From the ABC :
Scientists from the Australian National University have proved what many have thought for years - platypuses are really weird.
In the international Nature journal today they report a platypus has five chromosones determining sex, not one - like the rest of the species in the world.
Professor Jennifer Graves says platypus have five X and five Y chromosomes, and when sperm are made it gets even stranger.
"What we've discovered is that these five Xs and five Ys line up in a great big long chain, that go XY XY XY XY XY XY, and then all the X chromosomes move to one pole, and all the Y chromosomes move to the other," she said.
Professor Graves says there is another unexpected finding.
"One end of the chain looks like human sex chromosomes but the other end of the chain looks like bird sex chromosomes, so the chain is actually linking a very ancient system of sex determination in birds and probably reptiles too," she said.
The unique status of the Australian mammal is now unassailable.
The Australian takes it further :
The new research puts platypuses closer on the sex chromosome scale to spiders, termites and birds than their fellow mammals.In fact, the Echidna and Platypus differ not just from mammals, birds and reptiles, they differ from all other vertebrates in having this arrangement. It was thought that they were the last representatives of the primitive mammals that pre-dated the dinosaurs. (yes, mammals came first, but got supressed for a few eons), sort of halfway between reptile and mammal. This research shows that they are vastly more, er, peculiar.
Echidnas, the only other member of the monotreme club, have a similar "weird" chromosome make-up.
Only in Australia. (And in fact, there's a thriving colony of them not too far from where I'm writing this).