Mammalodon colliveri

Sketch of the head of Mammalodon colliveri, an extinct whale from the Oligocene of Victoria, Australia.

It's actually a weird baleen whale, although it still had teeth. It's theorised that it sucked mud from the ocean floor, a precursor of sorts to the filter-feeding of modern baleen whales. Its teeth were used to crunch up larger prey like big shellfish. Mammalodon was also a tiny whale, only about 3m long.

I wish I had done the teeth better. I was going off an image of the skull, but the teeth weren't shown very well.