Frolicking Ichthyosaurs

ichthyosaurs frolic in the waves
("Animals of the Prehistoric World", Heinrich Harder)

The ichthyosaur is a marine reptile that reigned the seas during the Mesozoic Era, the age of dinosaurs. It looks very similar to the shape of a dolphin. 

This illustration is from a series of 30 collector cards from "Tiere der Urwelt" or Animals of the Prehistoric World that were created in 1916.