Fish species are some of the most diverse and numerous here on Earth, with so much diversity being present from ocean to ocean, lakes to lake and rivers to rivers. While it has been noted in the past that fish fossils are rare prior to the Cretaceous Period, there were no studies into the reasons why or just how many species existed.
Researchers studying ray-finned fishes began looking at the fossil records, and searching the seabeds for marine sediments found in deep sea core samples. Sergio Cortes implied what they discovered was that fish fossil remains were present in the core samples, but there was a significant spike after the Cretaceous period when the dinosaurs went extinct. Pre-Cretaceous samples showed a wide diversity of species, with lower counts of fish fossils present. As soon as competing species died off the number of ray finned fish species began to increase until it reached levels that we are familiar with today.
It is believed that this occurred because there was a greater food supply available to the fish and they were able to expand into marine environments where their survival had been limited due to competitors. Interestingly, sharks were able to survive the mass extinction event that occurred on earth, right along with the fist they share their environment with. The increase in fish species in turn supported their survival through the extinction period so that they were able to survive to present day.