Ichthyosaurus
Ichthyosaurus bore live young. This we know from fossil evidence. It was thought at first that Ichthyosaurus laid eggs on land, but fossil evidence shows that the females gave birth to live young. Because they did, their life habit could be entirely aquatic. They were well-adapted to life as fully pelagic organisms because they never needed to come onto land. The babies were born tail first to prevent them from drowning in the water.