Monotremata

(vertebrate zoology) The single order of the mammalian subclass Prototheria containing unusual mammallike reptiles, or quasi-mammals.

The single order of the mammalian subclass Prototheria. Two living families, the Tachyglossidae and the Ornithorhynchidae, make up this unusual order of quasimammals, or mammallike reptiles.

The Tachyglossidae comprise the echidnas (spiny anteaters), which have relatively large brains with convoluted cerebral hemispheres. The known genera, Tachyglossus and Zaglossus, are terrestrial, feeding on termites, ants, and other insects. They are capable diggers, both to obtain food and to escape enemies. Like hedgehogs, they can erect their spines and withdraw their limbs when predators threaten. Commonly one egg, but occasionally two or even three, is laid directly into the marsupium (pouch) of the mother where it is incubated for up to 10 days. Species of Tachyglossus live in rocky areas, semideserts, open forests, and scrublands. They are found in Australia, Tasmania, New Guinea, and Salawati Island. Species of Zaglossus are found in mountainous, forested areas.

The duck-billed platypus, constituting the Ornithorhynchidae, has a relatively small brain with smooth cerebral hemispheres. The young have calcified teeth, but in the adult these are replaced by horny plates which form around the teeth in the gums. The snout is duck-billed. The semiaquatic platypus is a capable swimmer, diver, and digger. Two eggs are usually laid by the female into a nest of damp vegetation. After incubating the eggs for about 10 days the female leaves, returning only when the eggs are hatched. The platypus is found in Australia and Tasmania in almost all aquatic habitats. See also Mammalia; Prototheria.

The lowest order of mammals, including animals that lay eggs similar to those of reptiles, and nourish their young by a mammary gland that has no nipple, in a shallow pouch developed during lactation. The only living representatives are the spiny anteater and the duck-billed platypus.