The Early Cretaceous spans from 145 million to 100 million years ago. The Early Cretaceous saw the expansion of seaways, and as a result, the decline and extinction of sauropods (except in South America). Many coastal shallows were created, and that caused Ichthyosaurs to die out. Mosasaurs evolved to replace them as head of the seas. Some island-hopping dinosaurs, like Eustreptospondylus, evolved to cope with the coastal shallows and small islands of ancient Europe. Other dinosaurs rose up to fill the empty space that the Jurassic-Cretaceous extinction left behind, such as Carcharodontosaurus and Spinosaurus. Of the most successful would be the Iguanodon which spread to every continent. Seasons came back into effect and the poles got seasonally colder, but dinosaurs still inhabited this area like the Leaellynasaura which inhabited the polar forests year-round, and many dinosaurs migrated there during summer like Muttaburrasaurus. Since it was too cold for crocodiles, it was the last stronghold for large amphibians, like Koolasuchus. Pterosaurs got larger as species like Tapejara and Ornithocheirus evolved.
What caused Ichthyosaurs to thrive? (If the question is unanswerable, say "unanswerable")
unanswerable