How and why did the dinosaurs become extinct? About 66 million years ago, something happened that caused the extinction of 75% of all living species, including the dinosaurs. On this page, we will discover that it was a gigantic asteroid that suddenly put an end to the "reign of the dinosaurs", which lasted for about 160 million years...
End of the dinosaurs
Before examining the different theories put forward to explain the extinction of the dinosaurs, we must point out that in this article, when we talk about "dinosaurs", we mean "non-avian dinosaurs". If you have been following our various factual articles on dinosaurs, you know that many scientists today consider birds to be dinosaurs. That's because all living birds are descended from a branch of dinosaurs called theropods. 🦖
Birds form the class of animals "Aves" (a "class" is a large group of related species). The word used to describe members of the class Aves is "avian". Therefore, non-avian dinosaurs are those that did not evolve into birds. As we have seen in this article, dinosaurs lived much longer than they became extinct. They are one of the longest living groups of animals on Earth. What would it have taken to kill them off? 🤔
The extinction of the dinosaurs was not the first mass extinction. In fact, since the Cambrian explosion (about 543 million years ago, during which many complex life forms began to appear), there have been five major extinctions. These periods of rapid and widespread extinction are known as "extinction events."
Dinosaurs survived an extinction event
The Triassic-Jurassic extinction, which occurred about 201.3 million years ago, wiped out 70-75% of all species. But not the dinosaurs. In fact, it was a mass extinction that benefited the dinosaurs, as it wiped out many of their early mammal and reptile competitors!
Cretaceous and Paleogene extinction
However, when the Earth was hit by the Cretaceous and Paleogene extinction 134 million years later, the dinosaurs were not so lucky. Only those smart enough to have evolved into birds survived! 🐦
This mass extinction, which occurred about 66 million years ago, is also known as the "K-Pg" or "K-T" extinction. As we have already seen, it caused the extinction of 75% of all species on Earth, including all (non-avian) dinosaurs. It also marked not only the end of the Cretaceous period, but of the entire Mesozoic era. Information on the Triassic, Jurassic and Cretaceous, as well as the Mesozoic Era can be found here: Dinosaur Periods.
How did the dinosaurs become extinct?
Ever since we discovered that dinosaurs had been the dominant land animals on Earth, the logical question has been: what happened to them? Many theories have been put forth to explain the extinction of the dinosaurs. These range from the bizarre (were the dinosaurs kidnapped by aliens?) to more credible explanations like climate change and toxic volcanic gases. 🌋
KT extinction: not just the dinosaurs!
Let's not forget that many other species became extinct at the end of the Cretaceous. What killed the dinosaurs also killed the Pterosaurs, the Plesiosaurs, and many species of fish, mollusks, and plants. And not all mammals made it either.
Dinosaur extinction theory number 1: Nemesis 🌠
One of our favorite dinosaur extinction theories involves another star, called Nemesis. According to the theory, Nemesis, which has yet to be discovered, is part of a binary solar system with the Sun. Every 26 million years or so, their connected orbits bring the two stars closer together, causing asteroids and comets to crash into Earth. In case you are looking at the stars through a telescope, NASA rejected the Nemesis theory in 2011.
Dinosaur extinction theory number 2: Volcanoes 🌋
The Deccan Trapps are huge volcanic rock formations that cover 500,000 km2. They now form the Deccan Plateau in west-central India. The Deccan Trapps were formed by volcanic eruptions about 66 million years ago. The eruptions may even have lasted for tens of thousands of years. The smoke, ash and gases emitted by the volcanoes would have darkened the sky and poisoned animals and plants. Temperatures would have dropped, plants would not have been able to grow and food would have become scarce.
The Deccan Trap theory is credible, and may have been a factor in the extinction of the dinosaurs. However, today, most scientists believe that the root cause of the dinosaur extinction did not come from the Earth at all...
Dinosaur extinction theory number 3: Asteroid ☄️
In 1980, scientists Luis and Walter Alvarez, father and son, put forward a theory that the Cretaceous and Paleogene mass extinction was caused by an asteroid hitting the Earth. They calculated that an impact from an asteroid about 10 km in diameter would have caused an explosion millions of times larger than that caused by a nuclear bomb.
The effect of such an explosion would be vast and catastrophic. Everything in the vicinity of the explosion would have been vaporized instantly. The huge cloud of dust thrown up by the explosion would have darkened the sky and blocked out the sun. Temperatures would have dropped and plants would not have been able to grow in the darkness. This would have affected the entire food chain, from the smallest plant to the largest. Any animal unable to adapt would have perished.
Alvarez's team developed the asteroid impact theory after studying a narrow band of sediment that lies between rocks formed in the Cretaceous and Paleogene. ⛰
This band of rocks is known as the Cretaceous-Paleogene boundary. Alvarez's team found that it contained an unusually high amount of iridium, a type of metal usually found only in the Earth's core... or in asteroids. Alvarez's hypothesis suggests that the Cretaceous-Paleogene boundary was formed by debris thrown up by an asteroid strike, and that such a strike would have caused the extinction of the dinosaurs. 🦖
Crater of chicxulub
The main reason Alvarez's hypothesis was not immediately picked up by other scientists was that an asteroid impact of this magnitude would have left a huge crater. At the time, there was no evidence of such a crater. However, in the years that followed, a huge buried crater located in the Yucatán Peninsula of Mexico was recognized as evidence for the Alvarez hypothesis.
The Chicxulub crater, named after a nearby town, was discovered by geophysicists Antonio Camargo and Glen Penfield. Studies have shown that it was formed 66 million years ago, and scientists now believe it is evidence that the Earth was hit by an extraterrestrial object large enough to have caused the extinction of the dinosaurs. The Alvarez hypothesis continues to be the most likely explanation for the extinction of the dinosaurs.
So what caused the extinction of the dinosaurs?
Many theories have been put forth to explain why dinosaurs became extinct while other types of animals survived. Of course, as we've seen, it wasn't just the dinosaurs that went extinct. Many other types of animals also became extinct. Whether or not an animal survived in the years following the asteroid impact may simply have depended on its size. Larger species need more food and are therefore more vulnerable to food shortages. 🥩
Smaller animals are also better at taking cover, either by burrowing or hiding under rocks or plants. Larger animals would be much more exposed to adverse environmental conditions. Dinosaurs may have been further disadvantaged by the large difference in size between young and adults. Young dinosaurs must be small to fit in an egg, even if they eventually grow into a huge sauropod like a brachiosaurus. They have to grow a lot before they can outgrow all the other animals around them. 🦕
This was fine as long as food was plentiful. However, after the asteroid hit, resources would have been extremely scarce. Young mammals, on the other hand, can feed on their mother's milk immediately after birth. They also do not need to hatch from an egg, and are therefore "closer" to their adult size at birth.
Late Cretaceous mammals would have been able to compete with dinosaurs at a young age, and thus would have been much more likely to reach adulthood.
A tale of two species
Most scientists agree that the species that survived the extinction were less specialized in their food needs and habitat than those that perished. Suppose, for example, that two species lived shortly before the asteroid impact. One species is large and ferocious, the other small and shy. 🐇
The larger species could prevent other species from eating a particularly nutritious plant. That's why the larger species adapted so that they could only eat that plant. After all, there was no point in being able to eat lesser plants, they had control of the best food there was. The other, smaller animal would have loved to be able to eat the best plant, but instead it was reduced to fighting for whatever else it could find. Because of this, he evolved the ability to eat many kinds of plants. 🌱
In the cold, dark years after the impact, the best plant could not grow properly. Its leaves were not as large and nutritious as they used to be. Eventually, only a few wilted bushes remained. The large, ferocious species would be left with nothing to eat. The withered bushes could no longer support its large size. Since his adaptations only allowed him to eat the best plant, he could not eat any other food. Suddenly, his size and ferocity counted for nothing, and he would starve to death. 🦴
The smaller species, on the other hand, would thrive. Happy to eat the parched bushes, as well as forage for whatever else they could find, they were healthier and more likely than ever to survive.
Although we still can't be absolutely sure what happened at the end of the Cretaceous, thanks to the work of scientists like the Alvarez's, our understanding is becoming clearer and clearer. Unless the star Nemesis emerges from the depths of space again, or aliens return with a spaceship full of dinosaurs, then the Alvarez asteroid hypothesis currently remains the best way to explain why the dinosaurs became extinct.