The Paleozoic is the era before the Mesozoic. The Mesozoic is also called the age of dinosaurs. The Mesozoic was before the Cenezoic, also called the age of mammals.
At the beginning of the Paleozoic multicellular animals which clearly moved, had senses like sight, and preyed on one another appeared. Unlike mammals and birds they were cold blooded. Warm blooded animals first evolved in the last geologic period of the Paleozoic. So the Paleozoic might loosely be termed the age of cold bloods.
The Paleozoic was about 300 million years long. It started with the Cambrian explosion and ends with greatest mass extinction, the Great Dying, at the end of the Permian.
I have had some difficulty remembering the geologic periods of Paleozoic, which makes it difficult to read the literature. So I have tried to straighten the whole thing out in my mind, and now I am writing my thinking down on this page. That way both the general public and I will be able to refer to it. This page contains relatively little in the way of new insight, I am simply organizing information.
The Paleozoic has been split into six geologic periods.
Those six geologic periods are as follows:
Almost all phyla of multicellular animals appear in the geologic record at this time, including Mollusks: snails, Arthropods: trilobites, and Chordates: vertebrates.
For the first time organisms with hard body parts evolved, so for a long time scientists thought this was the beginning of the fossil record. Scientists gradually found fossils of the soft bodied animals of the earlier period. Animals in the Cambrian moved and preyed on one another.
Some particularly interesting marine groups evolved in the Ordovician: primitive fish, cephalopods, and coral evolved. Squid and octopuses are cephalopods.
Furthermore the Arthropods invaded the land. Insects and spiders are examples of arthropods that live on land.
The Ordovician ended with a massive ice age, which has been described as a snowball earth. It is considered the first mass extinction and the second most serious, so it was worse than the one that wiped out the dinosaurs.
Reptiles evolved from amphibians. The reptiles unlike the amphibians did not have to return to the water to lay eggs. They could spend their entire life on land.
The Carboniferous period ended with a giant glaciation, ice age, centered on the south pole. With all that coal being formed carbon was removed from the atmosphere, which reduced the green house effect, and brought on glaciations, or ice ages.
In the second half of the Permian the Therapsids evolved. Therapsids were thought to be warm blooded and mammals evolved from them. So from this point on the large land animals were warm blooded animals like mammals including humans and dinosaurs including birds.
The Permian ended with the third and most severe mass extinction, the Great Dying. The Permian is followed by the Mesozoic, a summary of which is here.
More summaries of natural history, and other biology web pages can be found in the biology index.