The Earth's History is broken up into a hierarchical set of divisions for describing geologic time. They were broken up in to increasingly smaller units of time. The generally accepted divisions are eon, era, period and age. The units were the Precambrian and Phanerozoic. The Precambrian was divided into three major divisions, Hadean Archean and Proterozoic. The Phaneozoic Era was divided into three major divisions, Paleozoic, Mesozoic and Cenozoic. At the time of the Dinosaurs, it was the Mesozoic Era. The Mesozoic Era was like the middle age of life. The Mesozoic Era started abround 245 million years ago and it lasted about 180 million years. During the Mesozoic Era, the super continent known as Pangea, which formed during the Paleozoic Era, was beginning to break apart into smaller continents. During the Mesozoic Era, the dinosaurs were evolving and dominating, causing the Mesozoic Era to also be known as the age of the reptiles.

The dinosaurs and the mammals appeared during the Triassic period, roughly 225 million years ago. The dinosaurs went extinct 65 million years ago.

MESOZOIC ERA

The Mesozoic is divided into three periods: the Triassic, Jurassic and Cretaceous Era. At the start of the Triassic Era, there was an enhanced extinction that changed the marine life. They discovered that ninety five percent of the marine species were gone. By the end of the Triassic, dinosaurs had spread rapidly through the land ecosystems on Earth. It was during the Jurassic that many of the most famous dinosaur species had evolved. They evolved quickly and adapted. There was evidence that the first bird was evolved during this era.It was a fossil record that showed it. It is believed that a combination of volcanic activity and an asteroid impact around 65 million years ago was the cause of the extinction of the dinosaurs, and many other species that were living in the Mesozoic Era.