In the 20th Century, most people thought that all dinosaurs were lizard-like and extinct. We now know that dinosaurs are not lizards and that birds are a type of dinosaur. Scientists now refer to modern birds as avian dinosaurs and to extinct dinosaurs as non-avian dinosaurs, many of which had feathers and beaks^[1].

1. Non-avian dinosaurs ↑

1.1 Triassic period ↑

Dinosaurs first evolved about 225 million years ago during the Triassic period. The Triassic period began about 250 million years ago, shortly after the Permian mass extinction wiped out about 96% of species on Earth. This was the third and most devastating mass extinction on Earth^[2a].

In the early Triassic period, most of the land on Earth was contained in a single continent known as Pangaea, which was generally hot and dry. Grass had not yet evolved, and the land was covered in conifers and ferns, with four-legged amphibians, reptiles, and mammals. The sea contained corral, worms, fish, shelled creatures like ammonites, and marine reptiles like turtles and plesiosaurs^[3a].

Some of the first dinosaurs to evolve were small, two-legged predators that lived on land. These were known as theropods^[4]. Theropods had similar skeletons to birds, and some were covered in feathers^[5][6].

The Triassic period ended about 200 million years ago with the fourth mass extinction on Earth when about 80% of species were wiped out. This is known as the Triassic-Jurassic mass extinction^[2b].

1.2 Jurassic period ↑

The Triassic period was followed by the Jurassic period when dinosaurs evolved to greater sizes and became the dominant animals on Earth. During this time, the Pangaea continent began to break up, eventually forming the continents we have today. This separated different species of dinosaurs so they could evolve independently^[3b].

The Jurassic period ended with a minor mass extinction, and many dinosaur species were wiped out, including Stegosaurus and many sauropod species, like Diplodocus^[7]. Sauropods were the largest known dinosaurs and the largest known animals to have ever lived on land.

Sauropods may have evolved to such large sizes in order to aid with digestion. They could survive on food that was less nutritious than that eaten by smaller animals because they spent more time digesting it^[8].

The largest sauropods were so tall that a person could walk underneath their stomachs. Sauroposeidon, for example, were about 18 metres (59 feet) tall^[9a]. This is around the height of a five-story building, and almost four times the height of Paraceratherium, the largest mammal to ever live on land.

Paraceratherium were hornless rhinoceroses, which lived about 30 million years ago. They are thought to be over 5 meters (16 feet) tall. This is slightly taller than a woolly mammoth and about 2 meters (7 feet) taller than an Asian elephant^[10].

Sauroposeidon were also about 34 meters (112 feet) long, which is about 10 meters (33 feet) longer than blue whales. They lived about 110 million years ago in the early Cretaceous period^[9b].

1.3 Cretaceous period ↑

The Jurassic period was followed by the Cretaceous period, which began about 145 million years ago. During this time, plants began to flower and bees evolved^[11]. Grass evolved about 70 million years ago^[12] and dinosaurs like Tyrannosaurus rex^[13] and Triceratops^[14] evolved about 68 million years ago.

There was about 82 million years between dinosaurs like Stegosaurus and Diplodocus, which evolved about 150 million years ago and dinosaurs like Tyrannosaurus rex and Triceratops, which evolved about 68 million years ago. This means that we are closer to the age of Tyrannosaurus rex and Triceratops than Tyrannosaurus rex and Triceratops were to the age of Stegosaurus and Diplodocus.

The Cretaceous period ended about 66 million years ago after the fifth mass extinction, the Cretaceous event, wiped out 76% of species on Earth^[2c]. This was most probably due to the impact of an asteroid or comet, known as the Chicxulub object^[15].

The Chicxulub object caused a large impact creator. The land it displaced was thrown into the atmosphere, and it is thought to have blocked out the Sun for months. During this time, plants couldn't photosynthesise, and so many became extinct. Most of the creatures that lived were in food chains that depended on dead plant material – detritus^[16].

2. Avian dinosaurs ↑

The Cretaceous period was followed by the Paleogene period. During the Paleogene period, mammals began to dominate the Earth, and the remaining dinosaurs evolved into modern birds - avian dinosaurs^[17]. The largest of these included Phorusrhacids, known as ‘terror birds'.

Terror birds evolved about 62 million years ago^[18] and went extinct about 1.8 million years ago^[19], around the time Homo erectus evolved. These were up to about 3 meters (10 feet) tall and couldn't fly.

Aepyornithidae, known as ‘Elephant birds', may have evolved around the same time. These were also flightless. They were around the same size as terror birds, and are thought to have gone extinct by the end of the 17th century^[20].

The largest bird currently alive is the ostrich, the tallest of which are only about 20 centimetres smaller than terror birds. The smallest bird currently alive is the bee hummingbird which is only about 5 cm (2 inches) tall^[21].

3. How do we know? ↑

3.1 Fossils ↑

The remains of preserved dead life forms, or signs of life forms like faeces or footprints, are said to be fossilised. This can happen if a body, plant, or sign of life is trapped in tar or amber, frozen, or mummified. It can also happen if one of these things is covered in water and a wet substance like mud falls onto it. If more and more layers of mud – sediments - cover it, the pressure causes the lower layers to harden, and the body is eventually replaced by rock^[22].

There's evidence that fossilised dinosaur bones have been discovered for thousands of years. In China, dinosaur bones were first thought to be dragon bones, and in Europe, they were thought to belong to giants and other life forms found in the Bible^[23].

Fossils were usually found in layers of sediments, known as strata, and in the 1790s, British geologist William Smith suggested that the bottom layers of strata are older than the top layers. This meant that fossils in the same layer were probably the same age^[24].

Dinosaurs were discussed scientifically in the 19th century, after remains were found in England in the 1820s and 1830s^[25][26][27]. British biologist and palaeontologist Richard Owen used the term Dinosauria to describe them in 1842^[28]. This meant ‘terrible lizard', although we now know that dinosaurs are not lizards, and they are not descended from them.

American palaeontologist William Parker Foulke identified the first American dinosaur fossil in New Jersey in 1858. Unlike previous dinosaur fossils, and unlike modern lizards, this dinosaur clearly walked on two feet^[29].

Shortly after this, American palaeontologists Edward Drinker Cope and Othniel Charles Marsh began a race to discover new American dinosaurs. This became known as the ‘Bone Wars', and did not end until Cope died in 1897. Cope and Marsh discovered over 140 new dinosaur species between them, although their methods were extreme and sometimes destroyed sites^[30].

At least thousands of dinosaur fossils have been found since then, appearing on all continents on Earth, and scientists have used this as a starting point in order to understand how non-avian dinosaurs looked and behaved.

Scientists can gain knowledge from fossilised bones, tracks, eggs, and faeces, as well as soft tissues, internal organs, skins, and feathers, although these are rarer. Theories are then developed from this information using the laws of physics, and our current knowledge of the physical and mental behaviours of animals.

We may never gain a complete picture, however, because the vast majority of animals are not fossilised. On top of this, some fossils are difficult to find and the majority of fossils are probably still far below the ground.

It may also be impossible to know if the images we create of extinct species are correct because most fossils do not contain complete skeletons or softer parts, like organs, fat, hair, fur, feathers, or external objects like spider's webs. Many animals look vastly different to what we would expect from their skeletons. This was explored by Darren Naish, John Conway, and C.M. Kosemen in their book All Yesterdays.

3.2 Dinosaurs and birds ↑

The first fossil of an apparently feathered dinosaur was found in 1861 and is known as Archaeopteryx. Archaeopteryx was thought to have lived about 150 million years ago, in the late Jurassic period^[31].

British biologist Thomas Huxley suggested that birds may have evolved from non-avian dinosaurs in 1868^[32]. However, this was not generally accepted until the late 20th century, when many more discoveries were made.

Scientists now think that some of the first types of dinosaurs, theropods, may have had feathers, and so it's possible that all their ancestors would have had them too, with birds first evolving from feathered dinosaurs during the Jurassic period.

Dinosaurs and birds are not just linked by feathers, however. Birds and theropods have similar skeletons, and some dinosaurs may have swallowed stones to aid digestion like some modern birds do^[33]. While non-avian dinosaurs were probably not warm-blooded like birds are, they may have been able to regulate their body temperature to some degree^[34].

Avian and non-avian dinosaurs may also have behaved similarly. Fossils show that some non-avian dinosaurs slept with their heads under their arms, like modern birds that tuck their heads under their wings^[35]. Both avian and non-avian dinosaurs are also thought to have laid eggs on land and built nests.

Some non-avian dinosaurs are thought to have laid eggs in communal nests that were incubated by males, and at least one of the parents may have cared for their offspring after they hatched^[36]. In some species, parents would have had to feed their offspring at first since they were born without teeth^[37].

Non-avian dinosaurs may also have been social animals. The remains of some non-avian dinosaurs, like sauropods^[38], Iguanodons^[39], or Hadrosaurids (duck-billed dinosaurs)^[40], have been found together, suggesting that they travelled in flocks. Some types of carnivorous dinosaurs may have also worked together to bring down prey^[41].

While avian dinosaurs can make a variety of noises, including mimicking human speech or machinery, it's not known if non-avian dinosaurs could make any vocal noises. However, they may have been able to hiss, or used their environment to make noises, like splashing or beating their wings. They may have also used visual communication, using coloured horns, frills, and feathers^[42].

It may be possible to clone a non-avian dinosaur one day, however it's very unlikely. To clone a non-avian dinosaur you would need a well-preserved piece of DNA, which we have never found, as well as a species close enough so that the DNA could be implanted in one of its member's egg cells. While modern birds are related to non-avian dinosaurs, it's unlikely that their eggs would develop.

There's a greater chance that we'll be able to clone extinct animals like woolly mammoths, which are better preserved, with closer living relatives.