plesiosaur: extinct marine reptile with a small head on a long neck a short tail and four paddle-shaped limbs; of the Jurassic and Cretaceous
archosaur: extinct reptiles including: dinosaurs; plesiosaurs; pterosaurs; ichthyosaurs; thecodonts
canal: (astronomy) an indistinct surface feature of Mars once thought to be a system of channels; they are now believed to be an optical illusion