Monotremata (class Mammalia, subclass Prototheria) An order comprising the duck-billed platypus (Ornithorhynchus anatinus, see ORNITHORHYNCHIDAE) and the echidnas or spiny ant-eaters (Tachyglossus and Zaglossus, see TACHYGLOSSIDAE). There were some extinct forms, of which very few are known in detail, though of those some attained large sizes. The echidnas have no fossil record older than the Pleistocene, but a fossil platypus, Obdurodon, is known from the Miocene and in the early 1990s teeth of an undoubted platypus-like form, Monotrematum sudamericanum, were discovered in Palaeocene deposits in Patagonia. Two Cretaceous genera, Steropodon and Kollikodon, are also known. In view of their reptilian affinities they are thought to represent a separate and direct line of descent from the earliest Mesozoic animals, possibly the Docodonta, independent of the line leading to other mammals, but the dental features of Steropodon are considered by some to point to affinities with the Eupantotheria. They retain many primitive features and are quite unlike marsupials or eutherians. The rectum and urinogenital system open to a common cloaca (the name ‘monotreme’ is derived from the Greek monos meaning ‘alone’ and trema meaning ‘hole’, although the feature is shared by marsupials and some insectivores). The male is heterogametic as in other mammals. The young are hatched from large, yolky eggs, incubated in a nest by the female platypus and in a pouch in the echidnas. The embryos develop a caruncle and egg-tooth. After hatching they are fed milk secreted by the female from specialized sweat glands which do not open through central nipples. The diaphragm is fully developed, and the heart possesses a single left aortic arch as in other mammals. The larynx is developed, and monotremes make sounds. The tarsus of the male has a grooved erectile spine to which (in the platypus) poison is fed from a gland in the thigh. The poison (said to be capable of killing a dog) may serve to immobilize the female during mating. Adults lack teeth but possess bills. The skull is specialized but retains primitive features. The jaw consists of a single bone. The cervical ribs are not fused, the shoulder girdle is reptilian in form, the pelvis is reminiscent of that of marsupials. The body is covered with hair, in the echidnas partly modified into spines on the back. The limbs are modified for digging (echidna) or swimming (platypus); it has recently been discovered that both platypus and echidna locate their prey by detecting weak electrical fields around the snout. Echidnas hibernate, and in hot weather all monotremes shelter in burrows or caves. Monotremes are known only from Australia and New Guinea, where they may have survived because of their high degree of specialization and the isolation of that continent.

Monotreme

Monotremes are an ancient group of mammals in the order Monotremata, which probably split from the lineage leading to marsupials (those with no placenta and having a pouch in the abdomen) and placental mammals early in mammalian evolution. The earliest fossil occurrence of monotremes is in the lower Cretaceous, approximately 110 million years ago.

Monotremes retain some of the primitive characteristics of mammalian ancestors, the therapsids. Monotremes lay eggs, have a somewhat reptilian posture, and retain a cloaca , a body cavity into which the reproductive, urinary, and excretory systems empty. Monotremes lack teeth as adults and have an unusual cranial shape. However, monotremes possess several critical mammalian features. They have fur, four-chambered hearts, single dentary (lower jaw) bones, and mammalian ear structure, and they lactate, or produce milk. Females lay one to three small, leathery eggs and incubate them outside of the body. Upon hatching, the young lap milk from the mother's mammary glands, which lack a nipple.

There are two families and three species of monotremes. The family Tachyglossidae includes two species: the spiny anteater, found in Australia, Tasmania, and southern New Guinea; and the long-nosed anteater, found only in New Guinea. The family Ornithorhynchidae includes a single species, the duck-billed platypus, an aquatic species that is found in eastern Australia and Tasmania. All three species eat primarily invertebrates and are prodigious burrowers. Populations of the long-nosed anteater are currently threatened by overhunting. Platypus is a protected species, and both the spiny anteater and platypus populations seem stable as of 2001.

see also Mammal; Marsupial

Tanya Dewey

Bibliography

Anderson, Sydney, and J. Knox Jones, Jr., eds. Orders and Families of Recent Mammals of the World. New York: John Wiley & Sons, Inc., 1984.

Nowak, Ronald M., ed. Walker's Mammals of the World. Baltimore, MD: Johns Hopkins University Press, 1991.

Vaughan, Terry A. Mammalogy. Philadelphia, PA: Saunders College Publishing, 1986.

monotreme , name for members of the primitive mammalian order Monotremata, found in Australia, Tasmania, and New Guinea. The only members of this order are the platypus , or duckbilled platypus, and the several species of echidna , or spiny anteater. Although monotremes possess the distinguishing mammalian features of hair and mammary glands, they are unique among mammals in laying eggs rather than giving birth to live young. The eggs are like those of reptiles, with large yolks and leathery shells. Like birds and reptiles, monotremes have a single opening, the cloaca , for the passage of liquid and solid wastes, the transfer of sperm, and, in the female, the laying of eggs. In addition, certain features of the skeletal structure are like those of reptiles, and the regulation of body temperature is less effective than in other mammals. Adult monotremes are toothless. The males possess spurs on their hind feet; these are connected to poison glands and are presumably used as weapons. Mammals are known to have evolved from reptiles; the monotremes probably branched off at an early stage of mammalian evolution and have retained many reptilian features. They are classified in the phylum Chordata , subphylum Vertebrata, class Mammalia, order Monotremata.

Monotremata (class Mammalia, subclass Prototheria) An order comprising the duck-billed platypus (Ornithorhynchus anatinus) and the echidnas or spiny ant-eaters, Tachyglossus and Zaglossus. There were some extinct forms of which very few are known in detail, though of those some attained large sizes. The echidnas have no fossil record older than the Pleistocene, but a fossil platypus, Obdurodon, is known from the Miocene and in the early 1990s teeth of an undoubted platypus-like form, Monotrematum sudamericanum, were discovered in Palaeocene deposits in Patagonia. Two Cretaceous genera, Steropodon and Kollikodon, are also known. In view of their reptilian affinities they are thought to represent a separate and direct line of descent from the earliest Mesozoic animals, independent of the line leading to other mammals. They retain many primitive features and and are quite unlike marsupials or eutherians (placental mammals).

monotreme One of an order of primitive mammals that lay eggs. The only monotremes are the platypus and two species of echidna, all native to Australasia. The eggs are temporarily transferred to a pouch beneath the female's abdomen where they eventually hatch and are nourished by rudimentary mammary glands. See also marsupial

monotremes See MONOTREMATA.

monotremes See Prototheria.

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