As you may know, the Platypus is one of only a handful of living Monotremes-- primative mammals that lay eggs. It was long believed that the Platypus had a very direct evolutionary story, one with very minimal branches. Obdurodon tharalkooschild, which was found in North-west Queensland (an area absent of modern Platypuses) was over a meter long and had teeth.
In fact, it was the fossilized tooth that put this whole story together. Modern Platypuses have teeth only as infants. They lose them as they grow up and they are never replaced. O. tharalkooschild had incredbily large teeth that it kept it's entire life. It had a carnivorous diet and could've used its giant chompers to feed on turtles and lungfish.
A few other extinct Platypuses have been discovered over the years, but all had smaller, and fewer teeth as they got chronologically closer to the present. O. tharalkooschild, which lived between 15 adn 5 million years ago, bucks that trend, which is why scientists believe it was part of a different Platypus evolution branch!
Status : Extinct, lived 15m-5m years ago
Location : Australia
Size : Length up to 3.3ft (1m)
Classification : Phylum : Chordata -- Class : Mammalia -- Order : Monotremata
Family : Ornithorhynchidae -- Genus : †Obdurodon -- Species : O. tharalkooschild
Image : Peter Schouten