The oldest probable fishes are Early Cambrian.
Had fins, fish musculature and gills, but no jaws.
Remained insignificant until the Silurian.
The Silurian saw a rapid diversification of jawless fish, as well as the origin of jawed fish.
Lampreys and hagfish are surviving jawless fish.
Left-hand image courtesy of Wikimedia Commons.
Right-hand image courtesy of NOAA.
Lamprey mouths on aquarium glass.
Hagfish on the seafloor off Oregon.