National Geographic has an interesting story about the discovery of an ancient but extinct species, the Giant Platypus, (Obdurodon tharalkooschild).
Here's how some artist imagines it, chowing down on a turtle:
So... did they find a fossil skeleton? Not exactly. They found one molar - pictured in the inset, above. Apparently platypus teeth are very distinctive, and this molar was obviously of the platypus design, but it was way too big. So... they extrapolated, and anointed it a species.
Behold the Giant Platypus:
its fur and bill are fabulous.
But how close is the sketch to truth?
It's all based on a single tooth!