When the silurian hypothesis comes up most people imagine the eponymous lizard people with big brains -- but, we are only asking: Could there have been intelligent life in some earlier age of earth, undetectable to us now in the fossil record? It need not be humanoid. It need not live on land.

Hence, I will suggest a global sea-spanning ultra-intelligent siphonophore who studied the stars, the earth and itself with biotech. One great mind, many individuals. 1/