What Killed the Dinosaurs? The Asteroid and How We Know

What killed the dinosaurs? The leading answer, backed by decades of evidence, is that a giant asteroid struck what is now Mexico’s Yucatán Peninsula about 66 million years ago. The impact set off fires, darkened the skies and collapsed food chains around the world, and every dinosaur except the birds disappeared. So, do we know for sure what killed the dinosaurs? Not every detail is settled, but the main answer rests on several separate lines of evidence. This guide follows our video on the last day of the dinosaurs and lays out the clues scientists used to piece the story together.

The short answer: what killed the dinosaurs was most likely an asteroid. An impact at Chicxulub, in what is now Mexico, ended the age of the dinosaurs. Huge volcanic eruptions in India were under way at the same time, but a 2020 study in the journal Science concluded that the impact, not the volcanoes, was most likely the main cause of the mass extinction.

Watch the full story below, then read on for the evidence behind each step. The dinosaur extinction is one of the best-documented events in Earth’s history, and it began with a thin layer of clay in the hills of Italy.

Watch The Last Day of the Dinosaurs on the Mr Global Rankss YouTube channel.

The dinosaur extinction at a glance: the key clues

Clue	Where	What it shows
Clay layer	Gubbio, Italy	A thin clay band between two layers of limestone marks the end of the Cretaceous period
Iridium	The same clay layer	A metal rare in Earth’s crust but more common in space, pointing to an outside source
Chicxulub crater	Yucatán, Mexico	A buried crater about 180 km (110 miles) wide, formed about 66 million years ago
Tanis fossils	North Dakota, U.S.	Fish with tiny glass beads in their gills, buried in a rush of mud and water
Survivors	Worldwide	Small animals, and the birds, outlasted every other dinosaur

How do we know what killed the dinosaurs?

The clay clue in Italy

Tilted layers of sandstone rock under a blue sky — Rock layers record Earth’s history. A thin clay layer in such strata held the clue.

The story begins in Gubbio, a village in Italy about halfway between Rome and Florence. There, the geologist Walter Alvarez found a thin band of red clay sitting between two layers of limestone. The clay marked the boundary between the Cretaceous period and the one that followed, the moment when the great extinction took place. He found it in 1977, and his question was simple: how long did the clay take to form, and what did it say about how quickly the dinosaurs vanished?

The iridium test

To answer it, Walter turned to his father, the physicist Luis Alvarez, and together with the chemists Frank Asaro and Helen Michel they measured the chemistry of the clay. The result was striking. The layer was rich in iridium, a metal that is rare in Earth’s crust but far more common in space rock.

A bold idea

A meteor streaking across a starry night sky above a mountain lake — A meteor streaks across the night sky. The Chicxulub impactor was vastly larger.

In 1980 the team published their conclusion in the journal Science, under the title “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.” Their proposal was that a huge asteroid had struck the Earth and that the iridium in the clay was the fallout from that impact. People often ask whether it was an asteroid or a meteor: the object was an asteroid, a rocky body from space, while a meteor is the streak of light seen when space rock burns up in the sky. It was a bold idea, because it meant the dinosaurs had not faded away slowly but had been undone by a single event from space. The one thing missing was the crater.

Where is the Chicxulub crater?

The crater was found, and it was hiding in plain sight. By the early 1990s scientists had tied the iridium clay to a buried impact structure off the northern coast of the Yucatán Peninsula, now known as the Chicxulub crater. It is about 180 km (roughly 110 miles) across and about 66 million years old, and part of it lies beneath the Gulf of Mexico.

In 2016 a team of scientists drilled into the heart of the crater, targeting a ring of hills inside it called the peak ring. Drilling there let researchers study rock that was thrown up from deep in the crust by the impact, and test how such a structure forms. The Chicxulub crater remains the key physical evidence for the asteroid that killed the dinosaurs.

The volcano debate: did eruptions kill the dinosaurs?

Glowing lava flowing from a volcano across dark rock in Iceland — Lava from the Fagradalsfjall volcano in Iceland, a modern eruption.

Among the theories about what killed the dinosaurs, not everyone accepted the asteroid at first, and there is a serious rival explanation. At about the same time, enormous lava flows called the Deccan Traps covered much of what is now western India. Volcanic gases can change the climate, so some researchers argued that the eruptions, not an impact, drove the extinction.

A 2020 study in Science, led by Pincelli Hull, tested the idea against temperature records from the deep sea. The team found that the warming linked to the volcanoes came well before the impact and that it did not line up with the die-off. More than 90 percent of plankton species vanished abruptly after the impact. The researchers concluded that the asteroid was most likely the main cause, and that the volcanoes mostly shaped how surviving life recovered.

The fish of Tanis: a snapshot of the impact?

A fossilized fish preserved in pale stone — A fossil fish preserved in stone. Fossils record events in remarkable detail.

One of the most vivid clues comes from a fossil site called Tanis, in the Hell Creek Formation of North Dakota in the U.S. Led by the paleontologist Robert DePalma, researchers described a layer packed with fish, including sturgeon and paddlefish, in a study published in the Proceedings of the National Academy of Sciences in 2019. Tiny glass beads, formed when rock melted in the impact and fell back to Earth, were found in the gills of some fish, and an iridium-rich clay layer lies above the deposit.

The authors argued that seismic waves from the impact sloshed the local water and buried the fish within hours. Other scientists called the sequence plausible but noted that the exact timing is hard to verify, so it is best read as a strong clue rather than a proven stopwatch. A later study in Nature (2022) examined growth rings in the bones of Tanis fish, much like tree rings, and concluded that they died in spring in the Northern Hemisphere.

How the world changed after the asteroid hit

What killed the dinosaurs on Earth was not only the impact itself but the chain of events that followed. The asteroid was miles wide. Within minutes, hot glass and rock debris rained down and fires broke out, and animals in North America were hit especially hard. Farther away, dust and soot filled the air and blocked out the sky. With fires destroying plants and sunlight dimmed, plant-eaters starved, and soon the meat-eaters lost their food too. Estimates suggest that roughly 70 to 75 percent of species were lost.

Who died out, and who survived?

Who died out

Tyrannosaurus rex skeleton mounted in a museum gallery — A mounted Tyrannosaurus skeleton. T. rex was among the last non-avian dinosaurs.

All the non-avian dinosaurs died out, including Tyrannosaurus rex and Triceratops. The seas lost as much as the land: more than 90 percent of the plankton species disappeared, which weakened ocean food chains from the bottom up.

Why big animals suffered most

Size was a handicap. On land, almost nothing larger than a beaver made it through, and paleontologist Nicholas Longrich has said there is no evidence of any animal over 10 kilograms (about 22 pounds) surviving. Small animals needed less food and found shelter more easily, and some could burrow underground while the surface burned.

Who survived

A small reddish-brown mammal perched on a log — Small mammals like this one are the kind of animal that survived the extinction.

Survivors tended to be small, adaptable and able to eat almost anything. Some small mammals and crocodiles made it through, and turtles and salamanders fared best, losing only about a quarter of their species. Birds with beaks could eat seeds, a calorie-rich food that lasts for years, while toothed birds without beaks appear to have struggled and died out. Scientists are still debating exactly why some animals lived and others did not, and luck in surviving pockets of habitat probably played a part.

Are birds living dinosaurs?

A red cardinal perched on a branch — Birds are living dinosaurs, the only dinosaur lineage that survived.

Yes. Birds are the only dinosaurs alive today. They descend from small, meat-eating theropod dinosaurs, the same group that includes Tyrannosaurus rex. Early birds such as Archaeopteryx, about 150 million years old, had feathers but also teeth, a bony tail and clawed wings; later birds lost their teeth and evolved beaks. Small size, a varied diet and the ability to fly probably helped one group of birds survive, and today there are at least 11,000 bird species. So the last day of the dinosaurs was not quite the end: the story goes on outside your window.

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