Cretaceous Period (145 – 65 million years ago)
During the Cretaceous the land separated further into some of the continents we know today, although in different positions. This meant that dinosaurs evolved independently in different parts of the world, becoming more diverse.
Other groups of organisms also diversified. The first snakes evolved during this time, as well as the first flowering plants. Various insect groups appeared, including bees, which helped increase the spread of flowering plants. And the mammal group now included tree climbers, ground dwellers and even predators of small dinosaurs.
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Sea levels rose and fell during the Cretaceous. At the highest point there were many shallow seas separating parts of the continents we know today. For example, Europe was made up of many smaller islands. Thick layers of sediment built up at the bottom of these seas as single-celled algae died and their skeletons fell to the seabed.
This is how most of the chalk we use today was first formed. So much so, that ‘Cretaceous’ comes from the Latin for chalk, creta.
Jurassic Period (200 – 145 million years ago)
At the end of the Triassic Period there was a mass extinction, the causes of which are still hotly debated. Many large land animals were wiped out but the dinosaurs survived, giving them the opportunity to evolve into a wide variety of forms and increase in number.
The single land mass, Pangaea, split into two, creating Laurasia in the north and Gondwana in the south. Despite this separation, similarities in the fossil records show that there were land bridges between the two continents.
Temperatures fell slightly, although it was still warmer than today due to higher amounts of carbon dioxide in the atmosphere. Rainfall increased as a result of the large seas appearing between the land masses.
These changes allowed plants such as ferns and horsetails to grow over huge areas. Some of this vegetation became the fossil fuels that we mine today. Elsewhere there were forests of tall conifer trees such as sequoias and monkey puzzles.
The plentiful plant supply allowed the huge plant-eating sauropods – such as Apatosaurus, Diplodocus and Brachiosaurus – to evolve. These are the largest animals to have ever walked the Earth. By the end of the Jurassic their herds dominated the landscape.
Triassic Period (250 – 200 million years ago)
All continents were part of a single landmass called Pangaea during the Triassic Period. This meant that there weren’t many differences between the animals or plants found in different areas.
The climate was relatively hot and dry, and much of the land was covered with large deserts. Unlike today, there were no polar ice caps.
It was in this environment that the reptiles known as dinosaurs first evolved. Reptiles tend to flourish in hot climates because their skin is less porous than mammal skin, for example, so it loses less water in the heat. Reptile kidneys are also better at conserving water.
Towards the end of the Triassic, a series of earthquakes and volcanic eruptions caused Pangaea to slowly begin to break into two. This was the birth of the Atlantic Ocean.
Dinosaurs lived between 230 and 65 million years ago, in a time known as the Mesozoic Era.
Scientists divide the Mesozoic Era into three periods: the Triassic, Jurassic and Cretaceous. During this time, the land gradually split up from one huge continent into smaller ones. The associated changes in the climate and vegetation affected how dinosaurs evolved.