Ichthyosaur
It is true, though, that the ichthyosaurs had declined from their peak. 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. From what we know now, the ocean anoxic event must have ended the last ichthyosaurs.