Since the earth formed an estimated 4500 million years ago, it has been constantly changing.
The earth's surface is made of slowly moving plates of rock, and the continents sit on top of these plates and move with them over the earth's surface. Sometimes the plates bump into or scrape past each other, pushing up mountains, or causing earthquakes.
Millions of years ago, even the shape of the continents were different, all the continents formed a single land mass (termed Pangea) which slowly drifted apart to their present positions over millions of years.
Even now, the existence of earthquakes and volcanic activity tells us that the earth is still constantly shifting and changing
Since the earth's beginning, the shape and location of the continents has altered, as the continents have drifted over time.
One way in which we are able to categorise and measure all the incredible changes that were occurring on earth over these millions of years is to place them into timeslots or era's
The Paleozoic era saw the early history of complex life in the oceans. The fishes arose early during this time, and some of these gave rise to amphibians - the first backboned animals on land. The reptiles came soon after
The last period of the Paleozoic Era was the Permian period (245 to 286 million years ago) and is known as the "The Age of Amphibians" because amphibians were prolific at this time. The Permian period ended with the largest mass extinction ever, wiping out approximately 50% of all animal families, 95% of all marine species, and many trees.
This extinction was perhaps caused by glaciation or increased volcanism on Earth. This depleted state was followed by an explosion of new life forms in the Mesozoic Era.
The Mesozoic era is also known as the 'Age of the Dinosaurs' as it was a time in which these prehistoric reptile types literally dominated the earth
Millions of years ago, long before there were any people, there were dinosaurs. Dinosaurs were one of several kinds of prehistoric reptiles that lived during the Mesozoic Era.
During the Mesozoic era, the Earth was very different than it is now. The climate was warmer, the seasons were very mild, the sea level was higher, and there was no polar ice. Even the shape of the continents on Earth was different. At the beginning of the Mesozoic Era the continents were jammed together forming the supercontinent of Pangea. Without wide seas to block their path, the land-living dinosaurs could and did spread across the continents. Towards the middle of the Mesozoic Era, continental drift caused Pangea to start to break apart.
The Mesozoic Era lasted is divided into three periods, the Triassic, the Jurassic, and the Cretaceous.
The Cenzoic Era began after the great extinction of the dinosaurs and many other Mesozoic groups , both on land and in the sea. The scene became dominated by mammals on land. The latest major group to appear were humans approximately 5 million years ago.
Hence the Cenzoic era is often referred to as the "Age of Mammals"
What caused the extinction of the dinosaurs?
Scientists are still trying to decided exactly how the extinctions occurred, some of the questions still answered are:
As yet, nobody knows the exact answers to these questions but people have thought up many different theories to explain how the dinosaurs became extinct.
Dinosaurs became extinct at the end of the Cretaceous Period. Perhaps we will never know why the group of animals that had been dominant for 150 million years could not survive.
Scientists are baffled by the fact that many groups of animals died out along with the dinosaurs, but others survived.
We're also not sure of how long it took for extinction to take place. Some believe it happened over half a million years and others say it only took a few days. The extinction event that killed the dinosaurs affected the entire earth; plants and animals on both land and in water were also affected.
The disappearance of other living things was so great that scientists knew about the extinction 30 years before the first dinosaur was discovered. Excavations of rocks from the late Cretaceous period have shown that there was a wide variety of dinosaurs living at the time, then at the end of the Cretaceous period they all died out.
Experts disagree about what caused their extinction.
There are two groups of extinction theories:
Catastrophic extinction would have been caused by a sudden, external event, such as the collision of the earth with an asteroid, or the eruption of a series of gigantic volcanoes.
Some believe the extinction was caused by a catastrophe such as a meteorite or comet hitting the earth. Many small meteorites have been found on the earth, and the giant craters mark major collisions. The shock of such a massive collision would have surrounded the earth in a huge cloud of dust and steam, blocking out the sun for months or even years.
Many animals would have been killed by the explosion and the resulting alteration in weather patterns. Dinosaurs may have been particularly vulnerable to these changes in the ecology of the earth. Other animals, however were able to survive the crisis.
The most popular 'disaster theory' is that the earth was hit by an asteroid or a comet 65 million years ago. The asteroid would have exploded deep into the earth forming a massive crater hundreds of kilometers wide, and shot out up to 400 million million tonnes of rock and dust! The dust that resulted would have travelled round the earth and stayed for many months, effectively blocking out the sun and possibly resulting in raising atmospheric temperature.
The best evidence in support of the meteorite theory so far is found when
examining Cretaceous sedimentary rocks. In several places around the world,
geologists have found large amounts of the metal iridium in clay bands.
Iridium is a rare metal which generally only comes from space. Normally we only
find very small amounts - which come from tiny meteorites that land on earth.
But there are large amounts of iridium at the time the dinosaurs died out. Many scientists believe this is good evidence for an asteroid, yet others suggest that iridium could have come from large volcanoes
Image of a meteorite crater
Others believe a more gradual process was responsible for the mass extinction
65 million years ago.
Gradual extinction would have been the result of changes in the earth's land mass and climate shifts, such as glaciation.
The earth is constantly changing as continents move and climates alter.
65 million years ago the continents had seperated completely. Some seas had spread into the continents, and there was a higher sea level across the world. Warm tropical conditions gave way to cooler, more seasonal climate. Tropical plants and rainforests were replaced by more modern woodland plants. These slow changes may have been too much for the dinosaurs, and their place as rulers of the world was taken over by mammals.
One recent idea is that there were changes in temperature world-wide. In many areas it seems that the air and sea became hotter at the end of the Cretaceous (evidence for this is seen from changes in the plants and from measurements in the chemical composition of certain rocks that were being laid down at the time). An increase in temperature could have seriously upset the life of many animals and sensitive ones (like the dinosaurs) died out.
Large dinosaurs could simply have overheated. Similarly, any decrease in temperature would also have resulted in the death of many dinosaurs - as it has been theorised that the majority of dinosaurs where cold blooded (ie could not regulate their own body temperature, and depended upon the ambient temperature to regulate their own body warmth).
Additionally it is worth considering that temperature change might have had a different effect to that outlined above. The sharp heating of the Earth's surface was followed by a gradual cooling. The extinction might have been gradual and caused by slow climatic change over hundreds of thousands of years
b) Survival of the Fittest
Another popular branch of the Gradual extinction theory is often termed "Survival of the fittest", as it suggests that new and better animals won in the struggle for existence.
'Survival of the fittest' theories are certainly valid when trying to explain the mass extinction at the end of the Cretaceous period (as can be seen time and time again in present day - to explain evolutionary trends and models, see worksheet 4 - evolution), however these theories fall short when trying to explain it as the sole reason for extinction.
Many of these theories can only partially explain dinosaur extinction and usually tend to ignore the extinction of the other groups of animals which occurred simulataneously.
The key to the great extinction mystery may be found in the end by looking at which plants and animals died, and survived. Any theory has to be able to explain why dinosaurs and the great sea reptiles perished, but crocodiles, turtles, lizards, snakes, birds and mammals survived.
The world that came after the dinosaurs was more like our own. It was dominated by mammals and birds, animals that are familiar today. During the Cenzoic era, mammals increased in size and variety and soon became the dominant land animals. Plants too, became more like those of today. Not all the dinosaur's close relatives died out, for example; the crocodile is a living cousin of the dinosaurs. But in number and variety - the most successful descendants of the dinosaurs are the birds. (see worksheet 4 - evolution)
Dinosaurs Index | Lesson Idea 1 - Art Primary | Lesson Idea 2 - Art | Lesson Idea 3 - English | Lesson Idea 4 - Science | Worksheet 1 | Worksheet 2 | Resource 1 | Resource 2 | Resource 3 | Activitysheet 1 | Associated Web Links