Around 244 million years ago, sails were a major fashion trend. No less than three extinct crocodile cousins found in different parts of the world – Arizonasaurus from the southwestern United States, Xilousuchus from China, and Ctenosauriscus from Germany – walked across the early Triassic landscape with prominent billboards of flesh and bone on their backs. This was not just a bizarre twist in evolutionary history. Together, these creatures mark the rise of the “ruling reptiles”, and may help pin down the humble beginnings of their great dinosaurian cousins.

Arizonasaurus, Xilousuchus, and Cte...