Dinosaurs: The 165-Million-Year Story That Never Really Ended

What Exactly Is a Dinosaur?

The word dinosaur is often used for any large prehistoric animal, but scientists use it much more precisely. Dinosaurs are a group of reptiles that belong to the clade Dinosauria. A clade is a group made up of one common ancestor and all of its descendants. That definition has a surprising consequence: because birds descended from dinosaurs, birds are dinosaurs too. Paleontologists therefore divide the group into avian dinosaurs (birds) and non-avian dinosaurs, which include every dinosaur that was not a bird, from Triceratops to Tyrannosaurus.

Many famous prehistoric reptiles are not dinosaurs at all. The flying pterosaurs, the sea-going ichthyosaurs, plesiosaurs and mosasaurs, and the sail-backed Dimetrodon all belong to other groups. Dimetrodon is actually more closely related to mammals than to dinosaurs, and it lived millions of years before the first dinosaur appeared.

The features that set dinosaurs apart

What makes a dinosaur a dinosaur is mostly hidden in the skeleton. The most important trait is posture. Lizards and crocodiles hold their legs out to the sides of the body, at least partly, while dinosaurs held their legs straight underneath the body, like pillars. This upright stance made walking and running more efficient and helped some dinosaurs grow enormous. Linked to this is an unusual hip socket: in dinosaurs it is open, with a hole through the middle, rather than a closed cup of bone. Experts also look at details of the skull, the arms and the ankle to decide whether a fossil belongs to Dinosauria.

A Name Born in Victorian Britain

People found dinosaur bones long before anyone knew what they were. Some historians suggest that ancient fossil discoveries may have inspired legends of dragons and griffins, although this is hard to prove. The scientific story begins in England in the early nineteenth century. In 1824 the geologist William Buckland described Megalosaurus, the first non-avian dinosaur to receive a scientific name. A year later Gideon Mantell named Iguanodon after noticing that its teeth looked like those of a modern iguana, only far bigger.

In 1842 the anatomist Richard Owen realised that these animals shared features that set them apart from living reptiles, and he coined the name Dinosauria. It comes from the Greek words deinos, meaning terrible or fearfully great, and sauros, meaning lizard. The name stuck, even though dinosaurs are not lizards. Owen later helped create life-size dinosaur sculptures for the Crystal Palace park in London, which opened in the 1850s and can still be visited. By modern standards they look odd, with Iguanodon shown as a heavy, four-legged beast with a horn on its nose. That “horn” was later found to be a spiked thumb.

In the late nineteenth century the United States became the centre of dinosaur hunting. Two rival paleontologists, Edward Drinker Cope and Othniel Charles Marsh, competed so fiercely that their feud is remembered as the Bone Wars. Their teams uncovered many famous dinosaurs in the American West, including Stegosaurus, Triceratops and Diplodocus.

When Dinosaurs Lived: The Mesozoic Era

Dinosaurs lived during the Mesozoic era, which began about 252 million years ago and ended 66 million years ago. Geologists split it into three periods:

• Triassic (about 252 to 201 million years ago): the first dinosaurs appear.
• Jurassic (about 201 to 145 million years ago): giant long-necked sauropods spread and the first birds evolve.
• Cretaceous (about 145 to 66 million years ago): flowering plants spread, and dinosaurs such as Tyrannosaurus and Triceratops appear near its end.

Dinosaurs appeared during the Triassic, more than 230 million years ago. Some of the oldest well-known examples, such as Eoraptor and Herrerasaurus, come from rocks in Argentina. These early dinosaurs were mostly small or medium-sized, walked on two legs, and shared their world with many other reptiles, including relatives of crocodiles, which were often larger and more common.

The turning point came about 201.3 million years ago with the Triassic–Jurassic extinction event, one of the largest mass extinctions in Earth's history. Many of the dinosaurs' competitors vanished, and dinosaurs became the dominant land vertebrates. They kept that position through the whole Jurassic and Cretaceous, a reign of well over 100 million years.

The world they lived in kept changing. At the start of the Mesozoic almost all land was joined into a single supercontinent called Pangaea. Over millions of years it broke apart, and dinosaur populations were separated on different landmasses, where they evolved in different directions. Fossils show that dinosaurs reached every continent, including Antarctica, which was much warmer then than it is today.

A surprising fact about time

The age of dinosaurs was so long that it is easy to misjudge. Stegosaurus lived in the Late Jurassic, around 150 million years ago, while Tyrannosaurus rex lived at the very end of the Cretaceous, about 66 million years ago. That means more time separated Stegosaurus from T. rex than separates T. rex from us.

The Great Dinosaur Family Tree

In 1888 the British paleontologist Harry Seeley divided dinosaurs into two large groups based on the shape of their hip bones. This split is still widely used, although some researchers have proposed rearranging the main branches.

Saurischians: the “lizard-hipped” dinosaurs

Saurischians include two major lineages:

• Theropods: mostly meat-eaters that walked on two legs, with sharp teeth and claws. They range from small, nimble hunters such as Velociraptor to giants such as Tyrannosaurus, Giganotosaurus and Spinosaurus. Birds belong to this group.
• Sauropodomorphs: plant-eaters that include the sauropods, the long-necked, long-tailed giants such as Brachiosaurus, Diplodocus and Argentinosaurus.

Ornithischians: the “bird-hipped” dinosaurs

Ornithischians were almost all plant-eaters, and many had a beak at the front of the mouth. They include:

• Thyreophorans: armoured dinosaurs such as the plated Stegosaurus and the tank-like Ankylosaurus, which had a bony club at the end of its tail.
• Ornithopods: a group that includes Iguanodon and the duck-billed hadrosaurs, which had hundreds of teeth packed together for grinding plants.
• Marginocephalians: the horned and frilled ceratopsians, such as Triceratops, and the dome-headed pachycephalosaurs.

There is a famous twist in these names. Birds descended from the “lizard-hipped” saurischians, not from the “bird-hipped” ornithischians. The bird-like hip of ornithischians evolved separately.

Giants, Hunters and Tiny Dinosaurs

The largest dinosaurs were the sauropods, the biggest animals ever to walk on land. Giants such as Argentinosaurus and Patagotitan, both from Argentina, are estimated to have reached 30 metres or more in length and to have weighed well over 50 tonnes. Because their skeletons are usually incomplete, these figures are estimates and vary between studies.

How could an animal so large support itself? Part of the answer lies in a feature they shared with birds: many of their bones contained air spaces connected to a system of air sacs. This made the skeleton lighter without making it much weaker, and it also helped them breathe efficiently. Their long necks let them reach a wide area of plants without moving their massive bodies.

Among predators, Tyrannosaurus rex is the most famous. It lived in western North America at the end of the Cretaceous, grew to about 12 metres long, and had one of the most powerful bites of any land animal known. Its small arms have become a joke, yet they were strongly muscled. Spinosaurus, from North Africa, was even longer, and many paleontologists think it spent much of its time hunting fish in rivers, although how much it lived in water is still debated.

Not all dinosaurs were huge. Many were the size of a chicken or a turkey. Contrary to its film image, Velociraptor was roughly the size of a turkey, and fossils show bumps on its arm bones where feathers were attached. The smallest living dinosaur is the bee hummingbird of Cuba, which weighs only about 2 grams.

Feathers, Nests and Warm Blood: How Dinosaurs Lived

Feathered dinosaurs

In 1861 workers in a limestone quarry at Solnhofen, Germany, found Archaeopteryx, an animal about 150 million years old with feathers and wings but also teeth, clawed fingers and a long bony tail. In the 1860s the biologist Thomas Henry Huxley pointed out the strong similarities between birds and small meat-eating dinosaurs. The idea gained wide support after 1969, when John Ostrom described Deinonychus, an agile, bird-like predator. This started what is often called the “dinosaur renaissance”, a new view of dinosaurs as active, energetic animals rather than slow, cold, swamp-bound lizards.

The final proof came from China. In 1996 scientists described Sinosauropteryx, the first non-avian dinosaur found with traces of feather-like covering. Since then, feathers or simple filaments have been found on many dinosaurs, including the four-winged Microraptor and Yutyrannus, a large relative of T. rex. Feathers probably first served for warmth and display, and only later for flight. By studying tiny pigment structures called melanosomes preserved in fossil feathers, researchers have even estimated the colours of some dinosaurs; Sinosauropteryx, for example, appears to have had a reddish-brown, striped tail.

Eggs and parenting

All known dinosaurs laid eggs. In the 1920s, expeditions led by Roy Chapman Andrews to the Gobi Desert in Mongolia found some of the first nests recognised as dinosaur nests. A nearby skeleton was named Oviraptor, meaning “egg thief”, because scientists thought it had died while stealing eggs. Decades later, fossils of related animals were found sitting on nests with their arms spread over the eggs, in the same pose that birds use when brooding. The supposed thief was most likely a parent. In Montana, nesting grounds of the duck-billed Maiasaura, whose name means “good mother lizard”, suggest that some dinosaurs cared for their young after hatching.

Were dinosaurs warm-blooded?

For a long time dinosaurs were imagined as cold-blooded like modern lizards. Today many lines of evidence suggest that many of them, at least, had high metabolic rates more like birds and mammals. Growth rings and the structure of their bones show that many species grew quickly. Dinosaurs lived in polar regions with long, dark winters, and many were covered with insulating feathers. Scientists still debate how warm-blooded different groups were, and the answer may have varied from one group to another.

The Day the Sky Fell: The End-Cretaceous Extinction

About 66 million years ago, the age of non-avian dinosaurs ended suddenly in the Cretaceous–Paleogene extinction event. Roughly three-quarters of all plant and animal species on Earth disappeared, including every non-avian dinosaur, the pterosaurs and the great marine reptiles.

In 1980 the physicist Luis Alvarez and his son, the geologist Walter Alvarez, with colleagues, proposed that a large asteroid had struck Earth. Their clue was a thin layer of clay, found around the world at the boundary between Cretaceous and later rocks, that contains unusually high levels of iridium, an element rare in Earth's crust but more common in asteroids. In the 1990s the impact site was identified: the Chicxulub crater, buried beneath the Yucatán Peninsula in Mexico and more than 150 kilometres wide. The asteroid is estimated to have been about 10 kilometres across.

The impact threw huge amounts of dust and gases into the atmosphere, blocking sunlight, cooling the planet and disrupting food chains. At around the same time, enormous volcanic eruptions in India, which formed the Deccan Traps, were also releasing gases. Most scientists consider the asteroid the main cause, while discussion continues about how much the volcanoes added to the crisis.

Birds: The Dinosaurs That Survived

One dinosaur lineage made it through the catastrophe. Small, feathered, beaked birds survived, possibly helped by their small size and ability to eat seeds. They went on to diversify into the more than 10,000 bird species alive today, from penguins and ostriches to eagles and sparrows. Next time you see a pigeon, you are looking at a living dinosaur. Among living animals, the closest relatives of birds are crocodilians, which share a common ancestor with dinosaurs.

The extinction also opened the way for mammals. Mammals had lived alongside dinosaurs for well over 100 million years, but most stayed small. With the large dinosaurs gone, mammals evolved into many new forms, eventually including whales, elephants and humans.

How We Know: Fossils and Modern Paleontology

Everything we know about non-avian dinosaurs comes from fossils. Bones and teeth are most often preserved, usually when an animal was buried quickly in mud or sand that later turned to rock. Scientists also study trace fossils: footprints that reveal how dinosaurs walked and whether they moved in groups, fossilised dung called coprolites that shows what they ate, and gastroliths, stones that some plant-eaters swallowed.

Modern tools have transformed the field. CT scans let researchers look inside skulls to study brains and inner ears. Microscopes reveal growth rings in bones that show how old an animal was when it died. Computer models test how fast dinosaurs could run or how hard they could bite. Paleontologists have named more than a thousand species of non-avian dinosaurs, and new ones are still described every year, many from Argentina, China and the United States.

Science also corrects itself. Brontosaurus was long considered just another name for Apatosaurus, but a 2015 study argued that the two are different enough to be separate genera, and the famous name came back into use. Each new fossil can change the picture, which is why the study of dinosaurs remains one of the liveliest areas of science.