Cartilaginous fishes were very diverse during the Permian period. However, after severe losses among cartilaginous fishes during the Middle Permian extinction, bony fishes experienced a massive diversification in the subsequent... Today, ray-finned fish, which belong to the bony fish, are by far the most biodiverse fish group in both salt- and freshwater. Their spectacular variety of forms ranges from eels, tuna, flounders and angler fish all the way to seahorses. With around 1,100 species, the second most biodiverse group is the cartilaginous fish, which are almost exclusively marine and include sharks, rays and chimaeras. Exactly why bony fish managed to prevail in different habitats is the subject of debate: Do they have a better body plan, which is suited to more ecological niches than that of the cartilaginous fish? Or are other factors involved in their successful distribution? Paleontologists from the University of Zurich now reveal that climate catastrophes in the past played a crucial role in the dominance of ray-finned fish today.