The Paleozoic Era

THE PALEOZOIC ERA

This era happened over a 290-million-year time period and started with what's called the Cambrian Explosion. This was when the amount and diversity of life on earth really exploded. It ended with the Permian extinction, which likely destroyed most of those life-forms for all eternity. This era had six distinct periods that we know of currently.

There were two types of bacteria living in the millions of years before this eon began. Some were called heterotrophs, which survived on other living things —mostly other bacteria. Others were called autotrophs —they did not need other living things but relied on the sun's energy to get their energy. Autotrophs were so important because they used photosynthesis and made oxygen as a toxic byproduct.

At one point, a smaller bacteria must have been eaten by a larger one. The smaller one produced energy and became symbiotic enough to become the energy factories in eukaryotic cells we now call mitochondria. These are all part of nearly every eukaryotic life form and cell today. As cells became more specialized, they formed multicellular organisms that were probably initially tube-shaped water dwellers without any mobility.

The Cambrian explosion meant that many new species were formed. Most phyla that existed during that time have since become extinct. You would have seen a lot of arthropods like trilobites, mollusks, brachiopods, and others. You have probably seen many brachiopod fossils, even if you haven't known it. Figure 12 shows a brachiopod fossil:

Figure 12.

You can see brachiopod fossils everywhere, even in a parking lot where you can find tan or gray rocks that have these fan-shaped creatures imbedded as fossils within them.

There were a number of periods in this Paleozoic era. These were the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Rather than remember these, you should know that the earliest fossils were in this order: mollusks, brachiopods, trilobites, crinoids, and then fish. The crinoids were interesting because their fossils look a lot like flowers. These were animals that were the ancestors of the sea urchins, sea cucumbers, and starfish you see today. Figure 13 shows a typical crinoid fossil:

Figure 13.

Notice that most of these were not widely mobile in any substantial way. Their main goals were to eat, consume oxygen, reproduce, and move around their general area.

This was when the main supercontinent was called Gondwana. It was closest to the South Pole. North America was not even part of the continent at that time. There was a small area where the North American continent could be found nearer to the equator. Most life was not initially on land in the beginning of this era.

The Silurian period was from about 419 to 443 million years ago. This was when land animals and plants could be found. There were still few significant plants or animals until the Carboniferous period at the end, when giant forests could be seen everywhere. Imagine so many plants that the oxygen level on earth was 14 percent higher than it is now.