How do we know what killed the dinosaurs?

The leading explanation is that an asteroid struck what is now the Yucatán Peninsula in Mexico about 66 million years ago. The case rests on several independent lines of evidence: a chemical signal in rocks worldwide, a buried crater of the right age, a global layer of impact debris, and fossil patterns that fit a sudden catastrophe. Some details remain debated, including how much other factors such as volcanism and climate change contributed. [1] [2]

In the late 1970s, Luis and Walter Alvarez found high levels of iridium in rocks dated to the end of the Cretaceous. Iridium is rare in Earth's rocks but common in asteroids, which pointed to an impact from space. The idea met resistance at first but was gradually accepted over the following decade, according to the Natural History Museum. [3]

Geologists then searched for a crater that was both old enough and large enough. In 1990, Hildebrand and colleagues identified the Chicxulub crater in Mexico, using magnetic and gravity anomalies and oil drill cores to date it. Its size and age matched expectations. The crater is about 150 kilometres across, the second largest on Earth, and the asteroid is thought to have been 10 to 15 kilometres wide. [4] [1]

A 2010 review in Science by Schulte and colleagues described a single ejecta-rich deposit, compositionally linked to Chicxulub, found around the world at the Cretaceous-Paleogene boundary. The ejecta layer matches the onset of the extinctions, and ecological patterns in the fossils agree with modelled effects such as darkness and cooling. The authors concluded that the impact triggered the mass extinction. [2]

The date has been refined. The Natural History Museum says the clay layers are dated to within a couple of thousand years, and recent redating puts the extinction at 66.0 million years ago rather than the older figure of 65 million. A study in Science analysed ruthenium isotopes in rocks from Spain, Italy and Denmark and found the impactor resembled carbonaceous chondrites, which come from beyond Jupiter. Ruthenium levels fit an asteroid strike better than a comet or massive volcanic eruptions. [1] [3]

The Natural History Museum states that the exact nature of the event is still open to scientific debate. Evidence suggests an impact was the main culprit, but volcanic eruptions causing large-scale climate change, and gradual climate shifts over millions of years, may also have played a part. The 2010 Science review notes the extinction occurred during Deccan flood basalt volcanism in India, yet concluded the impact was the trigger. [5] [2]

The once popular idea that dinosaurs declined gradually is not supported by a Scientific American review of the evidence: major dinosaur groups persisted into the latest Cretaceous, and the extinction was abrupt in geological terms. The same authors found that big plant-eating dinosaurs did decline somewhat near the end, possibly linked to a sea-level drop, which may have made ecosystems more vulnerable when the asteroid hit. [6]