The Cretaceous Mass Extinction killed off many non-avian dinosaurs, as well as toothed birds and other creatures. There are many theories as to what caused it, but the answer is much more complicated than we could imagine.

The Competing Theories

There are many theories that are floating around about how the dinosaurs were wiped out. Some of them are ridiculous, and have no supporting evidence, which we will talk about.

ALIENS!

One theory on why dinosaurs went extinct was that aliens came down and murdered them. They left no trace, other than lots of iridium in the rocks. Iridium is a substance that is very rare on Earth, but in other parts of space is quite common. The spacecraft used by aliens may have brought lots of iridium to the planet, which others thought was evidence of an asteroid. However, they likely would've left behind something, maybe a space-bagel or a piece of machinery. In any case, there is no evidence that says aliens came, but there are plenty of reasons why aliens would murder the dinosaurs. They may have done so for food, hide, or fun!

Supernova of Doom

Another theory about why dinosaurs went extinct, which has less evidence than an alien invasion, is that a supernova occurred, sending lots of radiation at Earth. This killed off the dinosaurs and many other animals, as it broke the ozone. However, if this happened, nothing would survive. Even bacteria and archaea would die! However, this event would've most certainly turned all of the dinosaurs into a good meal!

Mammalian Thieves

One theory that is much more plausible than the former, but still likely didn't happen, is that mammalian egg-thieves grew in population, and ate too many dinosaur eggs. Birds, however, wouldn't survive either because mammals were adept climbers. Also, dinosaurs had been defending their nests successfully for so long, so this probably was not real.

Volcanoes in India

This theory as to what killed the dinosaurs definitely happened, but may not have killed the dinosaurs themselves. There is lots of evidence that volcanoes were erupting in India for millions of years, worse than ever before seen by dinosaurs! The planet became hotter, and a mass extinction similar to the Great Dying happened. The larger dinosaurs definitely would've died, but what about raptors, or dinosaurs on the other side of the world?

The Asteroid

One theory, which is talked about in most dinosaur books, is that a big comet or asteroid the size of Mount Everest smashed into the planet at super high speeds. There is plenty of evidence that backs this, such as lots of iridium, and a large crater that has been buried over time. This alone would not have killed the dinosaurs if it struck at a different time, though.

So, what happened? Scientists see that certain key herbivores, such as Triceratops and Edmontosaurus were already beginning to die out. Their populations were dwindling, which would cause lots of extinctions with carnivores. For millions of years before any asteroid attacked Earth, volcanoes in India erupted constantly. The ecosystem was at its weakest point, and one bad thing could cause it to collapse. Then, an asteroid smashed into Earth. Here is what would've happened at Hell Creek.

All of the dinosaurs were grazing or feasting or napping. A pack of Tyrannosauruses have been noticing a tiny sun that appears and disappears throughout the day. It's been getting larger, and brighter. Today, it has been brighter than the original sun.

A flash of blinding, noiseless light assaults every dinosaur's eyes, and the world becomes a shaky trampoline. Vibrations launch even adult T. rexes several feet in the air. Mammals retreat in their burrows, turtles and crocodilians into the water. Small raptors hide in rotting logs. Two hours later, the shaking stops. Those small raptors we talked about come out, and feast upon the dead Tyrannosauruses. Then, a boom rings out. A sound so loud it launches everything that came out of hiding. Those poor animals smacked into trees, hit the ground, each other, and were crushed. Then, something horrible came. The worst of the collision. Chunks of burning rock and glass rain down at super high speeds, searing flesh and heating the planet from the friction. The asteroid itself has incinerated, creating lots of dust and ash that blocks the sun. The planet burns, then becomes super cold. Plants die, causing regions to completely die. The few plucky plants that stayed were the only food sources for most herbivores. Toothed birds such as Balaur bondoc died out. Modern beaked birds survived by eating seeds that other animals couldn't eat. They could also fly away to safer places during the rock rain. They are the only dinosaurs remaining on this planet today.

In the end, crocodiles and turtles survived by hiding in the water. Supersized crocs such as Deinosuchus or Sarcosuchus were too large to seek shelter in underwater crevices, so they were pelted by the rocks and murdered. Thanatos guided all of the non-avian dinosaurs to Hades.

Mammals, the tiny little critters that were overshadowed by the dinosaurs, came out of their little holes. Their omnivorous diet and warm-bloodedness gave them an advantage over their reptilian competitors. They evolved many diets, lifestyles, shapes, and sizes. Eventually, they blossomed into us, a bunch of killing machines that are ruining the planet and killing each other for no reason.