OAC Biology - Geologic Timeline
History in Rocks:Rocks are important sources of information about the diversity of life that has existed on the planet Earth. By examining layers of sedimentary rock and dating the fossils that can be found between the layers, scientists have been able to put together a chronology of the Earths history called the Geologic Timescale.
A key Geologist that emphasized this was a man named"CHARLES LYELL" during the 1800s. This was the idea that natural geologic processes were uniform in frequency and magnitude throughout time, an idea known as the "principle of uniformitarianism".
Demo The Geologic Timescale
The Geologic Timescale begins with the formation of the Earth around 4.6 billion years ago.
The Geologic Timescale is divided intoFOUR ERAS. The Precambrian, the Paleozoic, the Mesozoic and the Cenozoic.
Each ERA can be further divided into PERIODS. The divisions in the timescale are distinguished by organisms that lived during that particular time period. The fossil record indicates that there were several (five) mass extinctions at various times during the Geologic Timescale. In fact, it has been estimated that about 99% of all species that ever existed are now extinct.
What is a fossil and in what kinds of fossils can exist?
How Can Fossils Be Dated?
The fossils in different layers of sedimentary rock vary in age. Scientists use a variety of methods to determine the age of fossils.
Potassium-40: a radioactive isotope that decays to argon-40 to date rocks containing very old fossils. It has a half-life of 1.3 billion years.
Carbon-14: to date fossils less than 50,000 years old. It has a half-life of 5730 years.
Uranium-238: has a half-life of 4.5 billion years and is reliable for dating rocks and fossils within the rocks millions of years old
You can also date amino acids (measure the ratio or L-form (living) to D-form amino acids (non-living))
What is PANGAEA?
Near the end of the PALEZOIC ERA, plate movements of the Earth brought all of the landmasses together into a "super-continent" called PANGAEA.
About 245 million years ago, the continents of the Earth were joined together is a landmass known as PANGAEA. The geological explanation for how the continents move is called PLATE TECTONICS. They still move today at a rate of about 6 cm per year (the same rate as your hair grows). During the Mesozoic Era, the continents moved apart thus affecting the places where species lived and their distribution on Earth. Near the end of the Mesozoic, 66 million years ago, the continents had taken on their modern day shapes. This explains why matching fossils have been recovered from widely separated areas (Ghana and Brazil have similar reptile fossils).
Precambrian Era: Jan 1 Oct. 15
Paleozoic Era: Oct. 15 Dec. 10(Age of Fishes)
View the continents of the EARTH at various times during the "Paleozoic Era" below:
Mesozoic Era: Dec. 10 Dec. 26(Age of Dinosaurs)
- many changes in the Earths organisms and geology occurred during this Era
Cenozoic Era: Dec. 26 Dec. 31(Age of Mammals)
What is a mass extinction? A mass extinction is one in which it is a catastrophic, widespread (often-global) event in which large groups of existing species are wiped out. Mass extinctions are usually followed by periods of recovery called "ADAPTIVE RADIATIONS" in which numerous new species have evolved over several millions of years to fill new or vacated ecological niches. For example, when dinosaurs disappeared, there was an evolutionary explosion for mammals. Fossil records indicate that it takes around 10 million years after a mass extinction to rebuild biological diversity via adaptive radiations.
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