File:Ichthyosaur swimming backbone.gif

Summary
The narrow backbone of the first ichthyosaurs suggests that they undulated their bodies like eels (bottom). This motion allowed for the quickness and maneuverability needed for shallow-water hunting. As the backbone thickened in later ichthyosaurs, the body stiffened and so could remain still as the tail swung back and forth (bottom). This stillness facilitated the energy-efficient cruising needed to hunt in the open ocean.