What are dinosaurs?
Dinosaurs are an extremely varied group of animals that developed from archosaurs, their ancestors of the clade Dinosauria. They first appeared during the Triassic period, 231.4 million years ago, and were the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic period (approximately 201 million years ago) until the end of the Cretaceous period (66 million years ago), when they became extinct. According to a 2006 study, over 500 non-avialan dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered. An earlier study predicted that about 3400 dinosaur genera existed, including many which would not have been preserved in the fossil record. By September 17, 2008, 1047 different species of dinosaurs had been named.
Where did dinosaurs live?
Palaeontologists have discovered that dinosaurs lived on all of the continents – but not all the types lived everywhere. The reason for that lies in the past - about 230 million years ago, at the beginning of the age of dinosaurs, the Earth was very different from the way it is today. There was only one continent, a supercontinent called Pangaea. Unlike the present Earth, much of Pangaea was in the Southern Hemisphere. Because of plate tectonics, which caused volcano eruptions, earthquakes, mountain building, and sea-floor spreading, Pangaea started breaking apart. Its pieces then spread across the globe. The earliest dinosaurs may have only lived in certain parts of Pangaea, so as a consequence different dinosaurs were scattered around the globe. The oldest known specimens were all discovered in Argentina and most dinosaur bones – an estimated 7,600 samples – were reported to have been found in China.
What did dinosaurs eat?
Because of popular films, featuring ‘Tyrannosaurs rex’ and ‘Velociraptor’ many people think that most dinosaurs were carnivores – meat-eating dinosaurs. They are wrong – about 65% of dinosaurs were herbivores – plant-eating dinosaurs. Scientists know this because they have studied the content of the dinosaurs' stomachs and their teeth - most plant-eating dinosaurs had flat teeth that enabled them to snip and strip vegetation. They did not eat grass, because grass hadn't evolved yet. They ate leaves of different trees, such as the ginkgo tree, and flowering plants. The last of the dinosaurs – before they became extinct - ate fruit, too. The carnivores had long, sharp teeth designed for tearing through tough meat. They ate other dinosaurs, lizards, eggs, insects, turtles and early mammals. Because they had different kinds of teeth, some scientists think that some dinosaurs adapted to both types of food, which would make such dinosaurs ‘omnivorous,’ meat- and plant-eaters.
How big were dinosaurs?
Dinosaurs varied greatly in size. While scientists agree that the smallest dinosaurs were a bit bigger than a chicken (Compsognathus - “pretty jaw”; 1 m long from head to tail and probably weighed about 2.5 kg), they disagree about the biggest dinosaur. This is because not many complete skeletons have been found. The largest complete dinosaur found was Brachiosaurus (“arm lizard”); which reached 23 m in length and 12 m in height. Sauroposeidon was the tallest one ever to roam the earth; its neck alone was almost 12 m long and it weighed about 50 or 60 tons. Argentinosaurus measured about 37m from head to tail and may have weighed almost 100 tons. The biggest animals on Earth today look like dwarfs next to the biggest dinosaurs. Here are two of the theories that try to explain this phenomenon: the prehistoric period was very rich with vegetation so there was plenty of food; or the biggest dinosaurs (all herbivores) were so big so that they were immune to predators which were much smaller. Recently, evidence of squirrel-sized dinosaurs with feathers on all four limbs has been found in China.
Why did dinosaurs become extinct?
Dinosaurs became extinct about 65 million years ago, after living on Earth for about 165 million years. Since the discovery of the first dinosaur fossils, scientists have been trying to find out why this happened. The most common theory is that an enormous comet hit the Earth, causing so much dust that the sunlight couldn't get through, so the animals and plants died. Another theory claims that because of the volcanic eruptions there was so much ash in the air that it blocked the sun from shining on Earth, or the volcanic gases killed them. Some scientists believe that it was the ice age that killed the dinosaurs, as well as more than 50% of other animals and plants that lived at the time. There are also some other theories, such as that a fatal disease caused the extinction, or that the climate gradually changed, which caused the dinosaurs to die out. The Earth's orbit or magnetic field may have changed and caused their extinction. The sea level changed, dinosaurs had a low reproduction rate, or food chain imbalances lead to the starvation of the dinosaurs… So far, none of the theories have been proved.
More interesting facts
In spite of modern technology, many questions about dinosaurs remain unanswered. Here are some of the most frequently asked ones: How long did dinosaurs live? How fast could they run? What colour were they? The life span of the dinosaurs varied and depended on their metabolism and their body size; it was probably somewhere between tens of years to hundreds of years. To determine their speed, scientists have measured the distances between the fossil footprints and calculated their estimated speed by comparing the information with that of animals today. They think that an average sized dinosaur could walk at about 4-6 km/h, and that the fastest could run at almost 90 km/h. Fossils do not reveal the dinosaurs' skin colour. Scientists believe that they had protective colours, except for predators, who did not need to blend with their environment.
Text 1(What are dinosaurs)
Key words for Text 1
Dinosaurs, archosaur, Triassic period, extinct, fossil, 1047
Sample summary:
Dinosaurs developed from archosaur during the Triassic period. They became extinct about 66 million years ago. Many of them are preserved as fossils. Until 1047, scientists named 1047 different species of dinosaurs.
