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.
In Walking with... Series
Ophthalmosaurus was one of the main characters of this episode. It began with the females coming to the shallows (where the episode was talking place) to give birth. One of the females had problems with giving birth, and was eaten by a Liopleurodon. Most births went successfully, however, and many pups were born. They were in danger from sharks and other sea predators, so they hunted mainly at night, using their large eyes. They also had to survive a sea storm that killed the old male Liopleurodon, but most of ichthyosaurs survived and became large and old enough to live in the open sea instead. One day, however, the females will return to the shallows, carrying the next generation with them.
The second half of this episode featured a Cymbospondylus. This large sea reptile harassed Nigel Marven but was presumably scared away by a tazer.