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At the beginning of the Paleozoic Era, there were already living organisms on the earth. The earth's temperature had long ago dropped low enough for clouds to form and rain to fall, and this had set the stage for the development of organic compounds and-eventually-cells with chlorophyll that had the ability to photosynthesize.
During the early Paleozoic Era, life began to flourish and become more complex. The Paleozoic oceans swarmed with primitive arthropods called trilobites (relatives of the horseshoe crab). Clams and snails already inhabited shallow waters, as did members of the subclass Xiphosurida, which included early members of the horseshoe crab family. Later in the era, primitive plants begin to appear on land and the first jawed fishes develop.
The world's first great flush of vegetative growth appears in the Devonian period (360 to 408 million years ago), with huge ferns and forests of scale trees. Fish and land plants become abundant and diverse.
The next 35 million years, the Carboniferous Period, is marked by a tropical climate and a swampy environment. The first winged insects and reptiles develop in this time period, though they are much larger than those today. The first dragonflies had 29" wingspans and the first cockroaches were four inches long.
The end of the Paleozoic Era is the Permian Period, sometimes called the "Age of Amphibians" Amphibians and reptiles are the dominant fauna and gymnosperms the dominant plant life. The Permian ends with a large mass extinction, perhaps caused by glaciation or volcanic activity. The trilobites become extinct, as do 50% of all animal families, 95% of all marine species and many trees. The horseshoe crab survives.
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