Mass Extinction of the Dinosaurs: Definition, Events & Causes

Instructor: Mary Ellen Ellis
Most everyone loved learning about dinosaurs as kids, and many of us have imagined what it would be like if they still roamed the Earth. Sadly, the Earth lost dinosaurs in one mysterious massive extinction event. What happened?

When Did the Dinosaurs Disappear?

The history of life on Earth has included several large extinction events. These are brief periods when a huge number of species disappeared from the planet at the same time. The extinction event that eliminated dinosaurs is the most famous of them all, but it is far from the only one. It also wasn't the largest extinction event that ever took place on Earth.

The largest extinction event was the end-Permian extinction. Extinction events are named for when they occurred on the geologic time scale. Geologists and paleontologists divide time on earth into eons, eras, periods, and even smaller periods of time. At the end of the Paleozoic was the Permian period. The Permian ended with the biggest extinction the world has ever seen. Based on the fossil record, paleontologists estimated that between 93 and 97 percent of all species on Earth went extinct.

The end-Permian extinction left room for dinosaur species to evolve and thrive. And that's exactly what they did during the Triassic, Jurassic, and Cretaceous periods of the Mesozoic era. The diversity of dinosaur species that evolved during this time period was staggering. It all came to an end with the end-Cretaceous extinction that wiped out dinosaur species entirely. This was approximately 66 million years ago, and it marked the start of the Cenozoic era.

The dinosaur extinction occurred at the end of the Cretaceous period.
geologic time scale

What Causes Mass Extinctions?

Throughout the history of life on Earth, species have gone extinct when they could no longer compete for resources against other species. Mass extinctions are different. These take place when one or more catastrophic events occur and cause a number of species to go extinct at one time.

We can never be exactly sure what causes these major die-offs, but we have evidence to support various hypotheses. Volcanic eruptions spewing gas and dust into the atmosphere, massive lava flows, and impacts from asteroids, meteoroids, and comets are most often implicated in mass extinctions.

Over the years, many ideas have circulated about what caused the extinction of dinosaurs at the end of the Cretaceous period. It was a mystery of interest for many years because dinosaurs had such a strong foothold on the Earth. It seemed amazing that these versatile creatures could be undone by one event. It would had to have been a truly massive one to bring about their permanent end.

The End-Cretaceous Extinction: An Asteroid Impact?

Several ideas that might explain the end-Cretaceous extinction have been raised and dismissed, but at least two remain. The evidence to support an asteroid impact is compelling. The idea of a huge asteroid impact wiping out the dinosaurs began with the discovery of a thin layer of rock found in formations around the world. This layer can be dated to around 66 million years ago (the time period of the event), it is found all over the Earth, and it is rich in iridium.

Iridium is a metal that is rare on Earth, although it is found in the core. To see it so abundant in a layer of rock is unexpected, unless that rock came from outside of the earth. Asteroids, which orbit the sun along with the planets, are rich in iridium. If a large, iridium-rich asteroid hit the earth around 66 million years ago, it would have spewed dust and gas, from both the impact site and the asteroid itself, into the atmosphere. That dust would have settled all around the world and could explain the thin layer of rock found by geologists and paleontologists.

The hypothesis of the impact as extinction event became stronger when scientists discovered a huge crater in the Yucatan peninsula of Mexico. Much of it has been eroded, and half of it lies under the Gulf of Mexico, but the crater is there and is evidence of a truly enormous impact. A hit of this magnitude would have sent enough dust in to the air to block out the sun for months. Without sunlight, life quickly dies.

Clues such as troughs and sinkholes along its edge helped scientists find the giant crater on the Yucatan Peninsula.
Yucatan crater

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