The Paleozoic Era comes directly after Precambrian Time on the Geologic Time Scale. This era began about 542 million years ago and lasted until about 250 million years ago.
The Paleozoic Era begins with what is called the Cambrian Explosion. This relatively quick period of evolution and species development established many new and more complex organisms than the Earth had ever seen before. Many of today's species' ancestors were established during this period including arthropods and echinoderms.
The Paleozoic Era is broken up into six periods. The first period is known as the Cambrian Period, which is when the Cambrian Explosion occurred and it lasted a few million years. All life was found in the seas, including trilobites - the most abundant of the sea dwelling animals. The next period is the Ordovician Period. The beginning of this period was very similar to the Cambrian Period. The only life on Earth was found in the seas. However, by the end of the Ordovician Period, life moved to land. Plants began to colonize out of the water paving the way in later periods for animals to follow.
The Silurian Period saw various invertebrates move on to land. Some of the arthropods from the Cambrian Explosion evolved various characteristics to allow them to live both in water and on land. Now that plants had been established, there were plenty of food sources for these animals. The following period, the Devonian Period, saw vertebrates join the other types of animals on land. The first ones to leave the water were similar to amphibians and they eventually evolved into reptiles.
There were many empty niches to fill on land since it was early in its colonization. This led to a great evolution of plants to fill those niches. By the end of the Devonian Period, there were forests of conifers popping up in various places.
The next period, the Carboniferous Period, is sometimes broken into two separate periods called the Mississippian Period and the Pennsylvanian Period. It is named the Carboniferous Period because much of the coal we use today was made during the later stages of this time period. In terms of evolution, perhaps the most important change that happened during the Carboniferous Period is the development of the amniotic egg. Land animals could now lay eggs that could survive longer.
There were very big climate changes during the Carboniferous Period that led to some species rapidly evolving. For instance, fish in the Devonian Period were heavily armored. These types of fish became extinct and instead created species that resemble current day fish instead. It was during this time period that Pangaea was beginning to come together, creating different climates on all continents.
The final period of the Paleozoic Era is the Permian Period. Most species thrived during the early parts of the Permian Period and further diversified. However, at the end of the Permian Period, which also marked the end of the Paleozoic Era as a whole, the largest mass extinction ever on Earth happened. This mass extinction, known as the Permian Extinction, completely wiped out about 95% of all life in the oceans on Earth. Many species on land also went extinct, but not nearly to the extent of marine life. Even within the species that did manage to survive such a large extinction, numbers of individuals were very low. This paved the way for new types of species to take over during the next era on the Geologic Time Scale - the Mesozoic Era.
Scientists are unsure of how or why this mass extinction occurred. Some believe this mass extinction happened in different phases. It is hypothesized the earlier part of the extinction was less severe and due to climate change as the continents all came together to form Pangaea. The latter phase may have been caused by some catastrophic event that led to even larger numbers of species dying off completely.