Ichthyosaurs, now extinct, were aquatic reptiles with dorsal fins and tails, similar to those of fish. Their most recent ancestors were terrestrial reptiles that had neither dorsal fins nor aquatic tails. The dorsal fins and tails of ichthyosaurs and fish are . homologous structures adaptations to different environments adaptations to a common environment and examples of convergent evolution adaptations to a common environment and homologous structures