This was a fun focus on ancient creatures who don't get enough press: the sea equivalent of the dinosaurs. They were huge and very, very mean sea lizards. Big teeth. Well, at least it was fun if, like me, you enjoy reading about phylogenetic debates. The illustrations are great. There was perhaps too much information on paleontological controversy. I didn't feel the need to know about the 6 different successive theories of what this one fossil dude used its limbs for, and occasionally information was repeated redundantly.
That said, it's really a very good distillation of the subject for non-professional readers who have a basic knowledge already.