In 1980 the scientists Luis and Walter Alvarez proposed that a large asteroid hit Earth and ended the dinosaurs. Rocks laid down at the end of the Cretaceous hold a thin clay layer with a lot of iridium, a metal that is rare at Earth's surface but common in asteroids. The impact sent dust and soot into the air, which cut down the sunlight reaching Earth's surface and slowed plant growth. As of November 2026, most scientists accept that an asteroid impact was the main cause of the end-Cretaceous mass extinction. Volcanoes and slow climate change may also have played a part.

As of November 2026, the asteroid that hit Earth about 66 million years ago is estimated to have been roughly 10 to 15 kilometres wide (about 6 to 9 miles). That is much smaller than the crater it left. As of November 2026, the crater, called Chicxulub, is more than 90 miles (150 kilometres) across. As of November 2026, about three quarters of the animals on Earth, not only the dinosaurs, died out in the mass extinction 66 million years ago. As of November 2026, the end-Permian mass extinction, before the dinosaurs, was the biggest in Earth's history. The end-Cretaceous one was the second biggest.

Some scientists think volcanoes and climate change had already stressed life before the asteroid struck. Huge volcanic eruptions in what is now India, which made a lava outcrop called the Deccan Traps, were going on around the time the dinosaurs died out. Some evidence suggests dinosaurs were already in a slow decline late in the Cretaceous. But the rocks cannot show this precisely, so it is still debated. As of November 2026, new dating of the Naashoibito Member, a New Mexico rock formation that holds dinosaur fossils, put its youngest rocks at about 66.4 million years old. That is within roughly the last half million years of the Cretaceous.

Birds were the only dinosaurs that survived when the rest of the dinosaurs died out. Birds are placed inside the dinosaur family tree, so scientists say dinosaurs are not totally extinct and call the others non-avian dinosaurs. Birds evolved from small meat-eating dinosaurs called theropods. Surprisingly, birds belong to the lizard-hipped group of dinosaurs, not the bird-hipped group. Some mammals, birds, small reptiles, fish and amphibians survived the end-Cretaceous extinction, including the lines that led to animals alive today. The same mass extinction that ended the dinosaurs also ended the flying pterosaurs and many sea animals.

Carbon dating only works on things from the past 50,000 years or so, which is far too recent for dinosaur fossils. Becoming a fossil is rare, so the dinosaurs we know from fossils are only a tiny share of all that ever lived. Most fossils found are of sea animals such as shellfish, so a fossil of a big land animal is unusual. Most dinosaur fossils are found in rock laid down by ancient rivers, which buried the bones.

As of November 2026, in big weight-bearing dinosaur bones, some of the growth rings laid down early in life are erased as the marrow cavity widens. So scientists look to smaller bones that keep their rings to read how fast the animal grew. As of November 2026, a tissue called medullary bone was found in a Tyrannosaurus rex specimen known as MOR 1125. Modern female birds make this tissue to supply calcium for their eggs. Those who found it said it suggested this T. rex was a female.

As of November 2026, Sinosauropteryx was the first non-avian dinosaur found with feather-like structures on its body. Scientists can work out the colour of some fossil feathers by studying tiny pigment structures preserved in them. As of November 2026, in 2010 scientists reconstructed the colours of the feathered dinosaur Anchiornis by studying the shapes of fossil pigment particles, called melanosomes, in its feathers under a microscope. They compared them with pigment from modern birds. As of November 2026, pigment traces preserved in the fossil feathers of Sinosauropteryx indicate that it was probably reddish-brown and ginger in colour, with a striped, banded tail.

Words to know

• iridium: rare metal
• medullary: inner bone
• decline: slow fall
• specimen: one example