I am currently reading my way through a long list of science fiction and fantasy titles. (http://www.npr.org/2011/08/07/138938145/science-fiction-and-fantasy-finalists if you are interested in the list).
Ichthyosaurs were busy in the oceans while dinosaurs stomped on the land. They are an example of convergent evolution, being reptiles that developed many traits in common with the mammalian dolphin.
Now we know they have more similarities than shape!