The platypus is the sole living representative or monotypic taxon of its family Ornithorhynchidae and genus Ornithorhynchus, though a number of related species appear in the fossil record.

[12] When the platypus was first encountered by Europeans in 1798, a pelt and sketch were sent back to Great Britain by Captain John Hunter, the second Governor of New South Wales.

[16][15] The common name "platypus" literally means 'flat-foot', deriving from the Greek word platúpous (πλατύπους),[17] from platús (πλατύς 'broad, wide, flat')[18][a] and poús (πούς 'foot').

[27] The body and the broad, flat tail of the platypus are covered with dense, brown, biofluorescent fur that traps a layer of insulating air to keep the animal warm.

[36] As in all true mammals, the tiny bones that conduct sound in the middle ear are fully incorporated into the skull, rather than lying in the jaw as in pre-mammalian synapsids.

The female platypus, in common with echidnas, has rudimentary spur buds that do not develop (dropping off before the end of their first year) and lack functional crural glands.

[53] Digging in the bottom of streams with its bill, its electroreceptors detect tiny electric currents generated by the muscular contractions of its prey, enabling it to distinguish between animate and inanimate objects.

[55][56] The platypus can feel the direction of an electric source, perhaps by comparing differences in signal strength across the sheet of electroreceptors, enhanced by the characteristic side-to-side motion of the animal's head while hunting.

A temporal (ear side) concentration of retinal ganglion cells, important for binocular vision, indicates a vestigial role in predation, though the actual visual acuity is insufficient for such activities.

[59] The platypus is semiaquatic, inhabiting small streams and rivers over an extensive range from the cold highlands of Tasmania and the Australian Alps to the tropical rainforests of coastal Queensland as far north as the base of the Cape York Peninsula.

[77][78] The platypus rests in a short, straight burrow in the riverbank about 30 cm (12 in) above water level, its oval entrance-hole often hidden under a tangle of roots.

[79] The platypus is a carnivore, feeding on annelid worms, insect larvae, freshwater shrimp, and yabby (crayfish) that it digs out of the riverbed with its snout or catches while swimming.

[96] The oldest discovered fossil of the modern platypus dates back to about 100,000 years ago during the Quaternary period, though a limb bone of Ornithorhynchus is known from Pliocene-aged strata.

[98] The fossilised Steropodon was discovered in New South Wales and is composed of an opalised lower jawbone with three molar teeth (whereas the adult contemporary platypus is toothless).

The molar teeth were initially thought to be tribosphenic, which would have supported a variation of Gregory's theory, but later research has suggested, while they have three cusps, they evolved under a separate process.

[100] Monotrematum and Patagorhynchus, two other fossil relatives of the platypus, are known from the latest Cretaceous (Maastrichtian) and the mid-Paleocene of Argentina, indicating that some monotremes managed to colonize South America from Australia when the two continents were connected via Antarctica.

[102] The loss of teeth in the modern platypus has long been enigmatic, as a distinctive lower molar tooth row was previously present in its lineage for over 95 million years.

This competition may have selected for the loss of teeth in the platypus and their replacement by horny pads, as a way of specializing for softer-bodied prey, which the rakali did not compete with it over.

[100] Because of the early divergence from the therian mammals and the low numbers of extant monotreme species, the platypus is a frequent subject of research in evolutionary biology.

[106][107] A draft version of the platypus genome sequence was published in Nature on 8 May 2008, revealing both reptilian and mammalian elements, as well as two genes found previously only in birds, amphibians, and fish.

[113] The platypus is not considered to be in immediate danger of extinction, because conservation measures have been successful, but it could be adversely affected by habitat disruption caused by dams, irrigation, pollution, netting, and trapping.

[115] In January 2020, researchers from the University of New South Wales presented evidence that the platypus is at risk of extinction, due to a combination of extraction of water resources, land clearing, climate change and severe drought.

[117][118] The study predicted that, considering current threats, the animals' abundance would decline by 47–66% and metapopulation occupancy by 22–32% over 50 years, causing "extinction of local populations across about 40% of the range".

The authors stressed the need for national conservation efforts, which might include conducting more surveys, tracking trends, reduction of threats and improvement of river management to ensure healthy platypus habitat.

Mucormycosis can kill platypuses, death arising from secondary infection and by affecting the animals' ability to maintain body temperature and forage efficiently.

The leading figure in these efforts was David Fleay, who established a platypusary (a simulated stream in a tank) at the Healesville Sanctuary, where breeding was successful in 1943.

[132] In addition, European researchers captured and killed platypus or removed their eggs, partly in order to increase scientific knowledge, but also to gain prestige and outcompete rivals from different countries.

[112] Another Dreaming story emanate of the upper Darling tells of a young duck which ventured too far, ignoring the warnings of her tribe, and was kidnapped by a large water-rat called Biggoon.

[138][139] In the American animated series Phineas and Ferb, the title characters own a pet bluish-green platypus named Perry who, unknown to them, is a secret agent.

Such choices were inspired by media underuse, as well as to exploit the animal's striking appearance;[140] additionally, show creator Dan Povenmire, who also wrote the character's theme song, said that its opening lyrics are based on the introductory sentence of the Platypus article on Wikipedia, copying the "semiaquatic egg-laying mammal" phrase word for word and appending the phrase "of action";[141] however, the article did not include "egg-laying mammal" in the lead sentence until 2014, several years after the song released.