system
Given the following post, respond as if you're commenting on Reddit:
human
What killed the prehistoric sea monsters?

The asteroid killed the dinosaurs, but did that also kill off those massive sea creatures? If not, why are they mostly not around anymore?
gpt
The asteroid impact would have killed more than just the large animals. It would have killed off a lot of the megaflora (large plantlife) present at the time that helped fuel the large growth of animals.

That would have meant a temporary decrease in aquatic life as well, as the atmosphere's carbon dioxide and oxygen levels got thrown out of whack, which would have rippled from the plankton to the small fish to the large predators.

The ones that survived did so by either subsisting on land animals (crocodiles), diving into the depths (fangtooth), or through luck and determination (sharks).

But the largest animals, like the plesiosaurs, would have not been able to sustain themselves as easily, and would have been hunted to extinction by the mid-sized predators.