Ichthyosaurs and Dolphins, Mosasaurs, Plesiosaurs, Whales, Cetaceans

ICHTHYOSAURS:  based on fossil evidence, they first appeared around 250 million years ago and at least one species survived until about 90 million years ago

MOSASAURS: 99.6 million years ago - 66 million years ago
 

PLESIOSAURS: Plesiosaurs first appeared in the latest Triassic Period, about 203 million years ago.They became especially common during the Jurassic Period, thriving until their disappearance due to the Cretaceous–Paleogene extinction event at the end of the Cretaceous Period, about 66 million years ago.