Order Uranotheria

Order Uranotheria, proposed in 1997, contains hyraxes (which resemble rabbits or large rodents); the aquatic sirenians (manatees, dugongs, and the extinct Steller's sea cow—sea cows), and elephants, the largest living land mammal. Traditionally each group has been recognized as a distinct order (Hyracoidea, Sirenia, and Proboscidea), but their evolutionary histories can be traced back to a common ancestral stock of condylarths (primi tive herbivorous placentals) living in Africa during Paleocene times, and researchers have combined them into a single order to reflect their evolutionary origin.

Human interactions with these remarkable mammals has ranged from positive to disastrous. Hyraxes are appreciated as a part of the African ecosystem, especially as part of African nature safaris, but they are incidentally exterminated when their habitat is destroyed for agricultural activities. Sirenians have been persistently hunted and abused in other ways, reducing population size and geographic range, and forcing one species to extinction. Elephants have been tamed for transportation and work, but many have been killed for their ivory; they have been eliminated from great expanses of their former range to make way for farms and towns, a reflection of expanding human population in Africa and Asia.

Living hyraxes (Suborder Hyracoidea) are in Family Procaviidae and consist of three genera and seven species found throughout Africa (except the very arid northwest) and in southwestern Asia (from Turkey to the Arabian Peninsula). They are either grazing or browsing herbivores. Their evolutionary history extends back to the Early Eocene of Africa, from which point they spread to Asia and Europe. There are eighteen extinct genera containing species ranging in size from modern hyraxes to a small rhinoceros. They were the dominant members of African herbivore communities during the Oligocene and eventually expanded into Asia and Europe during Miocene-Pliocene times.

All hyraxes have a compact body (30 to 60 cm long); inconspicuous tail (1 to 3 cm long); short, sturdy legs; and small ears. The forefoot has four digits, the hind foot three. The inner digit on each foot bears a curved claw, all other digits ending in flattened nails resembling tiny hooves. Soles of the hind feet have spe cialized pads kept continually moist by glandular skin secretions. Muscles retract the center of the sole to form a hollow that acts as a powerful suction cup. Teeth consist of two pairs of lower incisors, a single pair of upper incisors, a premolar in each quadrant of the jaw, and three molars; canines are absent. The pointed, widely separated upper incisors are triangular in cross-section, ever-growing, with enamel front surfaces and softer dentine behind, so that wear produces pointed cutting edges; lower incisors are chisel-shaped with bicuspid tips. The chewing surface of the molars resembles that seen in rhinoceroses. The skull is short and stocky with a large and deep mandible. The three species of Dendro-hyrax live in forests, are arboreal and nocturnal, foraging in trees and on the ground. They form small family groups. The single species of Procavia and three of Heterohyrax are diurnal, and, although capable of climbing, are generally terrestrial; they inhabit cliffs, rock ledges, and lava beds in grasslands. Terrestrial species are gregarious, forming colonies of up to fifty individuals among rocky outcrops that bask in the sun when not feeding or avoiding predators. Hyraxes run, jump, or climb over rocks, cliff faces, and tree branches, and have acute hearing and excellent vision.

The four species of living and one of recently extinct sirenians are in families Dugongidae (extinct Steller sea cow, Hydromalis gigas; and living dugong, Dugong dugon) and Trichechidae (Amazon manatee, Trichechus inunguis; West Indian manatee, T. manatus; and West African manatee, T. senegalensis). The dugong is found only in coastal waters of the Old World tropics. The Amazon manatee inhabits the Amazon River basin of northern South America; the West Indian manatee occurs in coastal waters and rivers from the southeastern United States, around the Gulf of Mexico and the Caribbean Sea to eastern Brazil; and the West African manatee lives in coastal waters and larger rivers from Senegal to Angola. Steller's sea cow, the largest sirenian (reaching 8 m in length and weighing up to 10,000 kg) was first encountered in the shallow western Bering Sea during the 1700s; it was hunted to extinction by about 1768, twenty-seven years after its discovery. It is the only sirenian known in historical times to occur in cold ocean coastal habitats, and may have been the largest noncetacean aquatic mammal.

The evolutionary history of the group extends back to Eocene times, as documented by fossils found in Europe, Africa, Asia, and North America in tropical marine sediments. Weighing up to 1,500 kg, living sirenians have a massive, spindle-shaped body (2.5 to 4 m long), flipper-shaped front limbs, a horizontally flattened tail, and lack hind limbs and a dorsal fin. The nearly hairless, wrinkled skin is thick and tough. The rounded head has a stubby muzzle; the mouth is small, and lips are surrounded by stiff tactile hairs (vibrissae). Nostrils are separate, located on top of the muzzle, and can be closed. Eyes are small and external ear flaps are absent. The skeleton is composed of dense, heavy bone. Long, unlobed lungs lie horizontally in the body and are separated from the capacious gut by a long and horizontal diaphragm. The orientation of the lungs combined with the dense bone allows the sirenian to adjust its lung volume and maintain a horizontal position while feeding at different depths. Dugongs have large, columnar, ever-growing teeth without enamel but covered with cementum. The teeth of manatees are covered with enamel and rooted; as the front teeth wear out they are replaced by rear teeth pushing forward, so that at any one time five to eight teeth are functional in each quadrant of the jaw. All sirenians have horny plates covering the front of the palate and adjacent mandibular surfaces.

African elephants, Samburu Wildlife Reserve, Kenya (Darrell Qulin/Corbis)

Sirenians are heavy, slow-moving, and the only completely aquatic mammals that are also strictly herbivorous, consuming vast quantities of submerged vascular aquatic plants along with emergent and floating vegetation, sea grasses, and marine algae. Because their diet is low in nutrients, sirenians have a slow metabolism and generate meager body heat for their size. This physiology, combined with lack of a thick layer of blubber, promotes rapid loss of body heat in cool waters. They are thus restricted to tropical seas and rivers where water temperatures are warm and stable. Sire-nians travel in small groups, or in pairs, or are solitary, and they live their entire life in the water. Even the young suckle while the mother is submerged in a horizontal attitude.

Indian (Elephas maximus) and African (Lox-odonta africana) elephants are the only living representatives of Family Elephantidae. The Indian elephant inhabits tropical forests and savannas in continental Asia and some of the larger islands on the Sunda Shelf. The African elephant is now found only in sub-Saharan Africa. The woolly mammoth (Mammuthus primigenius), also a member of Elephantidae, apparently survived up to 3,700 years ago on Wrangel Island in the Arctic Ocean, off northeastern Siberia; elsewhere it became extinct about 10,000 years ago in Europe, and 8,000 years ago in North America. The evolutionary history of elephants is first documented by fossils in the Paleocene of North Africa. These were likely small animals similar to tapirs in size and body conformation. By the Late Eocene the species had evolved into forms more closely resembling elephants, with either a long proboscis or trunk and columnar limbs. Later, during Miocene times, proboscideans evolved into an array of species and genera and dispersed out of Africa into Europe, Asia, and North and South America, even making their way to numerous coastal islands along the major continents and in the Mediterranean and Indo-Australian regions. The Indian and African elephants are the living remnants of this spectacular evolutionary radiation.

Elephants are known to everyone by their great size (nearly 4 m tall and weighing up to 7,500 kg); huge head; short neck; massive body; thick, long, and columnar legs; expansive, fan-shaped ears; and the very long, muscular, and flexible trunk ending in nostrils and fingerlike projections. The feet are short and columnar, like the legs. The skin is thick and nearly hairless, and the moderately long tail bears a brush of wirelike hairs at the tip. Elephants do not have sebaceous glands, which are found in the hair follicles of most mammals and produce secretions to soften and lubricate the skin and hair. Limb bones are heavy and separate. The bones of the fingers and toes are short, spread out, and braced at the heel by a pad of dense connective tissue, which supports the elephant's weight. The animal actually walks on its digits (five fingers on the forefoot, three or four toes on the hind foot), supported by the heel cushion. The skull is short but high and contains large air chambers.

Dentition is highly specialized. Each tusk is actually the second upper incisor. There are six cheek teeth (three premolars and three molars) in each half of each jaw. The tooth replacement pattern is unusual among mammals. The cheek teeth erupt from front to rear so that only a single tooth and fragment of another is functional at any one time. As a tooth becomes worn, it is replaced by the tooth behind it. The first three teeth in each quadrant of the jaw erupt during the first four years, the fourth erupts at four to five years of age, the fifth when the elephant is twelve to thirteen, and the last tooth becomes functional at about age twenty-five and remains in the jaw until the animal dies (for about the next fifty years). The high (hypsodont) and wide tooth consists of a series of thin laminae formed of enamel surrounded by dentine with cementum between each lamina. The last molar has the most laminae. This complicated chewing surface and pattern of tooth replacement provides a dentition lasting the lifetime of the elephant, up to eighty years. Elephants need these wide grinding cheek teeth because they consume more than 200 kg of forage each day, consisting of trees, leaves, shrubs, grasses, fruits, and aquatic plants.

Asian elephants differ from the African species by having much smaller ears, a flatter forehead, a dome-shaped head that is the highest point of the animal, four nails on the hind foot, and a single, fingerlike projection at the tip of the trunk. The shoulders are the highest point for the African elephant, the hind foot has three nails, and the trunk has two processes at its tip. These differences can be seen in any zoo; what cannot be viewed is the difference in number of ribs: nineteen pairs in the Asian, twenty-one in the African.

During the last few years, analyses of protein and DNA sequences of African mammals have supported the close relationship among hyraxes, sirenians, and elephants, as reflected by aspects of their anatomy and fossil histories. Surprisingly, results of the molecular analyses also indicated that the aardvark (Order Tubulidentata), elephant shrews (Order Macroscelidea), and tenrecs and golden moles (Order Afrosoricida) are also closely related to hyraxes, sirenians, and elephants, and all have been combined into Superorder Afrotheria. Judged by the fossil record, the divergence and major part of the evolution of these six assemblages occurred in Africa, between 105

and 40 million years ago, when that continent was an island after separating from South America and before colliding with Eurasia. The origin of aardvarks and elephant shrews can be traced by fossils back to African condy-larth ancestors, but tenrecs are represented only back to the Miocene and still look shrewlike and molelike. The concept of Afrotheria is one of the most remarkable current hypotheses dealing with mammalian evolution, and it is being tested by anatomists, paleontologists, and molecular systematists. In a child's alphabet of animals, A is for aardvark, E is for elephant, H is for hyrax, and S is for sea cow; different letters signaling animals wildly unalike in form, function, and habitat. But the letters are related in being part of a linguistic whole—an alphabet—and the mammals, along with elephant shrews, tenrecs, and golden moles, are linked by their evolutionary origin in Island Africa.

—Mary Ellen Holden

See also: Biogeography; Deserts and Semiarid Scrublands; Evolutionary Biodiversity; Extinction, Direct Causes of; Herbivory; Mammalia; Preservation of Species; Species; Systematics; Tropical Rain Forests

Bibliography

Hedges, S. Blair. 2001. "Afrotheria: Plate Tectonics Meets Genomics." Proceedings of the National Academy of Sciences 98, no. 1:1-2; Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th ed. Vol. 2. Baltimore: Johns Hopkins University Press; Van Dijk, Marjon A. M. 2001. "Protein Sequence Signatures Support the African Clade of Mammals." Proceedings of the National Academy of Science 98, no. 1:188-193; Vaughan, Terry A., James M. Ryan, and Nicholas J. Czaplewski. 2000. Mammalogy, 4th ed. Orlando: Harcourt.

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