ymbospondylus was one of the largest ichthyosaurs, with fossils ranging from 6 to 10 metres (20 to 33 ft) long. It was also one of the least fish-like of the ichthyosaurs, lacking a dorsal fin and fluked tail. It did, however, have an elongated snout like other ichthyosaurs.
The one metre long head of Cymbospondylus, with large jaws, contained rows of teeth adapted for catching and holding on to fish, belemnites, and cephalopods such as ammonites. The long tail would have been excellent for swimming, and allowed Cymbospondylus to move at fast speeds and efficiently hunt down shoals of swimming fish. Adult Cymbospondylus probably spent much of their time hunting in deep offshore water, only venturing into shallower water to breed or to catch seasonally available prey. Like other ichthyosaurs, Cymbospondylus probably gave birth to live young, as it had no way to lay eggs. These, on reaching adult size, probably had few, if any, predators that could harm them.
The eel-like tail of Cymbospondylus made up almost half the total body length, and it is possible that the tail was used as a primary swimming mechanism. Like present day sea snakes, Cymbospondylus probably swam by wriggling its body from side to side. The paddle-like limbs of Cymbospondylus would primarily have been underwater stabilizers, and for slowing down the ichthyosaur's swimming speed.