Dinosaurs

In the beginning the Earth started as a molten ball of rock, which formed about 4.6 billion years ago and took at least 600 million years to cool. The very first living organisms were microscopic bacteria that appeared 3.5 billion years ago. Next, the bacteria probably evolved to life in the sea, such as corals, fish, shellfish, and the ancestors of crabs. The very first fish and vertebrate was the Pikaia. After that the fish formed into amphibians, or a type of animal that spent part of its life in water, and part of its life on land. Finally, about 320 million years ago, in the Permian time period, reptiles were formed. Next, was the Mesozoic era also known as the Age of the Dinosaurs. The Mesozoic era has three time periods, which includes the Triassic period, the Jurassic period, and the Cretaceous period.

The first period was the Triassic period that lasted from 205 million years ago to 250 million years ago, and had the first dinosaurs. The Earth looked different at the beginning of the Triassic period than it does today, because all of the land was in one super continent called Pangaea. The climate was hot and dry, and mammal-like reptiles dominated the land and some grew to lengths of almost 20 feet. The first dinosaurs also appeared during this time and these included the small meat eaters such as the Eoraptor. In the late Triassic period, plant eaters that got up to 20 feet long also emerged. Nothosaurs were the first large reptiles to go into the sea they lasted almost the whole Triassic period. They probably hunted fish in more shallow waters. These dinosaurs got up to 10 feet long.

The Jurassic period was the second period of the Mesozoic time in which dinosaurs became the dominant species. This period lasted from 205 million years ago to 145 million years ago. The Earth's single landmass, Pangaea, had begun to come apart into two continents that were Laurasia in the north and Gondwana in the south. Even with this separation dinosaurs were still able to move about with relative freedom and therefore similar creatures were found throughout the landmasses. During this period global temperature began to cool even though it was still very warm. Rainfall also increased causing many plants and forests to grow. The large sauropods began to appear and replace the smaller plateosaur plant eaters. In the meat eater category, dilophosaurs stayed alive and healthy until an extinction event occurred about half way through the Jurassic. Allosaurs then evolved to fill the absence created by the disappearance of the dilophosaurs to become the top predator. Allosaurs could get up to40 feet long.

The Cretaceous period was the last period of time in which dinosaurs lived. Beginning at the last part of the Mesozoic, the Cretaceous was between 140 million years ago to 65 million years ago. The single landmass