An interesting fact is that ichthyosaurs had died out by the middle of the Upper Cretaceous, and plesiosaurs and sea-going crocodiles were in decline. The reasons for this are not known: perhaps there was increased competition from large predator fish. It is this large marine predator niche that mosasaurs apparently occupied. They gave birth to live young, just as the ichthyosaurs had done. They flourished in the later Cretaceous, only to become extinct at the K/T extinction event.