Paleozoic

The Paleozoic is an era of geologic time that lasted from 541 to 252.2 million years ago.

Geologic periods
The Paleozoic is subdivided into six geologic periods. From oldest to youngest they were: A total of three mass extinctions occured during the Paleozoic: the Ordovician-Silurian extinction event, the Late Devonian extinction, and the Permian-Triassic extinction event (which caused the disappearance of over 90% of marine animals and 70% of terrestrial vertebrates ).
 * Cambrian (541 - 485.4 mya)
 * Ordovician (485.4 - 443.4 mya)
 * Silurian (443.4 - 419.2 mya)
 * Devonian (419.2 - 358.9 mya)
 * Carboniferous (358.9 - 298.9 mya)
 * Permian (298.9 - 252.2 mya)

Geography
At the start of the Paleozoic, the supercontinent Pannotia was breaking up and splitting into several smaller continents. By the end of the period, another supercontinent, Pangaea, had formed.

Climate
The climate of the Paleozoic varied considerably due to being such a large era. In the Cambrian, the climate was likely fairly moderate, but the Ordovician and Silurian periods had a warm greenhouse climate and the highest sea levels of the Paleozoic ever recorded. During the Carboniferous, carbon dioxide levels decreased to an all-time low, and oxygen levels correspondingly increased dramatically. This led to a large ice age. After the atmopshere recovered to more normal levels, the Permian was marked by a period of hot and dry climate due to the formation of Pangaea.

Flora
The first major plant life appeared in the early Paleozoic, though it was mostly aquatic. During the Silurian and Devonian periods, plants began to colonize the land, and by the Carboniferous entire rainforests of plants had formed.

Fauna
During the Cambrian, almost all the invertebrate phyla appeared, and quickly diversified. The first vertebrates evolved soon after, and primitive fish began to colonize the seas. Arthropods were the first animals to live on land, although the first tetrapods and amphibians appeared around this time and were dominant until the evolution of the reptiles. They were able to thrive in the dry climate of the Permian.