Dinosaurs and Mass Extinctions

For most of the time there have been animals on land, the largest of them were dinosaurs. They appeared about 230 million years ago, spread to every continent, grew into the biggest land animals ever and lasted for 165 million years. Then, 66 million years ago, nearly all of them disappeared in what may have been a single bad afternoon. Their rise and fall is the most famous chapter in a longer story: life on Earth has nearly been wiped out at least five times.

What a dinosaur is

Dinosaurs were reptiles, but not like the lizards and crocodiles of today. They stood with their legs directly beneath their bodies, like mammals and birds, rather than sprawling, and that posture let them grow large and move fast. Many were warm-blooded, or something close to it. A great many had feathers, which evolved first for insulation and display, long before any dinosaur flew.

They came in extraordinary variety. The long-necked sauropods such as Argentinosaurus stretched over 30 metres and weighed 70 tonnes, the largest animals ever to walk. Tyrannosaurus rex, 12 metres long with a bite that could crush bone, hunted the plains of North America at the very end of the dinosaur age. Triceratops carried a bony shield and three horns; Stegosaurus plates and a spiked tail. Others were the size of chickens. Dinosaurs laid eggs, and fossil nests show that some brooded them and cared for their young. The flying pterosaurs and the marine reptiles of the same era, often lumped in with them, were not dinosaurs but cousins.

The dinosaurs that lived

One group of small, feathered, fast-running dinosaurs evolved longer arms, lighter bones and the ability to glide and then fly. Archaeopteryx, from 150-million-year-old German limestone, has feathers and wings alongside teeth, clawed fingers and a long bony tail. By the end of the Cretaceous, true birds were common. They survived the catastrophe that killed their relatives, and there are about 11,000 species of them today. In the strict sense dinosaurs never went extinct. A pigeon on a windowsill is a living one.

The day the Cretaceous ended

Sixty-six million years ago an asteroid about ten kilometres across struck what is now the Yucatán Peninsula in Mexico, at perhaps 20 kilometres per second. The impact released energy equivalent to billions of atomic bombs, blasted a crater 180 kilometres wide, and triggered earthquakes, tsunamis and a rain of molten rock that set forests burning across the globe. Worse came after. Dust and sulphur hurled into the stratosphere blotted out the Sun for months or years. Photosynthesis collapsed, temperatures plunged, and the food chains that depended on plants and plankton fell apart. About three quarters of all species died, including every dinosaur larger than a dog, the pterosaurs and the great marine reptiles.

The evidence is a thin layer of clay found in rocks worldwide at exactly that age, rich in iridium, an element rare on Earth but common in asteroids, and containing shocked quartz and tiny glass spheres formed in the impact. The crater itself, buried under Yucatán, was identified in 1991. Enormous volcanic eruptions in India around the same time may have made things worse.

The other four

The end of the dinosaurs is the most famous mass extinction but not the biggest. About 252 million years ago, at the end of the Permian period, around 90 percent of marine species and most land vertebrates died in the "Great Dying", almost certainly caused by colossal volcanic eruptions in Siberia that poured out carbon dioxide, acidified the oceans and heated the planet. Three others, at the ends of the Ordovician, Devonian and Triassic periods, each killed off more than half of all species. Volcanism and rapid climate change are implicated in most of them. Each time, life recovered over millions of years, and each time the survivors were different. Mammals, small and marginal for 150 million years, inherited the Earth only because the asteroid cleared it.

A sixth?

Species are now vanishing at a rate estimated to be tens to hundreds of times the natural background, through habitat loss, hunting, pollution and climate change. Many biologists argue a sixth mass extinction is under way, the first caused by a single species.

Why it matters

The fossil record shows that dominance is temporary and that catastrophe is a normal part of Earth's history. It also shows how life rebounds, and how the accidents of extinction set the course of everything after. Had the asteroid missed, there might be no mammals larger than a cat, and no one to dig up the bones.