Palaeozoic

The Palaeozoic (or Paleozoic) era is the earliest of the three eras of the Phanerozoic. Its name means early life. It lasted from about 541 to 252 million years ago (mya) and ended with the greatest extinction event, the Permian–Triassic extinction event. The Palaeozoic was the beginning of plants and animals. In particular, fish dominate the Palaeozoic.

 Beginning 
The Cambrian explosion marks the era with thousands of new life forms in the ancient seas. The era saw the development of most invertebrate groups, life's conquest of land, the evolution of fish, reptiles, synapsids (the ancestors of mammals), amphibians, insects, and plants, the formation of the supercontinent of Pangea and at least two distinct ice ages. The Earth rotated faster than it does today, so days were shorter, and the nearer moon caused stronger tides.

 Oxygen and CO2 levels 

At the start of the long Carboniferous period oxygen was low and carbon dioxide was high. The huge growth of trees and other plants during this time reversed these data. The carbon dioxide level dropped throughout the Carboniferous, and the oxygen level rose continuously until it dropped in the Permian.Lane N. 2002. Oxygen: the molecule that made the World. Oxford University Press, diagram p83. ISBN 0-19-860783-0 

Oxygen reached a peak of about 30% about 280 million years ago.Lane N. 2002. Chapter 5: The Bolsover Dragonfly: Oxygen and the rise of the Giants That is much higher than today's 21%.

Periods
The six periods of the Palaeozoic are:
Cambrian
Ordovician
Silurian
Devonian
Carboniferous
Pennsylvanian
Mississippian
Permian

 References