The exact ancestors of the ichthyosaurs remains unknown - already by 240 MYA ichthyosaurs such as Cymbospondylus dominated the seas, as shown by the first episode of Sea Monsters, and by 210 MYA they began to acquire their better-known, compact body form (the earlier species were more elongated, eel-like in shape). Their tails changed by developing a lower and an upper fluke, their teeth began to dissappear, and by the late Jurassic, ichthyosaur species such as Ophthalmosaurus looked very much according to 'canon' (shown in the 3rd episode of Walking with Dinosaurs). By the end of Cretaceous, however, ichtyosaurs died out alongside other - marine etc. - Mesozoic reptiles, without leaving any descendants.