Prehistoric aquatic reptile with flat body, short tail, long neck, and flippers

The Plesiosaurus is the defining member of the plesiosaur family of prehistoric aquatic reptiles; they are not properly dinosaurs but are contemporaries. Plesiosaurs have flat bodies, short tails, and long necks; where a quadrupedal animal would have legs, they have flippers. As they are reptiles, they cannot breathe underwater, though they can hold their breath for an extended period. They are aggressive carnivores, subsisting on fish and small non-aquatic animals, which will be pulled below the surface and drowned. On any successful hit, a plesiosaur has established a hold on the victim, who will then be pulled into and under the water unless they can make a successful save vs. Death Ray to break free. Note that a plesiosaur is not strong enough to thus move any creature weighing more than 1,000 pounds (half-ton).

Jurassic carnivore that may have preferred smaller prey than a man

None of these creatures are technically dinosaurs, though all are found in "Lost World" areas. Plesiosaurs may have preferred smaller prey than a man but would still be dangerous. Jurassic carnivore.