In separate publications in 1934, 1947, and 1951, William King Gregory theorised that placental mammals and marsupials may have diverged earlier, and a subsequent branching divided the monotremes and marsupials. Later research and fossil discoveries have suggested this is incorrect. Modern monotremes are the survivors of an early branching of the mammal tree, and a later branching is thought to have led to the marsupial and placental groups. Both molecular clock and fossil dating suggest that the platypus split from echidnas around 19–48million years ago.
thumb|right|Reconstruction of ancient platypus relative Steropodon
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-epoch strata. The extinct monotremes Teinolophos, Steropodon and Kollikodon from the Cretaceous period are considered to be basal to the platypus and echidnas. The remains of Steropodon were discovered in New South Wales, 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 that, while they have three cusps, they evolved under a separate process. The fossil jaw of Teinolophos is elongated but unlike the modern platypus (and echidnas), lacks a beak.
In 2024, Late Cretaceous (Cenomanian)-aged fossil specimens of early platypus relatives were recovered from the same rocks as Steropodon, including the basal Opalios and the more derived Dharragarra, the latter of which may be the oldest member of the platypus family Ornithorhynchidae, as it retains the same dental formula found in Cenozoic platypus relatives. Monotrematum and Patagorhynchus, also 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. These are also considered potential members of the platypus Ornithorhynchidae.
The loss of teeth in the modern platypus has long been enigmatic, as a distinctive lower molar tooth row was present in its lineage for over 95 million years. Even its closest relative, Obdurodon, which otherwise closely resembles the platypus, retained this tooth row. More recent studies indicate that this tooth loss was geologically a very recent event, occurring only around the Plio-Pleistocene about 2.5 million years ago, when the rakali, a large semiaquatic rodent, colonized Australia from New Guinea. The platypus, which previously fed on a wide array of hard and soft-bodied prey, was outcompeted by the rakali for hard-bodied prey such as crayfish and mussels. 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, over which the rakali did not compete. The platypus genome also has both reptilian and mammalian genes associated with egg fertilisation. Though the platypus lacks the mammalian sex-determining gene SRY, a study found that the mechanism of sex determination is the AMH gene on the oldest Y chromosome. A draft version of the platypus genome sequence was published in Nature in May 2008, revealing both reptilian and mammalian elements, as well as two genes previously found only in non-mammalian vertebrates. More than 80% of the platypus's genes exist in other amniotes whose genomes have been compared.
Conservation
The International Union for Conservation of Nature categorised the platypus as "near threatened" in 2016, and throughout Australia by 1912, it continues to drown in the nets of inland fisheries. The use of "opera house traps" by recreational fishers for catching yabbies is banned in the ACT, South Australia, Tasmania and Victoria, and restricted in NSW and Queensland, due to the traps drowning non-targeted species including platypuses. The platypus was listed as endangered is South Australia, under the National Parks and Wildlife Act 1972. In November 2020 a recommendation was made to list the platypus as a vulnerable species across all states. In January 2021, Victoria officially adopted the vulnerable species designation, under the state's Flora and Fauna Guarantee Act 1988. The platypus is not covered under the federal EPBC Act.
The platypus continues to be adversely affected by habitat disruption caused by dams, pollution, urban expansion, and urban runoff. Droughts and the demands for water for human use are also considered threats.
A November 2020 report by scientists from the University of New South Wales, funded by a research grant from the Australian Conservation Foundation in collaboration with the World Wildlife Fund Australia and the Humane Society International Australia revealed that over the past thirty years platypus habitat in Australia has dropped by 22%, and supported listing the platypus as a threatened species under the EPBC Act, as the declines have been mostly in the Murray–Darling basin and NSW in general. including twins. In 1990-91, there was successful breeding of platypuses at Warrawong Sanctuary. and the facility has since bred more platypuses to be released into the wild in NSW. , the only platypuses in captivity outside of Australia are in the San Diego Zoo Safari Park in the U.S. state of California. Three platypuses were given to the Bronx Zoo in 1947, two females and a male. One of the females; Penelope had a false proto-pregnancy.
Human interactions
Usage
Aboriginal Australians hunted and ate platypuses, particularly for their fatty nutritious tails, while, after colonisation, Europeans killed them for fur from the late 19th century until 1912, when it was prohibited by law. 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. Australia gave live platypuses as diplomatic gifts to Allied nations as part of an initiative to increase military assistance. One of which was intended as a gift to Winston Churchill died from neglect while en route. Aboriginals from the upper Darling River region have a story of a large water-rat called Biggoon who kidnaps a duck what wandered too far from its tribe. After managing to escape, she returned and laid two eggs which hatched the first platypuses. They were all exiled and went to live in the mountains. In another story from the upper Darling, the major animal groups, the land animals, water animals and birds, all competed for the platypus to join their respective groups, but the platypus ultimately decided to not join any of them, feeling that he did not need to be part of a group to be special,