At the beginning of the Cretaceous period, fish took over the ocean

Seas in the Jurassic were much different from now. They were dominated by cephalopods related to squids and cuttlefish. Fish were much less abundant and diversified. Based on Jurassic otolith and statolith assemblages of Poland, Lithuania and the United Kingdom (and published data) we have reconstructed the structure of nekton community and the evolution of actinopterygians. We found out that the great shift in nekton composition, when fish became the dominant group, took place around the boundary between the Late Jurassic and the Early Cretaceous.

Figure: relative abundance of cephalopod statoliths (on the left, orange) and fish otoliths (on the right, yellow) in the Jurassic (geological time running from the bottom to the top of the diagram).