Now, researchers have found that the genome that underlies these disparate traits is as diverse as the platypus itself.
"The platypus genome, like the animal itself, is an amazing amalgam of reptile-like and mammal-like features," study co-author Jennifer Graves, of the Australian National University in Canberra, told National Geographic News.
Graves and more than 100 other researchers worked together to sequence the genome of a female platypus nicknamed Glennie, from Glenrock Station in New South Wales, Australia.
The platypus, which lives only in Australia, Papua New Guinea and Tasmania, is one of only two monotremes (the other is the echidna), an unusual order of mammal that produces milk and has fur, but like reptiles lays eggs rather than giving birth to its young. Male platypuses have another reptilian trait -- they produce venom that they can shoot from spurs in their hind legs.
Monotremes split off from other mammals and began to evolve separately almost 170 million years ago. That means that the platypus' DNA can tell us about a long-ago period in mammalian evolution, according to researcher Chris Ponting, of Oxford University.
"Knowing the platypus DNA now allows us to look way back in time ... to when all mammals laid eggs, and when none of the placental mammals yet existed," Ponting said in an e-mail. "It has retained many genes that other mammals have lost from a time when all mammals looked much like lizards."
For example, platypuses have X and Y chromosomes like humans, but in platypuses those chromosomes have nothing to do with determining sex. Instead, platypuses have a sex selection system that looks more like birds, with 10 sex chromosomes.
Before this study, researchers believed that sex determination in mammals evolved at the time birds and mammals split, about 350 million years ago, Australian National University researcher Paul Waters told the Australian Broadcasting Corporation. But if platypus sex chromosomes are more similar to birds' sex chromosomes than mammals', that means that human sex chromosomes evolved later.
"It changes our understanding of sex chromosome evolution," Waters told the ABC.
The researchers found other reptilian-like genes, for egg yolk proteins and venom production, as well as mammalian genes for traits like producing milk.
Some of these genes reflected the platypus's position at a midway point somewhere between reptiles and other mammals, the Washington Post explained. For example, while chickens have three genes for a particular type of yolk protein, platypuses have only one -- suggesting evolution had begun to shift some resources from providing nutrition while in the egg to after hatching.
"It is much more of a melange than anyone expected," researcher Ewan Birney, of the European Bioinformatics Institute in Cambridge, U.K., told Agence France-Presse.
Mapping the platypus genome was a particular challenge, Graves told the Washington Post, because although many other organisms' genomes have been sequenced -- from mice to humans to corn -- none look much like the platypus.
"It was quite a difficult thing," she told the paper. "The genome was completely unknown, and we knew it was going to be kind of weird. You'd look at some of these repetitive sequences and think 'What on Earth is that?'"
The research was published Thursday in the journal Nature.