Strange Tooth
Whales fall into two groups: those with teeth and those with mouths lined by baleen, or hair-like bristles that filter crustaceans and plankton from seawater.
Baleen is actually an evolved type of tooth, hyper-specialized for a food source that didn't exist on land. Whales that fed on these tiny but plentiful organisms were able to attain vast sizes; blue whales, the largest creatures ever to exist on Earth, dinosaurs included, can ingest 470,000 calories in a single gulp.
Image: Museum of New Zealand Te Papa Tongarewa, 2008