You are in the swamplands of Thailand, Asia, 224 million years ago.

The water of the Late Triassic swamp was murky, tinted with tannins and warmed by a relentless sun. You pushed through the shallow, oxygen-rich bank, keeping quiet. The bottom was soft mud, and you were looking down, watching small fish dart away from your feet.

Nothing else stirred on the surface, but a few feet away, a patch of what you thought was a flat, moss-covered rock—roughly triangular and entirely, unnervingly shaped like a toilet seat—shook off the silt.

You paused, frozen. Two small, pale eyes on top of that wide, flat head stared upward. Suddenly, the "rock" erupted.

The Gerrothorax didn't lunge forward like a crocodile. Instead, it did something terrifyingly unique: it snapped its head upwards with incredible speed, opening its mouth like a hinged trapdoor. You felt a massive suction of water, a violent cavitation that dragged your wader-clad foot toward it. It was a perfectly still ambush, turning its entire body into a predatory vacuum.