The evidence for this comes from several icthyosaur fossils which have been found with their young still inside the womb. This icthyosaur is part of the Bristol University Geology collection. It was collected from a quarry in Somerset. Thanks to Dr Liz Loeffler, who helped me to take the photo. Please don't lift this image, as it would cause copyright problems.
Plesiosaurs fall into two groups.
The 'plesiosauroid' plesiosaurs had elongated necks with quite small heads. Their mouths were filled with dagger-like teeth to stab into fish. They would have twisted their neck around to help them to catch their prey and probably attacked their prey by ambush. The 'pliosauroid' forms had much shorter necks and were heavily built and had huge heads. They would have also preyed upon turtles and ammonites.