Ichthyosaurs were giant marine reptiles. Fossils indicate that they possessed dorsal fins and tails, as do fish, even though their closest relatives were terrestrial reptiles that had neither dorsal fins nor aquatic tails. The dorsal fins and tails of ichthyosaurs and fish are examples of:

Adaptations to a common environment and examples of convergent evolution.

Explanation:

Ichthyosaurs were the giant marine reptiles but it possess tails and dorsal fins though this is not a common character of reptiles. But the presence tails and dorsal fins states that it was an adaptation which was required at that time.

This giant reptile was an mixture of modern fish and dophins. It also had dorsal fin for swimming in water and has vertical tail which was used for the propulsive stroke. They were air breathing but had some part of their life in water as well like other reptiles.

Convergent evolution can be defined as the evolution of similar features in the species found at different period.

Some of the fishes like dolphins and other fishes shows convergent evolution to ichthyosaurs.