The term fish is used for any any swimming vertebrate with fins. It includes a diverse group of animals, divided into four taxonomic classes, Agnatha, Chondrichthyes, Osteichthyes, and Placoderma. Taxonomists first divided all "fish" into two classes, Agnatha and Placoderma. The Placoderma are considered ancestors of all other fish. What fossil evidence helped taxonomists separate all "fish" into these two branches for classification?