Metatheria

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Metatheria

[‚med·ə′thir·ē·ə]
(vertebrate zoology)
An infraclass of therian mammals including a single order, the Marsupialia; distinguished by a small braincase, a total of 50 teeth, the inflected angular process of the mandible, and a pair of marsupial bones articulating with the pelvis.

Metatheria

 

(marsupials or pouch-bearing mammals), an in-fraclass of the most primitive of living viviparous mammals. The Metatheria comprise one order (Marsupialia) with nine families: Didelphidae (opossums), Dasyuridae (pouched, or marsupial, mice and rats, native cats, the Tasmanian devil, and the Tasmanian wolf), Myrmecobiidae (one species—the numbat), Perame-lidae (bandicoots), Notoryctidae (the marsupial, or pouched, mole), Caenolestidae (rat opossums), Phalangeridae (phalangers, cuscuses, and koalas), Vombatidae (wombats), and Macro-podidae (kangaroos and wallabies). Some taxonomists divide the Metatheria into several orders, sometimes as many as five.

The body length varies from 4–10 cm (flat-skulled marsupial mice [genus Planigale]) to 75–160 cm (giant kangaroo [Macropus giganteus]). The appearances are quite varied: from molelike (bandicoots) to graceful and long-legged (Tasmanian wolf [Thylacinus cynocephalus]) or jerboa-like (jerboa pouched mice [genus Antechinomys]). In most Metatheria the tail is well developed; in climbing forms (several opossums and Phalangeridae) it may be prehensile. The extremities are usually pentadactyl, the first and fifth toes usually reduced. In some members, for example, kangaroos, the second and third toes are fused together over their entire length. In many climbing forms (koalas, cuscuses, and others) the first one or two toes are opposed. In burrowing forms (bandicoots, marsupial moles) the claws on the forelimbs are much enlarged; in jumping forms (jerboa pouched mice and kangaroos) the hind limbs are elongated, the forelimbs shortened, and the tail long. The pelage is thick and usually soft; sometimes it is bristly. The vibrissae on the muzzle and extremities are well developed. The prehensile tail of several arboreal forms is hairless or has a hairless tip. The coloration is usually uniform and protective, less commonly spotted (genus Dasyurus) or striped (Tasmanian wolf).

Preanal glands are characteristic of the Metatheria. The female of most species has a brood pouch formed by a skin fold. The shape of the pouch and the extent of development vary considerably. The pouch may open forward (kangaroos, wombats, several opposums) or backward (bandicoots, several dasyurids); in some cases it consists of small lateral skin folds or is absent altogether (rat opossums, several opossums, numbat). There are two to six teats in the pouch. Species with no pouch have as many as 25 teats arranged on the sides of the body.

The skull varies in shape from slightly elongated to short and massive. An inwardly curved angular process of the lower jaw is characteristic. The dentition is heterodont, and the number of teeth varies from 22 (honey possum [Tarsipes spenserae]), to 50–52 (numbat). In the more primitive Metatheria (opposums, several dasyurids, and others) the permanent teeth include five incisors and four molars in each half of the upper jaw. In the more developed herbivorous Metatheria (kangaroos, wombats, phalangeridae) the first incisors in both jaws are enlarged, and the remaining incisors are reduced. The milk teeth are represented by a single premolar in each jaw.

Both sexes have characteristic marsupial bones, which depart from the pubic symphysis of the pelvic girdle. All segments of the spinal column are developed normally. All Metatheria except bandicoots have a clavicle. A movable articulation of the fibula and tibia is characteristic of Phalangeridae.

The brain is primitive and lacks a corpus callosum; the neocortex has virtually no sulci. The sensory organs are typical of higher mammals. Ducts of Cuvier are found in the cardiovascular system. The body temperature is 34°–36°C—somewhat lower than in placentals. Females have a double vagina and double uterus. A typical placenta does not develop except in bandicoots. The young nurse passively: milk is squirted from a teat by the contraction of special muscles. The arrangement of the palate and larynx in the young permits simultaneous breathing and feeding.

The most primitive Metatheria are the opossums, from which in the late Cretaceous all the other families evolved. Metatheria are distributed in the New World from Canada to Tierra del Fuego (opossums and rat opossums) and in Australia, Tasmania, and New Guinea, and on adjacent islands and on some of the Sunda Islands (all other families). Some kangaroo species have been introduced into New Zealand.

The adaptations exhibited by Metatheria are almost as varied as those of placentals. Members inhabit extremely diverse landscapes: open tracts (many kangaroos, wombats, marsupial moles), forests (Phalangeridae, dasyurids, opossums, and others), and mountains at elevations up to 5,000 m (some opossums). They may be arboreal (Phalangeridae and some dasyurids and opossums) or terrestrial (some dasyurids and opossums, the numbat, wombat, kangaroos). Specialized subterranean forms (marsupial moles) and a semiaquatic form (yapock [Chironectes minimus]) also exist.

Most Metatheria are capable of rapid locomotion. Those that inhabit open spaces (kangaroos, marsupial jerboas) are adapted to jumping. The lesser gliding possums (genus Petaurus) can soar in the air as much as 55 m. Methatheria may be either diurnal or nocturnal. Tree hollows and crowns, ground depressions, and rock crevices serve as shelters. Some members dig burrows; wombat burrows may reach 3 m in length. The diet is varied —plants, terrestrial vertebrates, and insects—and many members are omnivorous.

Metatheria reproduce once to several times per year; gestation varies from eight to 40 days. The young are born underdeveloped and range from 0.5 to 3 cm in length. Very soon after birth they are safely in the pouch, where they hang from a teat. The margins of the mouth grow together around the teat, and nursing continues more than two months. The baby marsupial remains in the pouch up to eight months.

Marsupials have little economic value; a few large forms are hunted for their meat and pelts. Some marsupials (kangaroos, some opossums, and dasyurids) may do damage in areas where agriculture is developed. Man and introduced predatory placentals have greatly decreased the populations of several species. Commercial hunting of many species is restricted, and several species are now protected. Twenty-three species from different families have declined in numbers so sharply that they are on the verge of extinction and have been entered in the Red Data Book.

The earliest fossil Metatheria are known from Lower Cretaceous deposits in North America. Metatheria appeared in South America during the Paleocene. South American fossil marsupials include giant specimens the size of bears (Prothylacinus and Borhyaena). In Europe the Metatheria existed from the Eocene to the Miocene. In Australia they date from the Oligocene.

REFERENCES

Sokolov, V. E. Sistematika mlekopitaiushchikh. Moscow, 1973.
Zhizn’ zhivotnykh, vol. 6. Moscow, 1971.
Troughton, E. Furred Animals of Australia, 9th ed. Sydney, 1967.

O. L. ROSSOLIMO