Mammal Classification
Monotremes
When the first duck-billed platypus arrived from Australia in Britain in 1798, it was assumed to be a fake, a fantastic creature with the small body of a mammal and the bill of a duck. Named Ornithorhynchus anatinus, the duck-billed platypus has long since been classified as a monotreme, along with two species of echidnas: the Australian, or short-nosed, echidna (Tachyglossus aculeatus) and the giant, or long-nosed, echidna of New Guinea. They are all believed to have originated in the Australian-Antarctic section of Gondwana, an ancient continental landmass that once overlay the southernmost region of the planet more than 100 million years ago (See Mammal Migration).
Monotremes are sufficiently different in their morphology and evolution for systematists to have placed them in their own subclass, called Prototheria (See Mammal Orders) —despite controversial new DNA analysis that suggests monotremes may be more closely related to marsupials than previously thought. (This is why some classification schemes locate monotremes in subclass Theria, along with marsupials and placental mammals.)
Monotremes amble like reptiles on laterally oriented limbs. Female monotremes lay soft reptilelike eggs, incubating them much as birds do. They lack nipples, or teats, from which to suckle their newborn. Instead, milk seeps from pores in the abdomen for the young to lap up. As adults, they lack teeth—although like all other mammals, they have a single dentary bone and three middle-ear bones.
Echidnas live on land and eat termites and ants. When enemies approach, they burrow directly into the ground or roll into a spiny, protective ball. The platypus, however, is aquatic. It has a long, leathery beak covered with electroreceptors capable of detecting the electrical field of small invertebrates. All male monotremes sport spurs on their ankles, but only those of the platypus are equipped with poison glands (See Mammal Mobility).
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