Definition
Ichthyosauria is an extinct order of marine reptiles that thrived during much of the Mesozoic Era. They resembled dolphins or fish in their body shape, an example of convergent evolution. Ichthyosaurs were not dinosaurs, but rather reptiles that adapted to a marine lifestyle. They possessed streamlined bodies, paddle-like limbs, and a vertical tail fin for propulsion. They were viviparous, meaning they gave birth to live young. Ichthyosauria represents a successful adaptation to aquatic life.