Plesiosaurs are an amazing example of a completely extinct group of animals with the potential to help us understand a great deal about patterns in evolution. Less famous than the dinosaurs who were around on land at about the same time, they too went extinct at the Cretaceous/Tertiary boundary about 66Mya.

This blog post (as well as flogging a new book) is a great overview of what we do and don't know, and also provides an interesting insight into what little information palaeontologists have to work with sometimes, and how we deal with that.