Dinosaurs

The dinosaurs were a group of reptiles that ruled the land for approximately 164 million years, from about 230 to 66 million years ago. They were astonishingly diverse: from house-sized armoured monsters and giant long-necked plant-eaters bigger than 10 elephants, to small fast feathered predators and bird-like running dinosaurs the size of a turkey. Then 66 million years ago, almost all of them died out in a sudden mass extinction triggered by a huge asteroid impact. Almost all of them, but not quite all. One group of small feathered dinosaurs survived, evolved and is still with us today: the birds.

• First dinosaursApprox. 230 million years agoLate Triassic period
• Last dinosaurs66 million years agoEnd of the Cretaceous
• ReignApprox. 164 million yearsFar longer than mammals so far
• Two main groupsSaurischians and OrnithischiansLizard-hipped and bird-hipped
• Largest knownPatagotitanApprox. 37 m long, 69 tonnes
• SmallestMicroraptorApprox. 60 cm long with feathers

What is a dinosaur (and what is not)?

Strictly speaking, a dinosaur is any member of a particular group of reptiles called the Dinosauria, defined by a set of skeletal features (especially an upright stance, with legs directly under the body rather than sprawled to the side).

Things that are NOT dinosaurs, despite often being called one:

• Pterosaurs (the flying reptiles like Pteranodon): related to dinosaurs but not in the dinosaur group.
• Mosasaurs and plesiosaurs (the marine reptiles): different reptile groups.
• Dimetrodon (the sail-backed reptile from the cartoons): lived approximately 250 million years before the dinosaurs and is actually more closely related to mammals.
• Mammoths and sabre-toothed cats: mammals, lived 60+ million years after the dinosaurs.

And, surprisingly, things that ARE dinosaurs:

• Birds: every modern bird is a dinosaur, technically. They are the only group of dinosaurs to survive past 66 million years ago.

The two main dinosaur groups

Dinosaurs were divided into two big groups based on the shape of their hips.

• Saurischians ("lizard-hipped"): the giant long-necked sauropods (Brontosaurus, Diplodocus, Brachiosaurus, Patagotitan), and almost all the predators including T. rex, Velociraptor, Allosaurus. Birds also evolved from this group, despite the name.
• Ornithischians ("bird-hipped"): the armoured Stegosaurus and Ankylosaurus, the horned Triceratops, the duck-billed Iguanodon and many others. All ornithischians were plant-eaters.

Famous dinosaurs

• Tyrannosaurus rex: the most famous predator. Up to 12 m long, weighed 8 tonnes, ruled western North America 68 to 66 million years ago.
• Triceratops: three-horned plant-eater, 9 m long, lived alongside T. rex (and was eaten by it).
• Stegosaurus: with the famous double row of back plates and spiked tail. Lived 155 to 145 million years ago.
• Velociraptor: fast feathered predator the size of a turkey. The Jurassic Park ones were oversized.
• Brachiosaurus: giant long-necked plant-eater that held its neck high, like a giraffe.
• Ankylosaurus: walking tank with thick armour plates and a club tail.
• Iguanodon: one of the first dinosaurs ever named (in 1825). Could walk on two or four legs.

Fact The largest dinosaur ever discovered is probably Patagotitan mayorum, a long-necked plant-eater found in Argentina in 2014. Estimates suggest it was around 37 metres long from nose to tail and weighed about 69 tonnes: the weight of 12 African elephants. Other contenders include Argentinosaurus and Dreadnoughtus, all huge plant-eating titanosaurs from South America.

Birds are dinosaurs

One of the biggest discoveries in modern palaeontology is that modern birds are dinosaurs. They evolved from a group of small feathered dinosaurs called theropods, the same group that includes T. rex and Velociraptor. The famous fossil Archaeopteryx (found in Germany in 1861) was a beautiful halfway creature: half dinosaur, half bird, with teeth, claws on its wings and a long bony tail, but also feathers and the ability to fly.

When the great asteroid hit 66 million years ago and wiped out most dinosaurs, one small group of feathered theropods survived. Their descendants spread, diversified and evolved into the over 11,000 bird species alive today. So next time you see a sparrow, robin or chicken, you are looking at a real living dinosaur.

The end of (most of) the dinosaurs

Approximately 66 million years ago, a giant asteroid about 10 km across slammed into what is now Mexico. The impact created the 180 km-wide Chicxulub crater and triggered a global disaster: huge wildfires, an ash cloud that blocked out the Sun for years, and a sudden cold snap. Plants died. Plant-eaters starved. Meat-eaters that ate plant-eaters starved. Within months to a few years, approximately 75% of all species on Earth were gone, including every non-bird dinosaur.

The disaster cleared the way for mammals (which had been small and unimportant during the dinosaur era) to grow, diversify and eventually produce humans. Without the asteroid, dinosaurs would probably still rule the world today.

Did you know? The first scientifically described dinosaur was Megalosaurus, named in 1824 by William Buckland from a fossil jaw and some bones found in an Oxfordshire quarry. The word "dinosaur" itself was coined in 1842 by Richard Owen, from the Greek words meaning "terrible lizard". Before then, no one had a name for these enormous extinct reptiles, even though their bones had been turning up for centuries.

Deeper dive: were the dinosaurs warm-blooded?

For most of the time since dinosaurs were discovered, they were assumed to be cold-blooded like modern reptiles: slow, sluggish, dependent on sunshine for warmth. The whole image of dinosaurs as plodding, slow-moving giants comes from this assumption.

That picture started to change in 1969 when American palaeontologist John Ostrom described Deinonychus, a small fast-looking predator. He argued the bones showed a creature that was active and well-coordinated, not at all like a slow reptile. He suggested some dinosaurs may have been warm-blooded.

Over the following decades, evidence steadily mounted that many dinosaurs really were warm-blooded (or at least somewhere in between). The evidence includes:

• Growth rings in bones: dinosaurs grew rapidly in their first years of life, similar to modern birds and mammals, not like slow-growing modern reptiles.
• Feathers: discovered on many small dinosaurs, mostly used for insulation. Insulation only makes sense if the body produces its own heat.
• Hunting in herds and group nesting: complex social behaviour usually associated with warm-blooded animals.
• Fossil chemistry: oxygen isotopes in bones suggest stable body temperatures, similar to warm-blooded animals.
• Lived in cold places: dinosaur fossils have been found inside the Arctic and Antarctic Circles, where cold-blooded animals would have struggled.

The current best understanding is that most dinosaurs were somewhere between cold-blooded reptiles and fully warm-blooded mammals, with smaller theropods (especially the bird ancestors) being the most warm-blooded. The line between "dinosaurs" and "birds" was much more gradual than people once thought.

For the wider extinction, see extinction. For more about birds, see birds. For the rocks dinosaurs are found in, see sedimentary rocks.