When did the dinosaurs go extinct?

The end, when it came, came suddenly. An asteroid or comet 10 kilometres across slammed into the Gulf of Mexico, gouging a 180-kilometre crater and unleashing firestorms, eruptions and mega-tsunamis across the globe. The debris blocked out the Sun for years. The dinosaurs – and the other 75 per cent of life that went down with them – didn’t stand a chance.

The story of the demise of the dinosaurs 65 million years ago is well known. But that of their origin is less so. Dinosaurs were the dominant animals on land for at least 135 million years, the longest reign of any group. Had the impact not happened, they might still be in control. Where did these magnificent beasts come from?

Where did dinosaurs come from?

For years, palaeontologists thought that dinosaurs rose rapidly to prominence about 200 million years ago by virtue of being evolutionarily superior to their competitors. The Triassic period in which they first evolved was seen as little more than a dress rehearsal for the true age of dinosaurs – a kind of ‘Jurassic-lite’.

We now know this isn’t how it happened. The secret of the dinosaurs’ success was luck: they were in the right place at the right time. And, like their demise, their origins and heyday were triggered by huge, catastrophic mass extinctions.

At the end of the Permian period 251 million years ago, more than 90 per cent of all life suddenly disappeared. The cause (or causes) of the wipeout is angrily debated, but there is no doubt about its devastating impact. Life itself nearly went extinct, leaving bleak and empty landscapes over the vast single continent of Pangaea. A few plants and large land animals somehow clung on, and over the next 50 million years they gradually refilled the empty planet with life.

The first to take advantage was a group of mammal-like reptiles called the synapsids. They dominated the Early Triassic, and gave rise to mammals. By the middle of the Triassic period a second group of reptilian Permian survivors called the diapsids were starting to take over. That’s when things began to get monstrous.

Some of these beasts took to the water and evolved into ichthyosaurs, plesiosaurs and the other familiar marine reptiles of kids’ dinosaur books (though they were not dinosaurs). Another lot evolved into snakes and lizards. But the most interesting evolutionary action was taking place in a group of land animals called the archosaurs – the “ruling reptiles”.

The Triassic period

The classic view is that archosaurs evolved in the Middle Triassic and quickly gave rise to crocodiles, dinosaurs and the flying pterosaurs. They produced a few assorted “others” too, but these were of no great significance. Almost as soon as dinosaurs evolved, they started throwing their weight around. Thanks to superior evolutionary adaptations, they quickly became the dominant land animals, making the Triassic the “dawn of the dinosaurs”.

Or was it? It is true that the earliest dinosaurs are found in Middle Triassic rocks. The oldest come from a 230-million-year-old formation in the foothills of the Andes in Argentina.

The early birds

The first to be identified was Herrerasaurus, a very primitive two-legged meat-eater. Discovered in 1959, Herrerasaurus was found to belong to a group called the theropods, which ultimately gave rise to T. rex, Velociraptor and modern birds. A few years later came Eoraptor, a member of the lineage that eventually evolved into the gigantic long-necked sauropod herbivores such as Diplodocus and Apatosaurus.

The discovery of Pisanosaurus completed the picture. It was a forerunner of the duck-billed dinosaurs, confirming that even at this early stage dinosaurs had split into their two major families: the “lizard-hipped” saurischians, including theropods and sauropods, and the “bird-hipped” ornithischians such as the duck-billed dinosaurs and the stegosaurs.

But more recent discoveries have challenged the idea that the dominance of the dinosaurs was already a done deal at this point. Far from being a supporting cast, the assorted “others” were in fact the stars of the show, and dinosaurs hardly got a look-in until another extinction struck at the end of the Triassic. For whatever reason, this catastrophe hit the others hardest. All sorts of large, bizarre reptiles disappeared for ever. And much as the death of the dinosaurs cleared the way for the rise of mammals, so the Triassic reptiles’ demise heralded the age of the dinosaurs. The Late Triassic was the heyday of the archosaurs.

The illusion of dinosaur dominance stemmed from the fact that fossils of Triassic land animals are rare and usually incomplete. When scientists found Triassic fossils that looked like they came from dinosaurs, they logically assumed that they were dinosaurs. That included the rauisuchians, long-legged predators shaped like bears or lions. The largest stretched 7 metres. Some were bizarre, such as the sail-backed Arizonasaurus. Another dominant group of predators were the phytosaurs, long-bodied reptiles with narrow crocodilian jaws that looked a bit like modern gharials.

The most common plant-eaters were aetosaurs, low-slung animals up to 5 metres long with small heads and armoured bodies, built like the ankylosaurs of the dinosaur age.

Triassic–Jurassic mass extinction

For the next 10 million years the world belonged to these little-known animals, with dinosaurs playing bit parts. Then along came the Triassic–Jurassic mass extinction of 200 million years ago. It was one of the five most devastating extinctions of the past 500 million years but has attracted little attention, partly because there is no obvious trigger and partly because it claimed no charismatic victims.

Except that it did: the archosaurs. For some unknown reason they got absolutely hammered, leaving the dinosaurs to inherit the Earth.