Why Did Dinosaurs Go Extinct?

The asteroid gets all the attention, and rightly so. But by 66 million years ago, non-avian dinosaurs were already under pressure. The Deccan Traps in what is now India had been erupting for hundreds of thousands of years before the impact, releasing enormous quantities of volcanic gases that were already altering the climate and making ecosystems less stable.

Some palaeontologists argue that dinosaur diversity had been declining for millions of years before the impact. Others argue the fossil record is too incomplete to be sure. The current consensus is that the asteroid was the killing blow to a world that may already have been struggling.

What is certain is that the impact itself, the Chicxulub event, was the decisive catastrophe. The geological boundary between the Cretaceous and Paleogene periods, found in rock strata worldwide, contains a thin layer of iridium, an element rare on Earth but common in asteroids. That layer is the fingerprint of the impact, visible around the entire planet.

Birds are dinosaurs. Not descended from dinosaurs. They are, technically, avian dinosaurs, a lineage of feathered theropods that survived the extinction. Recognising this means the question is not why did all dinosaurs die, but why did this particular branch survive when 99% of non-avian dinosaurs did not.

The leading explanation involves several advantages birds had in a post-impact world. They were small, meaning they needed far less food. Many could eat seeds, which can persist in soil for years after plants die. Some species may have been able to migrate long distances to find viable food sources. Burrowing or water-based lifestyles also helped many species weather the impact winter.

Toothless beaks were also likely an advantage. Most toothed bird species, which required fresh meat or soft plant matter, went extinct. The toothless species that survived were better at cracking seeds and nuts, foods that remained available when almost nothing else did.

The survivors tend to share a set of characteristics. Small body size, which reduces caloric needs. Dietary flexibility, the ability to eat whatever happened to be available after the impact winter. Some form of shelter, whether underground, underwater, or in dense vegetation.

Mammals were ideally positioned. Most Cretaceous mammals were small, nocturnal, and omnivorous. They could eat insects, seeds, and carrion. Some could hibernate. In the immediate aftermath of the extinction, mammals began filling the ecological niches vacated by dinosaurs, and within 10 million years had diversified into horses, whales, bats, and primates.

Crocodilians, turtles, and certain fish also made it through, primarily by living in or near water, where temperature fluctuations were buffered and food chains took longer to collapse. Some crocodilians can go months without food, a significant advantage when prey was scarce.

This question divides palaeontologists. Some analysis of the fossil record suggests non-avian dinosaur diversity was declining in the final millions of years of the Cretaceous, as climate change driven by Deccan Traps volcanism destabilised ecosystems.

Other researchers argue the fossil record is too sparse and geographically uneven to draw firm conclusions. The absence of dinosaur fossils close to the boundary in some regions may reflect poor preservation conditions rather than actual decline.

The debate does not change the core conclusion. Whether dinosaurs were thriving or struggling, the asteroid impact created conditions that no large terrestrial animal could survive. The impact winter, not the initial blast, was the mechanism of extinction, and it was indiscriminate.

In 1980, physicist Luis Alvarez and his geologist son Walter Alvarez published a paper that changed palaeontology forever. Studying a thin clay layer at the Cretaceous-Paleogene boundary in Italy, they found concentrations of iridium 30 times higher than normal. Iridium is rare in Earth's crust but common in asteroids.

Their hypothesis was radical at the time: a massive asteroid had struck Earth and caused the mass extinction. Many palaeontologists rejected it, preferring gradual volcanic or climate explanations. The controversy was fierce.

The discovery of the Chicxulub crater in the 1990s settled the debate. The crater matched the timing and scale predicted by the Alvarez hypothesis precisely. Luis Alvarez died in 1988 before the crater was confirmed, but Walter lived to see his father's hypothesis become scientific consensus.