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Perissodactyls are odd-toed ungulates (hoofed animals), in which the weight is borne over the center toe. They are also herbivores (plant eaters).

Horses, Today's Best-Known Perissodactyls
Horses, most of the evolution of which took place in North America, present a well documented evolutionary sequence. Modern horses (equus) originated in North America during the Pleistocene (about 2 million years ago) and spread to all the major continents.

They had disappeared from the New World (North and South America) before the advent of the white man; but, when reintroduced, they found ideal environments on the plains and pampas (South American treeless plains).

Elongation and strengthening of the lower segment of the limb, (in horses, the middle toe) is an adaptation favoring great speed. The ankle bone is deeply grooved on top to prevent its twisting under the shinbone. the side toes (1st and 5th) are absent, and the 2nd and 4th reduced to tiny functionless splints.

Equus Caballas, the domestic horse, has now been selectively bred by man. A species of true wild horse still lives in Central Asia.

A number of species of Equus were found on the great Plains during the Pleistocene. They had many of the characteristics of modern horses.

The cheek teeth of modern horses continue to grow through the life of the animal, as their grinding surfaces are worn away.

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Divergent Groups of Perissodactyls
One of these was the tapir (a large, hog-like mammal). There are four species of tapir, found in South and Central America, and Southeast Asia. Fossil remains indicate that tapirs have changed little in 20 million years. Tapirs are the least specialized of living perissodactyls; the forefeet have four toes and the hindfeet three.

Except for the tapir, none of the "odd-toed" ungulates retained more than three toes later than the early Oligocene (about 25 to 38 million years ago). The lower segments of the limbs generally elongate, a gap (diastema) usually develops between the front and back teeth, and the premolars tend to resemble the molars.

Rhinoceroses diverged into three major lines between 40 and 50 million years ago. Two of these families flourished for about 10 million years, and then became extinct. The rhinos in these two families evolved adaptations to live in a variety of environments, and ranged greatly in size.

There are five species of rhinoceros alive today. Two live in Africa, while the other three are found respectively in India, Java, and Sumatra.

Rhinoceros horns are made of hair-like fibers packed tightly together; horns grow continuously and are supported by thick nasal bones. Unfortunately, the horns that make rhinoceroses so unique are also much sought after by some humans. Rhinoceroses are illegally killed in large numbers to supply this demand. This has pushed all five species of rhinoceros to the brink of extinction.

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American Mastodon, Mammut americanum
This specimen, discovered on a farm near Owosso, Michigan, is the most complete, and one of the better preserved, mastodons discovered in the state. A Museum of Paleontology team investigated the discovery site, excavated the bones, and reassembled them here in 1947.

This species of mastodon existed during the Pleistocene (about 2 million years ago) until about 10,400 years ago, and ranged from Alaska and the Yukon down to Central Mexico. About 11,000 years ago, when this individual lived, they were common around the Great Lakes and the Atlantic coast, feeding on plants in open spruce or pine woodlands and along wetland margins. Mastodons were still here when prehistoric humans arrived in Michigan late in the Pleistocene.

Males of the species grew to be 3 meters (10 feet) tall at the shoulder and had tusks over 2.5 meters (8.5 feet) long, but females were significantly smaller. The state of development of this individual's bones, along with the wear on its teeth, indicate it was fully grown. Its body size and tusk size are well within the range for females. The worn edges on the broken tusk reveal that the breakage occurred during her lifetime.

Mastodons have had an evolutionary history separate from that of mammoths and elephants for at least 20 million years. Nonetheless, all of these animals resemble one another in overall body form and are placed in the same order of mammals.

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Adapted from exhibit descriptions by John Klausmeyer, Exhibit Preparator, University of Michigan Exhibit Museum