Early Paleozoic, Cambrian- Ordovician, The Age of Invertebrates Written By: Garrett Wiska and Jordan Yono Earth Science, Period 6 Fr. O’Neill C.S.B. 2-10-12

Introduction Life began on earth as the earth began to cool. In the past 600 million years, the earliest forms of life changed from one-celled organisms to the multi-celled, multi-systemed organisms of which humans are obvious examples.

Introduction Cont. In the same time the earth changed from a single continent or compact group of continents, to the current arrangement of land and ocean. Two time periods in which these changes took place were the Cambrian and Ordovician periods,which ranged from 540 million years ago to 443 million years ago.

The Cambrian time period was considered a booming evolution for marine life. Many living organisms in the shallow waters were invertebrates, or animals without a skeletal backbone. One of the species that thrived during this time period was the Trilobites. The Trilobites are considered to be a part of the arthropod family due to their very rough layer of outer skin; they were also considered one of the first animal species to develop the gift of eyesight. During this time period there were over 100 types of Trilobites, they were also considered to be the most prevalent species of life during this era. Other popular invertebrates during this time period were sponges, mollusks, worms, and corals.

Body The Cambrian period was the first geological time period of the Paleozoic era beginning around 540 million years ago. During this time period the earth’s geography was much different than it is presently. The continents were in the process of breaking off of one another forming four different, major land masses: Laurentia, Siberia, Gondwana, and Baltica. Most of which were located in the earth’s southern hemisphere. The climate during this time period began quite cold, and then eventually became rather warm; this sudden rise in the earth’s temperature caused most of the earth’s glaciers to melt. This widespread melting caused large parts of the four land masses to flood creating warm shallow seas. These warm seas were considered to be ideal for early marine species, and in result marine life began to evolve, and thrive.

The Cambrian time period was followed by the second geological time period of the Paleozoic era, The Ordovician time period. The earth’s continents continued to drift apart, breaking apart from one another and forming their own individual continents. We also see the first evidence of tectonic plate movement resulting in the formation of mountains and volcanoes. The earth’s temperatures continued to rise, and glacier melting continued at an even more alarming rate. This melting resulted in more continental flooding causing the Ordovician period to have the highest sea levels out of any other time period in the Paleozoic era. As the sea levels continued to rise the Marine and aquatic life continued to evolve and thrive.

Invertebrates continued to develop and rein dominant, specifically arthropods and Mollusks. The Ordovician period is also home to many new species that we are introduced to for the first time such as snails, clams, and brachiopods. During this time we also see the development of sponges, and the first coral reefs. We are also introduced to many different types of fish, bottom dwellers. These fish had no jaws or teeth, they lived and the bottoms of oceans and lakes and would feed off of bacteria and plankton.

The Ordovician period unfortunately ended as a result of mass extinction. Scientists have labeled the Ordovician extinction as the second largest extinction in the history of the world in terms of loss of life and species. Scientists estimate that about sixty percent of the marine population was wiped out as a result of this extinction. There are two main hypotheses on how this extinction occurred.

First is the belief that due to extreme greenhouse gas conditions there was a major decrease in atmospheric CO2. This caused a major reduction in the amount of oxygen in water sources such as oceans, lakes, and rivers which meant that the marine organisms living in such places could not receive the oxygen necessary to sustain life in that type of environment. The second widespread theory given by some scientists is the gamma ray burst hypothesis. This suggests that the extinctions could have been caused by a gamma ray burst from a hyper nova located a short distance from the planet earth. Scientists say that a ten second burst would have stripped away half of the o-zone which would expose all of the earth’s organisms to un-livable levels of ultraviolet radiation.

Conclusion Life began on earth as the earth began to cool. In the past 600 million years, the earliest forms of life changed from one- celled organisms to the multi-celled, multi-systemed organisms of which humans are obvious examples. In the same time the earth changed from a single continent or compact group of continents, to the current arrangement of land and ocean. Two time periods in which these changes took place were the Cambrian and Ordovician periods, which ranged from 540 million years ago to 443 million years ago. These two time periods are responsible for one of the biggest forms of evolution in the planet, and its species.

Sources http://www.fossilmuseum.net/Paleobiology/Paleozoic_paleobiology.htm http://www.ucmp.berkeley.edu/cambrian/cambrian.php http://www.ucmp.berkeley.edu/ordovician/ordovician.php http://park.org/Canada/Museum/extinction/ordmass.html http://earth.usc.edu/~stott/Catalina/Ordovician.html http://kusmos.phsx.ku.edu/~melott/87551.pdf