Geologists have created a chart to organize Earth's history into their own time periods of : Eon, Era, Period and Epochs. Concepts based on their findings through time help to make educated theories on what may have happened during these time periods. The geologic time scale is based on the rock record of the earth: Cenozoic Era, Mesozoic Era, Paleozoic Era, and The Precambrian. As geologist continue to improve the dating methods small changes to the scale are to come. As more and more sections of rocks are explored and discovered theories and findings are to continue to the updates of this scale. What is most important to this chart are the fossils that are found and benefit these time periods as to what was alive then and what exactly they were to roam the earth way before us.

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251.9 million years ago-66.0 million years ago

During this period Pangaea began shifting into modern continents and the Rocky Mountains rose. The mammals in this time period include the dinosaurs, crocodiles and pterosaurs. As the climate was changing plate tectonics were very active causing ocean basins, sea levels to rise and seas to expand all across the North of America. Large marine reptiles were active during this time frame known as plesiosaurs, and coiled shell ammonites. The Mesozoic Era is also further divided into three periods: the Triassic, the Jurassic, and the Cretaceous. The Cretaceous period contains massive dinosaurs and other reptiles. The Triassic period consisted of the first dinosaurs to walk the land and swim in the oceans. Many of the modern reptiles derived from here such as the crocodiles and lizards. Lastly, the Jurassic was the starlight for the dinosaurs and categories were set for their diets such as herbivore and carnivores. Limestone strata was also found for the first time in this period. During the Jurassic the increase of volcanic activity led to seafloor spreading that released large amounts of carbon dioxide resulting in a global temperature for these creatures.

Mesozoic

541 million years ago- 251.9 million years

During this time period fish diversified and the marine life were very abundant. The breakup of one supercontinent and the formation of another is what began this era. Plants had spread and the first vertebrate animals colonized the land. In North America retreats of shallow seas repeated continental collisions forming the Appalachian Mountains. The common Paleozoic fossils are the trilobites, and cephalopods. The greatest mass extinction in the history of Earth brought this era to an end. This Era is also further divided into 7 periods known as: the Cambrian, the Ordovician, the Sulurian, the Devonian, the Mississippian, the Pennsylvanian and lastly, the Permian.

Paleozoic