National Geographic : 2009 Oct
PHOTOS: MARIE READ, WOODFALL/PHOTOSHOT TOP ; DAVID DOUBILET, NATIONAL GEOGRAPHIC STOCK W ILDLIFE Reptilian Roots The platypus is so bizarre its discovery was first dismissed as a hoax. After an Australian specimen arrived in London in 1798, biologists had to make a call: reptile or mammal? On the mammal side, it was covered in thick fur and nursed its young---with milky patches on the belly instead of nipples. On the reptile side, it laid eggs. Scientists voted mammal. Now researchers have sequenced the platypus genome, confirming the classification but also finding much reptile-like DNA. Mammals and reptiles share common ancestors but headed down separate evolutionary paths 315 million years ago. The platypus is a rare window on those first mammals, which had reptile traits such as egg laying. When the platypus branched off from the ancestors of most living mammals, it kept both its mammalian and reptilian DNA. Like rats, the platypus has genes linked to a keen sense of smell, which may help it find food. Like snakes, the platypus developed venom---with genes inherited from their common ancestor. Male platypuses inject the venom through spurs (left) on their hind legs when they fight over mates. ---Karen E. Lange A freshwater oddity found only in Australia, the platypus resembles early reptile-like mammals.