Extinct Sea Monsters

Mosasaurs were elongate, reptilian, aquatic predators that lurked in the vast epicontinental seas of the late Creatceous.  They were incredibly common creatures 70 million years ago.  By the end of the age of dinosaurs, they has invaded freshwater habitats and were at the peak of their diversity.  These were not dinosaurs.  Phylogenetically, they were lizards, and possessed many of the same adaptations as snakes (which are basically lizards as well), including a doubly hinged jaw for devouring big chunks of food.  In many ways, they replaced ichthiosaurs and plesiosaurs, the former being extinct by the time of mosasaur ascendance, the latter in decline.  They gave birth to live young, and probably undulated through the water the same way snakes swim.  

Their ancestors were reasonably abundant lizards resembling present-day monitor lizards,

 and I think the real lesson here is that, given the right warm climate, and 20 million years, a planet can bristle with marine reptiles.  These creatures may have even outcompeted certain sharks.  All it takes is a lizard. 

A much earlier beast was the placoderm, Dunkeleoseus.  These things haunted the oceans 360-380 million years ago, when life on land was mossy and reedy, with a small arthropod or two.  In the ocean, however, huge beasts like this 35 foot, 3 ton monster roamed.  Its skeletal features are so primitive and strange, a member of the extinct placoderms, it amazes me that oceanic food webs of the time could support such a beast.