Ichthyosaurs were air-breathing, warm-blooded, and bore live young. They may have had a layer of blubber for insulation.

Did ichthyosaurs breathe underwater?

In fact, from their streamlined, fishlike bodies, it seems almost certain that ichthyosaurs could not leave the water. Yet they still breathed air and lacked gills, like modern whales. Ichthyosaurs were not dinosaurs, but represent a separate group of marine vertebrates. ... The first ichthyosaurs appeared in the Triassic.

How did dinosaurs breathe underwater?

For the first time, we have solid bone evidence for this kind of a lifestyle among dinosaurs.” Spinosaurus had a nostril on the back of its head. That made it possible for the creature to breathe when part of its head was under water. Its long neck and short, powerful back legs helped it swim.