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Most dinosaurs, including all vegetarian ones, died out during the Cretaceous-Tertiary extinction event 65.5 million years ago, likely caused by a 6-mile asteroid impact. The impact caused acid rain and killed off many plant and animal species, leading to the rise of smaller, warm-blooded animals.
All but a few dinosaurs that would later become modern birds died out about 65.5 million years ago, during the Cretaceous-Tertiary extinction event. No land animals larger than a cat survived. The sky was largely darkened over a period of years to decades, killing numerous plant species and microbes that rely on photosynthesis for survival. Species that depended on consuming plant matter, like all vegetarian dinosaurs, became extinct when their food sources died out and decomposed. In North America alone, 57% of plant species have gone extinct. In paleontology, the layer of rock that corresponds to this extinction is known as the KT boundary.
Even today, we don’t know with 100% certainty what really caused this massive extinction. But the prevailing theory, from Nobel Prize-winning physicist Luis Alvarez, is that a 6-mile asteroid hit Earth, releasing the energy of 2 million atomic bombs and showering molten magma all over the planet’s surface.
The Alvarez extinction theory is supported by two facts: First, the huge Chicxulub crater in Mexico’s Yucatan peninsula can be dated to the KT boundary epoch. Second, worldwide, the KT boundary rock contains an unusually high proportion of iridium, a substance that is very rare on the Earth’s surface, but more abundant in asteroids. This suggested that impact debris fell all over the globe. This illustrates the destructiveness of the impact. The rock beneath the crater is rich in gypsum, which would have produced clouds of sulfur dioxide aerosols in the aftermath of the collision, causing acid rain and killing marine creatures that depend on their shells for survival.
Being very large and exposed at the surface, as well as being more dependent on smooth ecosystems than less specialized plants and microorganisms, dinosaurs have felt the brunt of the effects of the extinction. Asteroid Chicxulub slammed into Earth from an angle, traveling northwest. This would have spewed incandescent, ballistically indented ejecta throughout what is now the central United States, an area known for its large dinosaur populations.
The direct effects of the heat could have killed 99% or more of the dinosaurs in a few hours. Those who were able to survive the magma rain would die over the next few years or decades, their food sources would dry up, and ecosystems would be completely destroyed. Life would never be the same: from then on, it would not be dinosaurs, but smaller, warm-blooded animals that would thrive on planet Earth.
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