The platypus is an aquatic mammal, one of only five monotremes - mammal species which lays eggs as opposed to a live birth. Its natural range is in eastern Australia and Tasmania. It has a bill, similar in shape to a duck, which is very pliable, and also has webbed feet. Like all mammals it has fur and suckles its young. The platypus can find its prey in water due to its ability to detect electric fields produced by muscular contractions. The male platypus has has a spur on its hindfoot which can deliver venom - a feature which very few mammals possess. It has no visible ears.
An excellent swimmer, the platypus spends much of its time foraging for food in water. It swims with an alternating left-to-right paddling of its front feet. Despite all four of its feet being webbed, its hindfeet remain held against its body, instead being used for steering (along with its tail) rather than propulsion.
The platypus (and its fellow monotreme the echidna) was believed to have evolved in isolation 225 million years ago when the land mass that would become Australia (Gondwana) broke away from the other continents. This idea of evolution in isolation followed the theory of Charles Darwin, whose affinity for evolution may also have been influenced by his early studies of the platypus during his time on The Beagle.
However, the isolation theory was overturned when, in the early 1990s, three platypus teeth were discovered in South America - virtually identical to fossil platypus teeth found in Australia. (Marsupials, too, were once considered to be exclusive to Australia, but their fossils have now been found on every continent). Modern adult platypuses do not have teeth, but the discovery of platypus fossils in Australia had already identified that their ancestors did indeed have unique and distinctive teeth.
The platypus is featured on the Australian 20 cent coin.