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Ungulates are large grazing animals whose toenails have become enlarged into hooves. There are two orders of ungulates: Perissodactyla and Artiodactyla.

Animals in the order Artiodactyla have an even number of toes (usually two) that form a cloven hoof. This order is relatively diverse, containing 82 genera and several hundred species. There are nine families in this order, the most familiar of which are the pigs (Suidae), peccaries (Tayasuidae), hippopotamuses (Hippopotamidae), camels (Camelidae), deer (Cervidae), giraffes (Giraffidae), sheep, cattle, antelopes (Bovidae), and pronghorn antelopes (Antilocapridae).

Animals in the order Perissodactyla have an odd number of toes (usually one) that form a single, large hoof. Examples of this order include horses (family Equidae), tapirs (Tapiridae), and rhinos (Rhinocerotidae), together comprising six extant genera and 16 species.

At the time of the European discovery of North America, the native fauna of ungulates included bison (Bison bison), pronghorn antelope (Antilocapra americana), collared peccary (Tayassu tayacu), muskox (Ovibos moschatus), mountain goat (Oreanmnos americanus), and mountain and Dall sheep (Ovis canadensis and O. dalli). The North American Cervidae includes white-tailed deer and mule deer (Odocoileus virginianus and O. hemionus), wapiti or elk (Cervus elaphus), moose (Alces alces), and caribou (Rangifer tarandus).

About 8,000-10,000 years before Europeans first came to America, North America supported a substantially larger number of ungulate species, including 10 species of horses, four species of camels, a species of cow, two additional species of bison, and the saiga antelope (Saiga tatarica). There were also other large, now-extinct mammals, including four species of elephants, such as the mastodon (Mammut americanum) and mammoth (Mammuthus primigenius), a giant ground sloth (Gryptotherium listai), and large predators such as the sabertooth cat (Smilodon fatalis) and the American lion (Panthera leo atrox). These large mammals disappeared during a great wave of extinctions that occurred at the end of the last ice age (about 8-12 thousand years ago). These extinctions may have been caused by overhunting by the first human inhabitants of North America, migrants from Asia who colonized the continent at about that time.

Some species of ungulates that have been domesticated are important in agriculture and sometimes as draft animals. The most abundant domesticated ungulates are sheep (Ovis aries), goats (Capra hircus), cows (Bos taurus), zebu cows (B. indica), pigs (Sus scrofa), horses (Equus caballus), camels (Camelus dromedarius and C. bactrianus), and water buffalo (Bubalus bubalis).

Many of the wild species of ungulates have recently become endangered and some have become extinct as a result of human influences. The most important of the human activities that endanger wildlife are the habitat losses associated with extensive conversions of natural ecosystems into agricultural or urban lands, and overhunting for the meat, hide, horns, or antlers of these large animals. It is critical that these human influences be rigorously controlled, if there is to be room on Earth to sustain all living species of ungulates. Natural ecosystems would be severely impoverished if only domesticated species of ungulates used in agriculture were to survive, along with the few species tolerant of habitats created by humans.

Bill Freedman


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—A deep-sea exploration vehicle or submersible consisting of a ballast-filled float (resembling a blimp or balloon) with a spherical metal gondola for carrying occupants and equipment suspended below it.


—A deep-sea exploration vehicle or submersible consisting of a sphere that carries a crew and equipment and is lowered to the sea floor on a cable.

Bathythermograph (BT)

—An instrument for measuring the differences in temperature in sea water depths.

Black smokers

—Hydrothermal vents on the sea floor that emit black clouds of hot, mineral-rich water much like a chimney belches black smoke.

Continental shelf

—A relatively shallow, gently sloping, submarine area at the edges of continents and large islands, extending from the shoreline to the continental slope.

Diving bell

—An enclosed device for carrying a single diver exploring relatively shallow waters; replaced by submersibles except in some work situations.

Global Positioning System (GPS)

—A system of satellites whose signals can be used to locate objects on Earth (including below sea level) very precisely.

Hydrothermal vent

—An opening of Earth's crust on the sea floor where hot springs bearing mineral-rich waters are emitted. Hydrothermal vents are important sources of minerals and warmth for species of life not found in other environments.


—The molten rock from the core of Earth that emerges on the surface through volcanic eruption and sea-floor spreading. When magma cools, it forms igneous rock.


—The science of measuring the ocean.

Plate tectonics

—The theory now widely accepted that the crust of Earth is composed of about 12 giant plates that form the land masses and sea floors and that grind slowly past each other, causing earthquakes, mountain-building, and other large-scale geologic occurrences.

Remotely operated vehicle (ROV)

—A deep-sea submersible that carries equipment only (no human occupants) and can be remotely operated from a surface ship.

Rift valley

—A large, deep valley, either on the land surface or beneath the sea, created by the movement of two plates composing Earth's crust away from each other. The Mid-Atlantic Ridge, the Marianas Trench, and the Galápagos Rift are examples of submarine rift valleys.

Sea-floor spreading

—The part of plate tectonics that describes the movement of the edges of two of the plates forming Earth's crust away from each other under the ocean. Sea-floor spreading results in the formation of new submarine surfaces.


—Soil and rock particles that wash off land surfaces and flow with water and gravity toward the sea. On the sea floor, sediment can build up into thick layers. When it compresses under its weight, sedimentary rock is formed.

Self-contained underwater breathing apparatus (SCUBA)

—Also called an aqualung. The mask, mouthpiece, valves, and oxygen or compressed air tank that can be worn by a diver to sustain breathing for periods up to several hours under water.


—The process of using dropped weights (weight sounding), sound waves (sonar), or seismic waves artificially induced by man-made explosions to produce waves that, when reflected back to their source, can be used to measure distances and the densities of the materials through which the waves pass.


—Deep-sea exploration vehicles that carry two or three human occupants, cameras, and other equipment to relatively great depths in the ocean. Submersibles can also carry equipment only and be remotely operated.


—A difference in temperature in sea water or in the atmosphere.

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