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.
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