If there was a poster animal for diversification, it would have to be the platypus. It looks like an otter that’s gone trick-or-treating as a duck.

It’s a mashup that inspired Mark Anthony Libre to ask Weird Animal Question of the Week: "How did [the platypus] evolve in this unlikely fashion?”

The platypus is an Australian mammal with some weirdly reptilian traits, like egg laying.

While we think of mammals and reptiles as very different, at one time they shared a common ancestor, says Wes Warren of the Washington University School of Medicine in St. Louis.

Probing Platypus Evolution February 6, 2009—National Geographic researchers are trying to collect DNA samples from these odd duck-billed mammals to determine whether there are separate subspecies.





Evolutionary Split

Mammal-like reptiles diverged from the lineage they shared with birds and reptiles about 280 million years ago.

Around 80 million years later, the monotremes—or egg-laying mammals—split off from the mammalian lineage, says Rebecca Young, a biologist at the University of Texas at Austin.

View Images Venomous males have sharp stingers on the heels of their rear feet and can use them to deliver a strong blow. Photograph by Jean-Paul Ferrero, Minden Pictures

All that remains of that branch of the family tree is the platypus and four species of echidna. (Related: "Which Animals Have Barely Evolved?")

This split happened before the evolution of the placenta, Young says, “so in that sense they are somewhere between a lizard and what we think of as a human-like placental mammal,” retaining some reptilian and mammalian features.

While modern platypuses are down under, fossil evidence also shows that an ancient platypus lived in South America.

View Images A zookeeper cradles rare twin platypus babies, which are known as puggles, at Healesville Sanctuary in Australia. Photograph by Jason Edwards, Nat Geo Image Collection

Your Electric Bill

But why platypuses “stopped evolving and losing these components that make a mammal a mammal,” remains a mystery, says Warren.

The platypus's milk seeps through pores in its abdomen, not through teats as in all other mammals. Another incredible adaptation is how they forage for food. Platypuses close their eyes, ears, and noses underwater and find prey by sensing electric currents with their ducklike bills.

Their venom is located in a spur in the males' heels—a unique method of delivery among venomous creatures. (Also see "Venomous Primate Discovered in Borneo.")

View Images These bottom feeders scoop up insects, larvae, shellfish, and worms in their bill along with bits of gravel and mud. Platypuses do not have teeth, so the bits of gravel help them to "chew" their meal. Photograph by Dave Watts, Nature Picture Library

Warren led a 2010 study that found 83 toxins in platypus venom, which contains genes that resemble the venom genes of other animals, including snakes, starfish, and spiders. It's likely an example of convergent evolution, in which unrelated species evolve similar traits.

So while many things about the platypus remain mysterious. Young notes that there is "some randomness to how we acquire things over time," plus mutations and adaptations that happen more quickly.