Final answer:
The extinction of the dinosaurs happened around 65 million years ago at the close of the Cretaceous period due to a mass extinction event most likely caused by an asteroid impact.
Step-by-step explanation:
The extinction of the dinosaurs occurred approximately 65 million years ago, marking the end of the Cretaceous period in geological history. This catastrophic event is considered a mass extinction, where more than half of the species on Earth at the time, including most dinosaurs, were wiped out. Notably, other mass extinctions have occurred throughout the fossil record, but the end-Cretaceous event is particularly significant as it represents the end of the age of dinosaurs and the rise of mammals, which eventually led to the appearance of humans.
Scientific consensus links the cause of this mass extinction to the impact of a large asteroid at the present-day Yucatán Peninsula, causing dramatic global climate changes that the dinosaurs could not survive. However, birds, which are descendants of a theropod clade of dinosaurs, did survive and continue to thrive today.