This little critter was only about 2 feet (60 cm) long and approximately 8-9 inches (20 cm) high at the shoulder. He had four toes on the forefoot and three on the hindfoot, each bearing a little “hoofie” with footpads like a dog. The major bones in his legs weren’t fused and his joints were designed to rotate more freely to maneuver on the cluttered forest floor. He had a flexible spine, rather like a cat or rabbit, with short crowned (bunodont) teeth and a short head with a short diastema (jaw space between the front and back teeth) and large, low eyes. In many ways, he resembled a bunny. As such, his build was suited for a forest life, nibbling on soft leaves and sprouts while dodging, scampering, and sprinting around trees trunks and branches to escape danger. His upwardly curved spine also suggested he had a rump-high, “popping” gait as well, like a bunny. Another curious aspect about Hyracotherium was the wings of his atlas bone and the crest of his axis bone were quite large, indicating that he may have moved his head in a snout-pushing, rooting motion, too, like a pig. Though nothing is known of his coloration, it might be thought, as a forest dweller, Hyracotherium may have had stripes, spots, mottling, ticking, or other such camouflage patterns.

Hyracotherium lived in the northern hemisphere, populating North America, Asia, and Europe, which had moist tropical or sub-tropical climates at the time. Indeed, during this period, Alaska had cycads, magnolias, and fig trees, and Siberia and Greenland both possessed giant Redwoods and deciduous trees. Very successful, this animal lived in a pleasant niche for most of the Eocene, a long 20 million years, so he and his close descendants remained relatively unchanged. During this time, some Hyracotherium may have migrated from North America to Europe over the Greenland bridge (a north Atlantic land bridge connecting Canada to Scandinavia during the early Tertiary. There was also another land bridge, Beringia, between Alaska and Asia at that time, yet no Ogliocene horse fossils have been found there—yet). This implies that this route might not have been used for Eurasian migration by these animals. On this new continent, however, Hyracotherium was the progenitor of many new species, such as Paleotherium. And when grasslands began to expand, lines of Hyracotherium ventured out to exploit this new habitat, foregoing the browser lifestyle. And it was these lineages that would undergo the convoluted metamorphosis that produced fantastic diversity of one-toed and three-toed types in the Americas, Europe, Africa, and Asia. 

Also, it’s believed that Hyracotherium gave rise to not just the horse, but to many other Perissodactyl relatives, too, such as rhinos, tapirs, and some extinct animals such as the clawed chalicotheres, horned brontotheres (sometimes called titanotheres) and the biggest land mammal in history, the Indricotherium or “Beast of Baluchistan," a creature that weighed about 15-25 tons and stood 18 ft. at the shoulder. Today, only sixteen species of Perissodactyla exist: Equidae (7 living species, all of which are believed to have diverged from a common ancestor about 4-5 million years ago), Rhinocerotidae, (rhinos with five living species) and Tapiridae (tapirs with four living species). Many of these species, particularly those of the rhino, are endangered and, like the tiny Hyracotherium, may become extinct. Of the multitudes of animals thought to be descended from Hyracotherium, so few exist today that Perissodactyla has one of the highest extinction rates among mammals. Indeed, of the myriad forms this order produced, only 16 species exist today compared to the estimated 172 existing species of Artiodactyls.

Conclusion to Part V, Evolution 1

The horse's evolution is important for an equine artist to understand because it provides the "why" the horse is built the way he is. When we know this, it becomes harder to stylize our work into exaggerations of breed type. It also deepens our appreciation for this complex animal by putting him into biological context. So often we simply take the horse for granted, forgetting that he has a whole backstory that's unique and worthy of our understanding. It also makes it easier for us to regard this animal on his own terms, and with his dignity and autonomy in mind so we tend to perceive life through his eyes rather than our own.

In Part VI, Evolution II, we'll discuss some specific changes he underwent to accommodate his new cursorial lifestyle on the plains. And all this from simply switching to grass as a food source!

So until then...let your appreciation for this magnificent animal evolve!

"As a general rule, the most successful man in life is the man who has the best information."

~ Benjamin Disraeli

Related Posts with Thumbnails