First appeared Mid-Ordovician, evolved from Conodonts (eel shaped, 4-20cm long). Earliest such as Jaymoytuis had no jaw, small primitive fins, sucking mouth. Silurian= armoured variations called Ostracoderms evolved. End of Silurian= jawless fish replaced by armour-plated fish with jaws= Placoderms= lived in both marine and fresh water, most= small (Hemicyclaspis) but several large predators (Dinichthys, 10m +) also existed. Placoderms remained common throughout Devonian but declined during Carboniferous and became extinct in Permian. Devonian= 2 different adaptations= enabled fish to swim more efficiently- (1) Some groups such as sharks evolved lightweight skeleton composed of cartilage= rarely preserved= only teeth remains, (2) most kept bone skeleton but developed internal buoyancy pack= swim bladder