The surprising evolution of the weirdest animals on Earth, according to a new study
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The story of two of the strangest animals on the planet just got a little stranger, thanks to clues revealed by a lone fossil specimen that scientists now say represents a long-extinct ancestor. The new research could upend what’s known of the evolution of the most primitive mammals alive today.

Found in Australia and New Guinea, the platypus and echidna are called monotremes, and they are unique for being the only mammals that lay eggs.
The amphibious platypus has a bill and webbed feet, like a duck, and a beaver-esque tail. The small creature spends much of its time hunting for food in the water. The echidna — fittingly known as the spiny anteater — lives entirely on land, is covered in pointy quills and has rear feet that face backward, kicking up dirt as the animal burrows into the ground. Neither animal has teeth, and though they both produce milk, they secrete it through their skin for babies (often called puggles) to lap at, because they lack nipples.

“There’s plenty of weirdness to go around on these little things,” said Dr. Guillermo W. Rougier, a professor in the department of anatomical sciences and neurobiology at Kentucky’s University of Louisville who studies early mammalian evolution.

“They are one of the defining groups of mammals,” Rougier said. “The typical mammal from the time of dinosaurs probably shared a lot more biology with a monotreme than with a horse, a dog, a cat or ourselves.” Therefore, he said, monotremes provide a window into the origins of mammals on Earth.