The Ordovician Period

Again, index fossils and relative dating are the proof of the creatures that inhabited the Earth during this period. Even though there was an explosion of marine life, sixty percent of the species became extinct due to climatic changes and glaciation. After the Ordovician period is the Silurian period (440-410mya), which is best known for life on land. Once the climate stabilized, land plants grew near the equator and the evolution of fishes began. The first known freshwater fish, and fish with jaws appears. The Silurian strata contains fossils that indicate the previous presence of life during that period. The law of superposition was used to determine the sequence of the rocks, hence the relative ages of the rocks. When speaking of the

Evidence of plate tectonics is supported by geologists findings of fossil records. These six periods of the Paleozoic era show a huge development of diversification of plants and animals on Earth through the findings of fossils. The Mesozoic era (245-65mya) has two important milestones, the domination of dinosaurs, and the split of Pangea. This era contains three periods, the Triassic period (245-208mya), the Jurassic period (208-146 mya), and the Cretaceous period (146-65mya) The Triassic period (245-208mya), the first period in the Mesozoic era, marked the beginning of important changes in this era. Dinosaurs became enormous, fast, and ferocious; while the first flying vertebrates arrived, and flowering plants appeared. A milestone at the end of the Triassic period was a mass extinction, resulting in extreme reduction of some living populations, importantly including, marine invertebrates, and ammonoids. Scientists were able to conclude this, because these species served as important index fossils to designate relative ages to various strata in the Triassic system of rocks. There are several theories explaining the mass extinction which are, climate change, rising sea levels reacting from the release of carbon dioxide, the release of carbon dioxide from volcanic activity, or the greenhouse