One of the most unusual marine lifeforms to exist in the seas. It appears during the episode Dangerous Seas.
The Tanystropheus is a very long necked marine reptile. It's neck is 20 ft long and used it to sweep it's head around looking for food. Similar to modern lizards it tail will detach when threatened by a predator from behind.
During Dangerous Seas Nigel hitches a ride on it when it detaches it's tail only for Cymbospondylus to eat it.