It was thought previously that the Kronosaurus was even longer.  It would be comparable in size to the whale shark, but had nice, big chompers that it used to eat more than little things like krill.  Imagine swimming around and seeing one of those!

By the way, these are considered Pliosaurs, which lived in the Cretaceous and Jurassic periods.

Also, Kronos is the same dude as Cronus.  He ruled for a while until he was overthrown by the guys most people are familiar with (probably thanks to Disney), like Zeus and Hades, who also happen to be his sons.