The Extinction of the Dinosaurs

How did the dinosaurs become extinct?
A brief explanation of the evidence



65 million years ago an asteroid about 10km in diameter collided with the Earth causing the Cretaceous-Tertiary (K-T) Extinction Event.

This catastrophic event caused 75% of the world's species living at that time to become extinct. The most famous of the doomed creatures were the dinosaurs, so this event marks the end of their 135 million year reign on the Earth.

The survivors emerged into a world with plenty of niches to be filled. The end of the Cretaceous Era marked the end of the dinosaurs. It also marked the beginning of the Tertiary Era and the age of birds and mammals. The catastrophe was bad for the creatures that were destroyed but it opened a door of opportunity that eventually led to the rise of humans.

This is fine story. But where is the evidence?

In terms of fossils, the Cretaceous-Tertiary Boundary has always been very abrupt. The layers below the boundary teem with fossils of many different types including the giant reptiles. In contrast, the region immediately above the boundary contains few fossils. This boundary has always been known as a major extinction event, one of several in the Earth's long history, and the most recent.

In the 1980s chemical studies of rocks along the Cretaceous-Tertiary Boundary found high levels of a metal called iridium. Typically the amounts were tens of times higher than in rocks on either side of the boundary.

Iridium is a metal closely related to platinum. It is a heavy, precious, inert metal. There is not much of it in the Earth's crust because on the Earth, the heavier elements tend to have sunk towards the centre - hence the planet's iron-nickel core.

Heavy metals like iridium are relatively rare in the Earth's crust.

They are far more common on asteroids.

Asteroids are rocks in orbit around the Sun. They are material left over when the solar system (the Sun and its planets) formed 4,700 million years ago. Bits hit the Earth on a regular basis; remember Siberia in 2013. Occasionally bigger bits hit - but luckily not that often.

The best explanation for the iridium layer is an impact by a fairly large asteroid.

The amount of iridium measured in the Cretaceous-Tertiary Boundary was found to vary around the world. It was lowest in Asia and Australia and India and highest in the Americas. This implied that any impact must have occurred on or close to the Americas.

The first clue to the location of the impact came when geologists were studying gravity maps while searching for oil. Gravity maps are satellite radar maps that show how gravity varies over an area with depth. They are used to locate denser material like metal deposits and lighter materials like gas, oil or coal.

These showed a gravity anomaly shaped like a ring. The ring was 180km in diameter partially along the coast of the Yucatan Peninsula in Mexico and partially in the adjacent seas.

The ring resembled an impact crater like the ones present on the Moon.

Studies in the area found deposits of shocked quartz as well as tektites, which resemble glass beads. Both are produced when rocks are subjected to high impact stress and have been seen around nuclear bomb test sites.

Wells drilled into the clay soil found a layer of andesite about 1.3km down. This is another rock produced by high temperatures and pressures. The Yucatan is a region lacking a volcanic history so the cause of this layer could not be volcanic.

Putting all the evidence together with the latest measurements and maps, we now know that the 180km gravity anomaly is actually the inner wall of a 300km wide crater. The crater is in dense tropical rain forest on land and under a shallow sea off the coast so is not visible above the surface.

From the amount of iridium found around the world and the size of the impact crater, the diameter of the asteroid was estimated to be about 10km. This would have delivered a blast of energy 2 million times larger than the biggest hydrogen bomb ever tested (in 1961 by the Soviet Union).

The blast would have sent a cloud of superheated steam, dust and ash into the air. The cloud of dust in the atmosphere would have blocked sunlight for several years reducing the amount of photosynthesis. This would have affected the entire food chain. Plant eaters and their predators would have been decimated. Creatures living on detritus (non-living material) or underground managed to survive.

The impact would have raised enormous tsunamis. Deposits from these tsunamis have been found in Mexico, Brazil and in mid-Atlantic sediments.

The blast wave would have triggered earthquakes and volcanic eruptions around the globe. The heat from the blast, plus ejecta heated when re-entering the atmosphere, would have generated fires even on distant continents. Many creatures and plants would have been incinerated where they stood.

The extinctions were rapid on a geological time scale, perhaps over a couple of thousand years. It was also a global event, affecting all continents. It affected the oceans as well as the land.

Some groups of organisms were decimated. Others eliminated entirely. There were even some groups that were mainly unaffected.

The dinosaurs were completely wiped out apart from one branch which became the modern birds. All the giant marine reptiles died out as did the flying reptiles. Dinosaurs had been the dominant vertebrates for 135 million years. Over 1300 species are known from fossils.

Many species of crocodile, turtle, and salamander survived. Crocodiles are the largest air-breathing survivors. They live in water and can scavenge. Their young grow slowly and can also scavenge. 80% of cartilaginous fish (sharks and rays) survived. 90% of bony fish also survived.

Mammals, birds, and lizards suffered high rates of extinction. Marsupials disappeared from Asia and North America but survived in South America from where they migrated across Antarctica to Australia. The mammals that did survive were small and rat-like.

Over 98% of tropical corals died out. Other victims were the ammonites whose spiral shaped fossils are very common. Nearly 60% of land plant species died out.

Interestingly, freshwater ecosystems were less affected by the decrease of photosynthesis. This is because many fresh water organisms depend on detritus washed down from hills and mountains. As a result, few amphibians became extinct as they tend to live in a freshwater environment.

There is evidence from the fossil record that the Earth's ecology took nearly two million years to recover from the disaster.

The mass extinctions allowed mammals and birds to fill the numerous environmental niches left behind. They did so by evolving rapidly as soon as the ecological systems had settled down.

But that is another story....

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