system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we believe an asteroid killed off the dinosaurs?
gpt
67 million years ago there’s dinosaur fossils everywhere in the rock layer.

65 million years ago there’s *zero* dinosaur fossils.

In between is a thin layer of ash and iridium.  Iridium is very rare on earth, but common in asteroids.  A global layer of ash would suggest something cataclysmic happened, incinerating forests on a global scale and/or setting off a massive volcanic event.

This theory was kicked around for some time, and then the real bloody knife was discovered in the Gulf of Mexico - an enormous crater that’s 66 million years old, obscured beneath the water and Mexican jungles.