Final answer:
The boundary between the Cretaceous and Paleogene periods was marked by a major comet or meteorite impact in Mexico, causing a mass extinction and the disappearance of dinosaurs.
Step-by-step explanation:
The major comet or meteorite impact that occurred in Mexico and marks the boundary between two geological periods is the Cretaceous-Paleogene boundary. This cataclysmic event led to a mass extinction, notably causing the disappearance of the dinosaurs. The impact created the Chicxulub crater in the Yucatán peninsula and left a distinctive layer of sediment across the globe that is rich in iridium, a rare element on Earth's crust but common in celestial bodies such as asteroids and comets. The end of the Cretaceous period is also characterized by fossil records showing a sharp decline in the number and diversity of species, including changes in plant life as evidenced by pollen grains.