Platypterygius
No firm reasons can be given for the decline of the ichthyosaurs. By the Cretaceous they certainly had more competitors than in the Triassic, and more elusive prey. The adaptive radiation of teleost fish meant their new prey were fast swimming and highly evasive. The teleosts also included fast-swimming predatory fish, which must have been competitors. The anoxic event would have removed much of the plankton at the base of the food chain. That would certainly result in the extinction of many larger predatory forms of life.