The Paleozoic "old life" era is characterized by trilobites, the first four-limbed vertebrates, and the origin of land plants. The Cambrian Period marks the time of emergence of a vast number of fossils of multicellular animals, and this proliferation of the evidence for complex life is often called the "Cambrian Explosion".
The time data from radiometric dating is taken from that source. This was done by making a linear time line on the left side of the time columns. By the beginning of the Quaternary Period, most of the major plate tectonic movements which formed the North American continent had taken place, and the main modifications past that were those produced by glacial action and erosion processess.
There is an observable end to this formation of iron ore, so the increase in atmospheric oxygen would have been expected to accelerate at that time. Paleogene Period, Cenozoic Era, Phanerozoic Eon [66 Myr - 23 Myr ] The Paleogene Period or the early part of the Tertiary Period represents the time period after the major extinction that wiped out the dinosaurs and about half of the known species worldwide.
By the end of the Permian Period the once dominant trilobites are extinct along with many other marine animals. The later part of the Ordovician saw the appearance of the first fishes. For example, many fossils that are commonly found in the youngest Paleozoic rocks are not found in overlying Mesozoic rocks.
The Geological Time Scale Now that you have learned about the hierarchical components of the geological time scale--eons, eras, periods, and epochs--consider again how all of these parts fit together.
This theory, known as " Plutonism ", stood in contrast to the "Neptunist" flood-oriented theory. Bold horizontal lines indicate major boundaries between eras. Similarly, the Paleogene-Neogene boundary is equivalent to the Oligocene-Miocene boundary.
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