What event characterized the paleozoic era?

The  phanerozoic geologic era is divided into three and the paleozoic era is the first among them. The paleozoic era occurred about 902 million years ago and it divided into six different periods. It was a time of dramatic changes in term of climate, geology and evolution.  
During the first period of the paleozoic era large number of creatures evolved, this is called Cambrian explosion. Most of the biological classes that are still prevalent today such as primitive fish and coral were evolved during this era. Mass evolution of fish also occured during this era.