Not a lot of people realize how extreme and how sudden the dinosaur's extinction was. Radiolab did an episode with several geologists about the extinction of the dinosaurs. Basically, when the meteor hit the Earth, a humongous amount of material was ejected into space, and most of it fell back down to the surface all around the planet. As it fell it left an incredible amount of heat in the air, raising temperatures to as high as 1200 Farenheit (650 Celsius). Anything too large to hide under dirt died (dirt is a surprisingly good insulator). Even plankton and animals near the surface of the ocean would have been decimated by the heat, destroying the food chain that supported the large mesosaurs and plesiosaurs. The whole process is thought to have taken 2 hours. The fact that anything lived through that at all is dumb luck, and even if a semi-aquatic dinosaur was deep enough to avoid the worst of the heat, the food supply would have been destroyed and they would have starved shortly after the impact.