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Paleozoic ("Ancient Life") Era
This is the first of three geologic eras squeezed into the last 10% of Earth's whole geologic history. But things are really starting to happen! This last 10% of Earth's history is called the "Phanerozoic Eon" or eon of "evident life." This means that rocks from the Phanerozoic contain fossils, lots of fossils!

The Paleozoic period lasted about 325 million years, from about 570 million years ago to about 245 million years ago. So much happened during the Paleozoic that it is divided into seven geologic time periods, shown on the red steps of the "Staircase of Time." Many different things happened during each period, but we can only give a summary of them here. You can learn more by going to your library or searching the Internet for words like "Paleozoic" or the names of each of the periods.

Here in the Paleozoic, Earth's interior has cooled down to something like modern levels, so that volcanic activity is usually about as humanity experiences it: a few minor eruptions like Mount St. Helens each year, and major ones like Krakatoa every century or so. However, gigantic "hot-spot" type eruptions still occur every hundred million years or so. Plate tectonics continues to push land masses across Earth's surface. At this particular time--the middle of the Silurian Period--most of the land is still locked in two supercontinents called Gondwanaland, which happens to be wandering over the South Pole in our view, and Laurasia, which is on the other side of the globe. Huge glaciers cover the interior of Gondwanaland, and Earth is experiencing one of its ice ages. Over the next hundred million years, Gondwanaland will move north over the equator and begin to break up, and the climate will warm up substantially (See the Mesozoic.).

The composition of the atmosphere has continued to slowly change, mostly due to the increase of oxygen produced by photosynthetic algae floating on the ocean. By the Paleozoic, the composition of the air has reached something like what we breathe now: about 4/5 nitrogen, 1/5 oxygen, and small amounts of carbon dioxide, water vapor, and other gases. At long last the air is capable of supporting large animals, and almost in response, life explodes into the Paleozoic!

At the beginning of the Paleozoic, life existed only in or near the ocean. Trilobites, shellfish, corals, and sponges appeared, followed by the first fish. Land plants appeared near the end of the Ordovician and for the first time we see the green of land plants in our global view. Huge forests and swamplands formed during the warm climate of the Mississippian and Pennsylvanian periods that later fossilized into the giant coal beds of the eastern United States. Animal life also moved onto the land, first the arthropods (spiders and insects to you), then the amphibians, and later the reptiles. The most abundant animals on land and sea during the Paleozoic were those like shellfish and insects that lacked backbones, so the Paleozoic is often called "The Age of Invertebrates."

The Paleozoic was also marked by several mass extinctions: geologically short periods of time during which large numbers of life forms died out. Mass extinctions occurred at the end of the Ordovician, the Devonian, and the worst one of all at the end of the Permian, when about 95% of all life on Earth died! These mass dyings were probably caused by climate changes and periods of giant volcanic eruptions.

We can follow the development of life in detail during the Paleozoic, because at the beginning of that Era, life forms developed hard parts like shells, teeth, bones, and woody parts that were easily preserved as fossils. Earlier life forms were single-celled and soft-bodied, so older rocks contain few fossils.

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Geologic Time
Cenozoic Era
Mesozoic Era
Image of a star. Paleozoic Era
The Precambrian Eon
Names on the Staircase of Time
How Old is That Rock?
Geologic Time Activity
What is a Million?
Finding an Event in Time
 
             
     
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Geologic Time | Cenozoic Era | Mesozoic Era | Paleozoic Era | The Precambrian Eon | The Staircase of Time | How Old is That Rock? | Geologic Time Activity | What is a Million? | Finding a Place in Time

Diversity | Adaptation | Plate Tectonics | Cycles | Spheres | Biomes | Geologic Time

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April 28, 2005

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