Six feet tall at the shoulder and 1,000 pounds, Merychippus cut a reasonably horse like profile, if you're willing to ignore the small toes surrounding its enlarged middle hooves. Represented by a dozen separate species, Riparian (“like a horse”) was hands-down the most successful equip of the latter Cenozoic Era, populating the grassy plains not only of North America but also Europe and Africa.
Pliohippus is the bad apple on the equine evolutionary tree: there's reason to believe that this otherwise horse-like ungulate was not directly ancestral to genus Equus, but represented a side branch in evolution. Finally, we come to the last “hippo”: the donkey-sized Hippidion of the Pleistocene epoch, one of the few ancestral horses known to have colonized South America (by way of the recently submerged Central American isthmus).
Ironically, in light of the tens of millions of years they spent evolving there, Hippidion and its northern relatives went extinct in the Americas shortly after the last Ice Age; it remained for European settlers to reintroduce the horse into the New World in the 16th century AD. Apart from a couple of bothersome side branches, horse evolution presents a neat, orderly picture of natural selection in action.
The basic storyline goes like this: as the woodlands of North America gave way to grassy plains, the tiny photo- horses of the Eocene Epoch (about 50 million years ago) gradually evolved single, large toes on their feet, more sophisticated teeth, larger sizes, and the ability to run at a clip, culminating in the modern horse genus Equus. But before we embark on this journey, it's important to dial back a bit and place horses in their proper position on the evolutionary tree of life.
The giveaway to Phipps' status was its posture: this perissodactyl put most of its weight on a single toe of each foot, anticipating later equine developments. Phipps was closely related to another early ungulate, Palaeotherium, which occupied a distant side branch of the horse evolutionary tree.
During the Miocene epoch, North America saw the evolution of “intermediate” horses, bigger than Phipps and its ilk but smaller than the equines that followed. One of the most important of these was Phipps (“marginal horse”), which was slightly heavier (possibly weighing a few hundred pounds) and equipped with more robust grinding teeth than its ancestors.
As you might have guessed, Phipps also continued the trend toward enlarged middle toes, and it seems to have been the first prehistoric horse to spend more time feeding in meadows than in forests. During the Miocene epoch, waves of tasty grass covered the North American plains, a rich source of food for any animal well-adapted enough to graze at leisure and run quickly from predators if necessary.
Riparian was the most successful horse of its day, radiating out from its North American habitat (by way of the Siberian land bridge) to Africa and Eurasia. Lesser known than Riparian, but perhaps more interesting, was Hippidion, one of the few prehistoric horses to have colonized South America (where it persisted until historical times).
Wikimedia Commons Modern horses have come a long way since their prehistoric ancestors roamed the grasslands and prairies of Cenozoic North America. On the following slides, you'll find pictures and detailed profiles of over a dozen prehistoric horses, ranging from the American Zebra to the Tarzan.
When its remains were first unearthed, in 1928, the American Zebra was identified as a new genus of prehistoric horse, Plesippus. On further examination, though, paleontologists determined that this stocky, thick-necked grazer was one of the earliest species of Equus, the genus that comprises modern horses, zebras and donkeys, and was most closely related to the still extant Gravy's Zebra of eastern Africa.
Also known as the German horse (after the town in Idaho where it was discovered), Equus simplifies may or may not have sported zebra-like stripes, and if so, they were probably restricted to limited portions of its body. Notably, this early horse is represented in the fossil record by no less than five complete skeletons and a hundred skulls, the remnants of a herd that drowned in a flash flood about three million years ago.
As successful as Anchitherium was--this prehistoric horse persisted throughout the entire Miocene epoch, or close to 20 million years--the fact is that it represented a mere side branch in equine evolution, and wasn't directly ancestral to modern horses, genus Equus. In fact, around 15 million years ago, Anchitherium was displaced from its North American habitat by better-adapted equines like Riparian and Merychippus, which forced it migrate to the less-populous woodlands of Europe and Asia.
Despite its dinosaur-worthy name (Greek for “terrible horse”), you might be disappointed to learn that Dinohippus wasn't especially big or dangerous--in fact, this prehistoric horse (which was once considered to be a species of Pliohippus) is now thought to have been the immediate precursor of the modern genus Equus. The giveaway is Dinohippus' primitive “stay apparatus”--a telltale arrangement of the bones and tendons in its legs that allowed it to stand for long periods of time, like modern horses.
Phipps. Florida Museum of Natural History Phipps (Greek for “marginal horse”); pronounced EPP-ee-HIP-us. As prehistoric horses go, Phipps represented a slight evolutionary advance over its immediate predecessor, Orohippus.
This small equine had ten, rather than six, grinding teeth in its jaws, and the middle toes of its front and hind feet were slightly bigger and stronger (anticipating the single, huge toes of modern horses). Also, Phipps appears to have thrived in the meadows of the late Eocene epoch, rather than the forests and woodlands inhabited by the other prehistoric horses of its day.
You may be under the mistaken impression that ancestral horses were restricted to North America, but the fact is that a few ancient genera prowled Eocene Europe. Eurohippus has been known to paleontologists for years, but this dog-sized perissodactyl (odd-toed ungulate) thrust itself into the headlines when a pregnant specimen was discovered in Germany, in 2010.
By studying the well-preserved fossil with X-rays, scientists have determined that the reproductive equipment of Eurohippus was extremely similar to that of modern horses (genus Equus), even though this 20-pound mammal lived nearly 50 million years ago. The mother horse, and her developing fetus, were likely felled by noxious gases from a nearby volcano.
Along with Hippidion and Merychippus, Riparian was one of the most successful prehistoric horses of the Miocene epoch, evolving in North America about 20 million years ago and spreading as far afield as Africa and eastern Asia. To the untrained eye, Riparian would have appeared almost identical to the modern horse (genus name Equus), except the two vestigial toes surrounding the single hooves on each of its feet.
Judging from its preserved footprints, Riparian probably ran much like a modern thoroughbred, though it likely wasn't quite as fast. This ancient horse was about the size of a modern donkey, and its most distinctive feature was the prominent ridge on the front of its head that housed extra-wide nasal passages (meaning it probably had a highly developed sense of smell).
Some paleontologists believe Hippidion properly belongs to the genus Equus, which would make it a kissing cousin of modern thoroughbreds. You might think from its amusing name that Hypohippus (“low horse”) was about the size of a mouse, but the fact is that this prehistoric horse was relatively big for Miocene North America, about the size of a modern-day pony.
Oddly enough, Hypohippus was named by the famous paleontologist Joseph Lady not for its short legs (which he wasn't aware of at the time) but for the stunted profile of some of its teeth! Hyracotherium. Wikimedia Commons Hyracotherium (formerly known as Phipps) was directly ancestral to modern-day horses, genus Equus, as well as numerous genera of prehistoric horse that roamed the plains of the Tertiary and Quaternary North America.
Merychippus. Wikimedia Commons The Miocene Merychippus was the first ancestral horse to bear a noticeable resemblance to modern horses, although this genus was slightly bigger and still had vestigial toes on either side of its feet, rather than single, large hooves. Mesohippus. Wikimedia Commons Mesohippus was basically Hyracotherium advanced by a few million years, an intermediate stage between the smallish forest horses of the early Eocene epoch and the large plains browsers of the Pliocene and Pleistocene epochs.
The skull of Miohippus. Wikimedia Commons Although the prehistoric horse Miohippus is known by over a dozen named species, ranging from M. accident to M. quarts, the genus itself consisted of two basic types, one adapted to life on the open prairies and the other best suited to forests and woodlands. One of the more obscure prehistoric horses, Orohippus lived at about the same time as Hyracotherium, the equine ancestor once known as Phipps.
The only (obvious) equine characteristics of Orohippus were the slightly enlarged middle toes on its front and hind legs; other than that, this herbivorous mammal looked more like a prehistoric deer than a modern horse. Not all the ungulates of the Eocene and Oligocene epochs were directly ancestral to modern horses.
A good example is Palaeotherium, which, even though it was related to genuine prehistoric horses like Hyracotherium (once known as Phipps), had some distinctly tapir-like characteristics, possibly including a short, prehensile trunk on the end of its snout. For all intents and purposes, Parahippus was an “improved” version of another prehistoric horse, the similarly named Miohippus.
Parahippus was slightly bigger than its immediate ancestor, and was built for speed on the open prairie, with relatively long legs and noticeably enlarged middle toes (on which it put most of its weight when running). The teeth of Parahippus were also well adapted to chewing and digesting the tough grass of the North American plains.
Like the other “hippos”-es that it preceded and followed, Parahappus lay on the evolutionary line that led to the modern horse, genus Equus. The skull of Pliohippus. Wikimedia Commons Pliohippus (Greek for “Pliocene horse”); pronounced PLY-oh-HIP-us.
Although Pliohippus closely resembled modern horses, there's some debate about whether the distinctive depressions in its skull, in front of its eyes, are evidence of a parallel branch in equine evolution. Generally speaking, Pliohippus represents the next stage in horse evolution after the earlier Merychippus, although it may not have been a direct descendant.
Quanta.public domain DNA extracted from the hide from a preserved individual proves that the now-extinct Quanta was a sub-species of the Plains Zebra, which diverged from the parent stock in Africa sometime between 300,000 and 100,000 years ago. The Tarzan.public domain A shaggy, ill-tempered member of the genus Equus, the Tarzan was domesticated thousands of years ago, by early Eurasian settlers, into what we now know as the modern horse--but itself went extinct in the early 20th century.
Early finds inspired legends and fairy tales, as people imagined that these bones belonged to giants or huge monsters. Some consider Barnum Brown, who began his career at the American Museum of Natural History in 1897, to be one of the greatest dinosaur hunters of the late 19th and early 20th centuries.
Many of his greatest discoveries, including the first specimens of Tyrannosaurus Rex ever found, are on display in the Museum’s dinosaur halls. Today, in addition to patience and sharp observation skills, paleontologists employ new technologies to solve unanswered questions about dinosaurs and other fossils.
Advanced imaging technology, such as CT scans, allow paleontologists to see the three-dimensional structure of fossils, often without having to remove the matrix. Paleontologists incorporate the research of biomechanics, applying the principles of both physics and engineering to reconstruct the biological movement of non-avian dinosaurs.
The information gleaned from fossil bones along with observations of both the movement and the musculature of living animal species help scientists model how non-avian dinosaurs may have moved. Dinosaurs evolved into a very diverse group of animals with a vast array of physical features, including modern birds.
There are several theories as to what may have contributed to the mass extinction of non-avian dinosaurs and other species at the end of the Cretaceous Period. But other factors, including changing sea levels and large-scale volcanic activity, may also have played a significant role in this mass extinction.
Paleontologists use fossil evidence preserved in ancient rock to discover how long-extinct animals lived and behaved. By comparing the skulls of Protoceratops of different ages (like in the image above), paleontologists can draw conclusions about how some dinosaurs grew.
Series of fossilized footprints, called track ways, reveal some intriguing evidence about dinosaur behavior and locomotion. Paleontologists looking for dinosaur fossils begin their work by surveying areas to find sedimentary rock from the Mesozoic era.
They also work in the lab, examining the specimens they’ve found as well as fossils collected years earlier. They spend a lot of time classifying specimens, examining their characteristics, and determining their biological relationships.
The sharp points pierced the meat, and the serrations helped slice it by catching and tearing muscle fibers. They also ingested small stones, called gastritis, most likely to grind up the food in their stomachs, much the same way modern birds, such as parakeets and chickens, do today.
Feathers evolved before flight and may have functioned as insulation to keep dinosaurs warm, or for display as a way to attract mates. This trip runs through an authentic slice of Americana that will give everyone, from the kids to the adults, a taste of what makes this part of the country so special.
Seedskadee National Wildlife Refuge is preserving a wondrous slice of wilderness along the Green River, which gives visitors an opportunity to meet some of its native residents. Animals like moose, elk, wild horses, swans, pelicans, antelope, and more can be sighted year-round, and the river itself provides world-class fly-fishing.
From the region's prehistoric days below an ancient lake to the Shoshone and Ute tribes who first populated the area to the Oregon, California, Mormon, Overland, Cherokee, and Pony Express Trails that brought thousands of intrepid pioneers through, there is a ton to learn about. Hands-on activities for kids, endless information for adults, cool artifacts, and rotating exhibits combine to make a visit to the Sweetwater County Historical Museum well worth your while.
Here you can fish for trout, take a walk around the Greenbelt Pathway and admire the landscaping, kayak, raft or tube at the Whitewater Park, picnic and more. Coast across log bridges, tackle jumps, get some air against the walls and truck through exciting rough terrain on your bike.
Scenic overlooks make for great breaks from the road, interpretive signs add context to the route, and, of course, the wild horses are an incredible sight to see. Western Wyoming Community College in Rock Springs is right off the highway and contains five massive dinosaur skeleton casts that are on display for the public.
Head back to the Western Wyoming Community College to check out another interesting display, the Warner Wildlife Museum. From bears and elephants to lions and rhinos to cheetahs and cross, there are loads of specimens to examine at the free museum.
Walking into a room stuffed to the gills with mounted creatures is not something you get to experience every day, and it's worth a quick stop! Between marveling at dinosaurs, watching wild horses run free, enjoying a root beer float at a vintage diner, and fishing in the Green River, there's something for everyone to love in Sweetwater County.