Perissodactyla
Tapirs, Horses, Rhinos and their Relatives
During the Pleistocene epoch, North America was home to several kinds of perissodactyls, also known as odd-toed hoofed mammals. The most abundant were horses of the genus Equus, which occupied a wide range of grassland and open habitats. Several Equus species were found across the continent, and three-toed horses were also found in this region at the start of the Pleistocene. Tapirs could also be found in the Midwest, primarily in wooded regions and hilly areas. Earlier rhinoceros relatives had once inhabited North America during the Neogene period but they were already extinct on the continent by the start of the Pleistocene.
Anatomy
(BELOW) A labeled diagram of the skeletal anatomy of the Miocene three-toed horse genus Merychippus. Drawing is based on a mounted skeleton at the University of Nebraska State Museum of Natural History
Skeletal Anatomy
Perissodactyls tend to walk on feet with either one, three or five toes. This is a result of a feature of their skeletal anatomy called mesaxony, a condition in which the body's weight is concentrated on the enlarged third digit of each limb. One-toed horses exhibit the most specialized derived form of this arrangement with only a single functional toe per foot, whereas tapirs and rhinoceroses retain several toes that help provide extra stability when walking over soft ground and distributing their body weights. The gradual loss of toes in horses seems to be an adaptation for running quickly over open grassland habitats. In highly cursorial species certain limb bones have become reduced or partially fused and, like many types of artiodactyls, horses have cannon bones in the areas of their legs just above their feet. The skulls of perissodactyls tend to be fairly elongated with expanded nasal regions. In rhinoceroses, the nasal bones are extra thick in order to support the attachment of their characteristic horns, while in tapirs the nasal openings have become retroverted in order to make space for a short trunk. Some ancient horse genera had a set of deep fossae along the edges of their snouts in front of their orbits which may have held scent glands in life. These fossae are not present in living horse species. The teeth of perissodactyls are specialized for herbivorous diets, with broad grinding surfaces and, in many lineages, high-crowned molars adapted for processing abrasive vegetation.
(BELOW) The skull of the extinct horse genus Pliohippus features a series of deep fossae along the sides of its muzzle. These fossae are present in the remains of many types of extinct horses but are absent in the living genus Equus
(ABOVE) The skull of a tapir. These animals have particularly heavily built incisors at the front of their jaws and a distinctive retroverted nasal opening which anchors the base of the tapir's trunk
External Anatomy
Perissodactyls share a number of common physical traits. Their heads are usually rather long and equipped with laterally positioned eyes. Having their eyes on the sides of their heads gives these animals a wider field of view with a larger amount of peripheral vision. Their ears are often tall, rigid and highly mobile and they are able to turn their ears in order to better pick up a sound coming from a particular direction. In some groups, namely tapirs, the front of the skull is equipped with a short, flexible snout used to grasp vegetation, while rhinoceroses have specialized muscles in their upper lips which they use to pull leaves off of branches. Some early horses who fed on leaves may have had similar types of browsing-adapted lips. Body coverings vary a lot between different types of perissodactyls. Horses typically have a coat of hair which can be thin or very thick depending on the local climate, whereas rhinoceroses are covered in thick, protective skin with relatively little hair. Some living tapir species who live at high elevations today have fairly thick coats of fur, which would be what we might expect to see in midwestern ice age tapirs.
(BELOW) Features of the external anatomy of a horse
(BELOW) Features of the external anatomy of a tapir
Horses (Equidae)
One-toed and three-toed horses
During the Pleistocene, the grasslands and open habitats of the Midwest supported several types of prehistoric horses. The most common were members of the genus Equus, including stout-legged horses such as Equus scotti, a widespread species found across parts of the Great Plains and central North America. Fossil evidence shows that these horses were grazing animals adapted to open environments, much like the wild horse and zebra populations still found around the world today. The region may also have been inhabited by more slender-limbed, “stilt-legged” horses such as Haringtonhippus, which lived alongside Equus species throughout much of North America. Relatives of older horse groups with extra hooves bracing the sides of their main toes could still be found in this area during the early Pleistocene.
Equus
Temporal Range: 5.3 million years ago to present (outside the Americas)
Geographic Range: Genus found on all continents
Midwestern species in the Pleistocene: E. scotti, E. simplicidens
Diet: Herbivorous grazer, grasses
During the Pleistocene, the Midwest was inhabited by multiple species of Equus, the genus that includes modern horses, donkeys and zebras. One of the best-known was Equus scotti, a medium-sized, stout-legged grazing horse that lived in grasslands, open woodlands, and wetlands across much of North America, including parts of the Great Plains and Midwest. Another species, Equus simplicidens, could also be found in this region. This animal, sometimes called a Hagerman horse, had skeletal features similar to those found in today’s zebras.
Our knowledge about the coat colors of extinct horses is somewhat limited. Cave art from Eurasia suggests that many Pleistocene wild horses might have appeared very similar to today’s Przewalski's horse, with tan fur and a white muzzle. It may be reasonable to assume that North American species of Equus could have had similar coat colors.
(BELOW) A skull from a modern horse on display at the KU Natural History Museum. This particular skull has been cut away to reveal the shapes of the roots of each tooth. Note that some of the rear teeth had not yet fully emerged in this animal.
Stilt-legged horse
Haringtonhippus francisci
Temporal Range: 3 million years ago to approximately 11,000 years ago
Geographic Range: Genus across North America from Alaska to Mexico
Species: H. francisci, H. achates, H. quinni, H. cedralensis
Diet: Herbivorous grazer, grasses
Haringtonhippus was an extinct genus of horse that lived in North America during the Pleistocene. The best-known and only recognized species is Haringtonhippus francisci. This animal is often called a “stilt-legged horse” because of its unusually long and slender lower limb bones, which distinguished it from the more heavily built horses of the genus Equus which would have lived alongside it. Fossils of Haringtonhippus have been discovered across a wide geographic range, from Mexico through the western and central parts of North America and into Canada.
For many years, these animals were classified within the genus Equus, but genetic analyses of their bones and anatomical studies of their skeletal features later showed that they formed part of a distinct evolutionary lineage. Research indicates that Haringtonhippus was more closely related to modern horses than to the South American genus Hippidion, yet it remained separate from the lineage that includes all of today’s living horses and zebras.
Dwarf three-toed horse
Nannippus lenticularis
Temporal Range: 3.3 million years ago to 1.8 million years ago
Geographic Range: Found throughout North America
Species: N. lenticularis, N. minor, N. morgani, N. westoni, N. penninsulatus
Nannippus lenticularis was a small species of three-toed horse that lived in North America from the late Miocene to the start of the Pleistocene epoch. It belonged to an older group of horses that had been widespread across the continent before the evolution of the larger, one-toed horses such as those of the genus Equus. Fossils of Nannippus have been discovered in Kansas, Nebraska, Alabama, North Carolina, Texas, and Alberta, indicating that it was well adapted to a variety of environments.
Unlike modern horses, Nannippus lenticularis retained three functional toes on each foot. The enlarged middle toe bore most of the animal's weight, while the two smaller side toes were positioned higher up and braced the middle toe from each side. Members of the genus Nannippus were relatively small, growing to be around the size of a sheep or a large dog.
Dental evidence suggests that Nannippus species were primarily browsers who fed on leaves, shrubs, and other softer vegetation rather than relying exclusively on grasses like today’s horses. This diet allowed them to thrive in woodland and savannah habitats found across North America during the late Miocene, although as grasslands began to spread they faced increased competition from other horses who had become specialist grazers.
Diet: Herbivorous browser, leaves, twigs, leaf buds
Tapirs (Tapiridae)
Ancient tapirs in the Midwest
During the Pleistocene, tapirs lived much further north than they do now, with some making their homes in the Midwest. These large herbivores inhabited forests, wetlands, and river valleys, where they fed on leaves, fruits, aquatic plants and other vegetation. Like modern tapirs, they possessed a short, flexible snout that was useful for grasping and manipulating plant material. Although they fall within the “Odd toed” ungulate group, tapirs actually have four main toes on their front feet. Their hind feet have five toes, each of which is tipped in its own hoof-like nail.
Tapirus veroensis
Temporal Range: 300,000 years ago to approximately 11,000 years ago
Geographic Range: Eastern North America
Subspecies: T. veroensis
Diet: Herbivorous browser, leaves
Tapirus veroensis was a species of tapir that lived in eastern North America during the Pleistocene. Fossils attributed to this species have been discovered in Florida, Georgia, Kansas, Missouri, and Tennessee. Although complete details of its external appearance are not known, Tapirus veroensis was likely similar in overall form to living mountain tapirs. We know that these animals had large trunk-like snouts based on the shapes of their nasal openings. The skulls of tapirs have nasal bones which are positioned further back, leaving more space for the soft tissues needed to support a flexible trunk.
Although Tapirus veroensis is one of the last types of tapirs to be found in the Midwest, their history in this region goes back millions of years, with the first fossils of ancient tapirs in the Great Plains going back to the Oligocene epoch.
Media Gallery
Suggested References: Perissodactyla
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Eric Scott (2006) “Extinct horses and their relatives, Fossil Treasures of the Anza-Borrego Desert: the Last Seven Million Years”, Sunbelt Publications, p. 253-271
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MacFadden, Bruce J (1984). "Systematics and phylogeny of Hipparion, Neohipparion, Nannippus, and Cormohipparion (Mammalia, Equidae) from the Miocene and Pliocene of the New World". Bulletin of the AMNH. 179.
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