WASHINGTON, MAY 8: When the British naturalist George Shaw received a weird specimen from Australia in 1799 — one with a mole’s fur, a duck’s bill and spurs on its rear legs — he did what any skeptical scientist would do: he looked for the stitching and glue that would reveal it to be a hoax.

“It was impossible not to entertain some distant doubts as to the genuine nature of the animal,” Shaw wrote of the seemingly built-by-committee creature, which he eventually named “platypus.”

Now, more than 200 years later, a team of scientists has determined the platypus’s entire genetic code. And right down to its DNA, it turns out, the platypus continues to strain credulity, bearing genetic modules that are in turn mammalian, reptilian and avian.

There are genes for egg laying — evidence of the animals’ reptilian roots. Genes for making milk, which the platypus does in mammalian style despite not having nipples. Genes for making snake venom, which the animal stores in its legs. And there are five times more sex-determining chromosomes than scientists know what to do with.

“It’s such a wacky organism,” said Richard Wilson, director of the genome center at Washington University in St Louis, who led the two-year international effort, described online Wednesday in the journal Nature.

Yet in its wackiness, Wilson said, the platypus genome offers an unprecedented glimpse of how evolution made its first stabs at producing mammals.

“As we learn more about things like platypuses,” Wilson said, “we also learn more about ourselves.”

Platypuses live on a relative sliver of Earth along Australia’s east coast, Tasmania and Papua New Guinea. But Ornithorhynchus anatinus has a global fan base, it seems, having been chosen as the mascot of countless companies, products and events.

The animal’s complete genetic code, or genome, turns out to have 2.2 billion molecular “letters” of DNA, or about two-thirds as many as the human genome, and contains 18,500 genes, about the same as humans. Finding the order of all those letters was gruelling, scientists said, because no similar animal has ever been sequenced.

“It was quite a difficult thing,” said Jennifer Marshall Graves of Australian National University in Canberra, who led part of the analysis after the St. Louis team derived the basic sequence. “The genome was completely unknown, and we knew it was going to be fairly weird,” Graves said. “You’d look at some of these repetitive sequences and think, ‘What on Earth is that?’ “

One of the more surprising elements was the animal’s system for sex determination. Most mammals have two sex chromosomes, either two X chromosomes (to make a female) or an X and a Y (to make a male). Not only do platypuses have 10 instead of two, but some of those resemble the Z and W chromosomes of birds.

Moreover, the key gene on the Y chromosome that confers maleness in most mammals is not present on any of the platypus’s sex chromosomes. It is on another chromosome, where it seems to have nothing to do with sex.

Other genes show how platypuses were transitional creatures on the road from egg laying to internal gestation. There is just one gene for one kind of yolk protein, for example, while chickens have three. That is consistent with the idea that the platypus represents a shift in strategy toward providing more nutrition after hatching, rather than during incubation, and lends credence to the poet Ogden Nash’s famous appreciation of the platypus’s approach to child-rearing: “I like the way it raises its family/ Partly birdly, partly mammaly.”