Periods of the Paleozoic Era

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Periods of the Paleozoic Era

Fossil remains of Archimedes, Bryozoa, Carboniferous Period. Getty/De Agostini Picture Library

Each major era on the Geologic Time Scale is further broken down into periods that are defined by the type of life that evolved during that span of time.  Sometimes, periods would end when a mass extinction would wipe out a majority of all living species on the Earth at the time.  After Precambrian Time ended, a large and relatively quick evolution of species occurred populating the Earth with many diverse and interesting forms of life during the Paleozoic Era.

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Cambrian Period (542 - 488 million years ago)

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Trilobite fossil from the Cambrian Period. Archbob

The first period in the Paleozoic Era is known as the Cambrian Period.  Many of the ancestors of the species that have evolved into what we know today first came into existence during the Cambrian Explosion in the early Cambrian Period.  Even though this “explosion” of life took millions of years to happen, that is a relatively short amount of time when compared to the entire history of the Earth.  At this time, there were several continents that were different than the ones we know today.  All of the landmasses that made up the continents were found in the southern hemisphere of the Earth.  This left very large expanses of ocean where sea life could thrive and differentiate at a somewhat rapid pace.  This quick speciation led to a level of genetic diversity of species that had never been seen before in the history of life on Earth.

   

Almost all life was found in the oceans during the Cambrian Period.  If there was any life on land, it was most likely in the form of unicellular microorganisms only.  Fossils have been found all over that can be dated back to this span of time.  There are three large areas called fossil beds where the majority of these fossils have been found.  Those fossil beds are in Canada, Greenland, and China.  Many large carnivorous crustaceans, similar to shrimp and crabs, have been identified.

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Ordivician Period (488 - 444 million years ago)

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Calymene tristani fossils from the Ordivician Period. Hectonichus

After the Cambrian Period came the Ordivician Period.  This second period of the Paleozoic Era lasted about 44 million years and saw more and more diversification of aquatic life.  Large predators similar to mollusks feasted on smaller animals on the bottom of the ocean.  During the Ordivician Period, many environmental changes happened.  Glaciers started to move onto the continents and, subsequently, the ocean levels decreased significantly.  The combination of the temperature change and loss of ocean water resulted in a mass extinction that marked the end of the period.  About 75% of all living species at the time went extinct.

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Silurian Period (444 - 416 million years ago)

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Spiny Conchicolites fossil from Silurian Period. Olev Vinn

After the mass extinction at the end of the Ordivician Period, diversity of life on Earth needed to work its way back up.  One major change in Earth’s layout was that the continents began to merge together.  This created even more uninterrupted space in the oceans for marine life to live and thrive as they evolved and diversified.  Animals were able to swim and feed closer to the surface than ever before in the history of life on Earth.  

   

Many different types of jawless fish and even the first finned fish with rays were prevalent.  While life on the land was still lacking beyond single celled bacteria, diversity was beginning to rebound.  Oxygen levels in the atmosphere were also nearly at our modern levels, so the stage was being set for more types of species and even land species to begin to appear.  Toward the end of the Silurian Period, some types of vascular land plants as well as the first animals, the arthropods, were seen on the continents.

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Devonian Period (416 - 359 million years ago)

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Trilobite fossil from the Devonian Period. Yannick Trottier

Diversification was rapid and widespread during the Devonian Period.  Land plants became more common and included ferns, mosses, and even seeded plants.  The roots of these early land plants helped to make weathered rock into soil and that created even more of an opportunity for plants to take root and grow on land.  Lots of insects began to be seen during the Devonian Period as well.  Towards the end, amphibians made their way onto land.  Since the continents were moving even closer together, the new land animals could easily spread out and find a niche.

   

Meanwhile, back in the oceans, jawless fish had adapted and evolved to have jaws and scales like the modern fish we are familiar with today.  Unfortunately, the Devonian Period ended when large meteorites hit the Earth. It is believe the impact from these meteorites caused a mass extinction that took out nearly 75% of the aquatic animal species that had evolved.

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Carboniferous Period (359 - 297 million years ago)

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Snail fossil from the Carboniferous Period. Wilson

Again, the Carboniferous Period was a time in which species diversity had to rebuild from a previous mass extinction.  Since the Devonian Period’s mass extinction was mostly confined to the oceans, land plants and animals continued to thrive and evolve at a fast pace.  Amphibians adapted even more and split off into the early ancestors of reptiles.  The continents were still coming together and the southernmost lands were covered by glaciers once again.  However, there were tropical climates as well where land plants grew large and lush and evolved into many unique species.  These plants in the swampy marshes are the ones that would decay into the coal we now use in our modern times for fuels and other purposes.

   

As for the life in the oceans, the rate of evolution seems to have been markedly slower than times before.  While the species that managed to survive the last mass extinction continued to grow and branch off into new, similar species, many of the kinds of animals that were lost to extinction never returned.

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Permian Period (297 - 251 million years ago)

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Crinoid fossil from Permian Period. Junpei Satoh

Finally, in the Permian Period, all of the continents on Earth came together completely to form the supercontinent known as Pangaea.  During the early parts of this period, life continued to evolve and new species came into existence.  Reptiles were fully formed and they even split off into a branch that would eventually give rise to mammals in the Mesozoic Era.  The fish from the saltwater oceans also adapted to be able to live in the freshwater pockets throughout the continent of Pangaea giving rise to freshwater aquatic animals.  Unfortunately, this time of species diversity came to an end, thanks in part to a plethora of volcanic explosions that depleted oxygen and affected the climate by blocking the sunlight and allowing large glaciers to take over.  This all lead to the largest mass extinction in the history of the Earth.  It is believed that 96% of all species were completely wiped out and the Paleozoic Era came to an end.