What is 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]], [[reptile]]s, [[synapsid]]s (the ancestors of [[mammal]]s), [[amphibian]]s, [[insect]]s, and [[plant]]s, the formation of the supercontinent of [[Pangea]] and at least two distinct [[ice age]]s. The Earth rotated faster than it does today, so days were shorter, and the nearer moon caused stronger tides. == Oxygen and CO<sub>2</sub> levels == [[File:Sauerstoffgehalt-1000mj2.png|thumb|Oxygen content of the atmosphere over the last billion years.<ref>{{cite journal |last1=Martin |first1=Daniel |last2=McKenna |first2=Helen |last3=Livina |first3=Valerie |title=The human physiological impact of global deoxygenation |journal=The Journal of Physiological Sciences |volume=67 |issue=1 |pages=97–106 |doi=10.1007/s12576-016-0501-0 |pmid=27848144 |pmc=5138252 |issn=1880-6546 |year=2016 }}</ref>]] At the start of the long [[Carboniferous|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.<ref name=Lane>Lane N. 2002. ''Oxygen: the molecule that made the World''. Oxford University Press, diagram p83. ISBN 0-19-860783-0</ref> Oxygen reached a peak of about 30% about 280 million years ago.<ref>Lane N. 2002. Chapter 5: The Bolsover Dragonfly: Oxygen and the rise of the Giants</ref> That i