A large marine reptile, ichthyosaurs had a porpoise-like head, a long toothy snout, and a tuna-like body. They swam with side-to-side movement like catfish. Of the seafaring reptiles, ichthyosaurs were the most meticulously adapted to their watery world.
Averaging 2 meters long and 80–100 kg, ichthyosaurs were a parallel development to the lineages that led to the modern-day dolphin and whale. One ichthyosaur species could dive down 610 meters. This was another instance of convergent evolution, in that all 3 families were air breathers, fast swimmers, capable of deep dives, and bore live young.
The varied diet of the ichthyosaurs, capable of hunting at night, gave them a long run: from 250–90 MYA. The first ichthyosaur evolved only a few million years after the Great Dying.
Ichthyosaurs died out as environmental volatility in the oceans overwhelmed their ability to adapt. Global changes profoundly reorganized marine ecosystems during the Cenomanian age (100.5–94 MYA).
Most reptiles are egg layers, but adaptation to living in the sea demanded viviparity. This was true of ichthyosaurs, nothosaurs, plesiosaurs, and mosasaurs. Having few large offspring meant that these creatures were likely to having been caring mothers.