The truth is that dinosaurs never went extinct. The order decided that it was time to move them to a different planet and every single species of animal that existed in a certain era was removed to a neighbor planet that is to remain undisclosed. The prehistoric species that inhabited the oceans proved to be very difficult to capture and this is the reason why a deep cleansing of the oceans was done and all species that existed at sea back then got erased from existence.
We needed to repopulate the oceans with creatures that did not pose such a terrible threat to humans and we only used moderately sized creatures. The only prehistoric animal that is still roaming the oceans is the Carcharodon Megalodon that was saved from the deep cleansing phase. This huge shark lives in the depths of the oceans and never comes out to the surface.