You are in the oceans of Europe, 150 million years ago.

The sunlit turquoise water of the sea was warm, but a sudden chill ran down your spine as the sunlight above was broken by a long, streamlined shadow. You—in your diving suit—held your breath, observing a Plesiosuchus.

It was easily seven meters long, and it looked more like a marine reptile than a modern crocodile. It lacked the armored plating of its river-dwelling cousins, and its skin smooth and streamlined for speed. Its tail ended in a powerful shark-like fin, that swayed with efficiency, driving a massive, deep skull forward with terrifying speed. It moved with shocking stealth for its size, propelled by small, paddle-like flippers.

As it swims seemingly towards you, you caught a glimpse of its jaws. They were deep and robust, packed with large, serrated teeth—like that of a killer whale—designed for crushing through prey…