A big meteorite crashed into Earth, changing the climatic conditions so dramatically that dinosaurs could not survive. Ash and gas spewing from volcanoes suffocated many of the dinosaurs. Diseases wiped out entire populations of dinosaurs. Food chain imbalances lead to the starvation of the dinosaurs.

When did Brachiosaurus become extinct?

A: Brachiosaurus went extinct roughly 156-145 million years ago.

How did brontosaurus go extinct?

Both of these theories hold ground due to the fact that the Apatosaurus (Brontosaurus) was so large it could not efficiently move through forests to feed on masses of vegetation, nor could it stand in swampy ground without sinking and thereby meeting a slow death.