Ichthyosaur, marine reptile

Ichthyosaurs had a strong tail fin that propelled them quickly through the water and flippers that helped them steer.

Breathing
Marine reptiles can’t get oxygen from the water the way fishes do—they have lungs, so they must breathe air. Just as modern whales come to the surface to breathe, so did ichthyosaurs, crocodilians, and plesiosaurs.

Giving Birth
Some marine crocodilians, such as Steneosaurus, probably came ashore to lay eggs. Other marine reptiles, like ichthyosaurs and plesiosaurs, couldn’t leave the water, and gave birth to live young.

The Shapes of Fast Swimmers
Although many ichthyosaurs resembled sharks and dolphins, they were actually reptiles. Thanks to convergent evolution, these unrelated animals evolved similar bodies because the demands of living underwater favored those shapes over others. Stiff, smooth bodies, tall dorsal fins, and strong tails allowed them all to be fast-swimming predators.