How did plesiosaurs become extinct?

PlesiOSaurs became extinct 65.5 million years ago, probably for the same reason the dinosaurs died out. The generally excepted theory is that an asteroid, 6 miles in diameter, crashed into the Yucatan Peninsula and threw dust into the atmosphere. The dust layer blocked the sun for months, killing plants and plankton. This meant that herbivores had nothing to eat, and died, leaving the carnivores with nothing to eat and they soon died, too. There are other theories as well, and you can check out the related link to read about those.