OAC Biology - Geologic Timeline

Mr. Galajda

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 Earth’s 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

For Geologic Timeline– click here

The Geologic Timescale begins with the formation of the Earth around 4.6 billion years ago.

The Geologic Timescale is divided into FOUR ERA’S. 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 extinction’s 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?

Trace fossils (footprints)

Casts (minerals in rocks fill a space and make a cast of the dead organism)

Petrified (minerals penetrate and replace parts of an organism, producing copies of them)

Imprints (leaf falling into sediment and leaving an imprint when the sediment hardens into rock)

Amber-Preserved (organism trapped in ice or tree sap)

Molds (organisms falls into sediment, then decays, leaving an empty space)


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.

Relative Dating: Like stacking newspapers at home. The older newspapers tend to be at the bottom of the pile (look at the date). Well, rock layers behave in much the same way and if undisturbed, you can approximate the age of the fossil by determining the age of the rock. Older fossils tend to be at the bottom of the rock layers. This is a principal "Geologic Law".

Absolute Dating - Radiometric Dating: Using radioisotopes for the specific ages of fossils. Remember that radioisotopes are atoms with unstable nuclei that break down or decay over time. A radioactive isotope forms a new element after it decays. Because every radioactive isotope has a characteristic decay rate, scientists use the rate of decay like a clock. The decay rate is known as the isotope’s half-life.


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).

View animation – click here!


Precambrian Era: Jan 1 – Oct. 15

accounts for 87% of the Earth’s history

oldest fossils are found in Precambrian rocks, 3.5 billion years ago (changed) due to fossils found in Western Australia

fossils resemble a modern species of photosynthetic bacteria known as Cyanobacteria

Stromatolites (dome shaped structures) were also found and still form today in Australia from mats of Cyanobacteria

Thus the stromatolite fossils are evidence of the existence of photosynthetic bacteria on earth 3.5 billion years ago

Prokaryotes appeared around 3 billion years ago

Around 1.8 billion years ago the fossil record shoes more complex eukaryotic organisms

By the end, multicellular organisms flourished (sponges, jellyfish)


Paleozoic Era: Oct. 15 – Dec. 10(Age of Fishes)

includes the CAMBRIAN PERIOD – which is known as the "Cambrian Explosion of Life" because the fossil record shows an enormous increase in the diversity of life forms at this time

Oceans teamed with many different life forms

Trilobites were dominant and other marine invertebrates during the first part of the Paleozoic Era

Fishes, land plants, amphibians, primitive trees, reptiles, insects made appearances during various periods

The largest MASS EXTINCTION occurred at the end of the PALEOZOIC era. About 90% marine species and 70% of land species disappeared at this time

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 Earth’s organisms and geology occurred during this Era

This era is best known as the time "Dinosaurs ruled the Earth", but remember, mammals also made an appearance during this time period

Mammals were, according to the fossil record, small and mouse-like

The Jurassic Period of time is known as the "Age of Dinosaurs"

Recent fossil evidence indicates that modern birds evolved from some of these dinosaurs (LAB – WEBQUEST)

The mass extinction of the dinosaurs marked the end of the Cretaceous Period

It has been estimated that 66% of all life on Earth was lost (meteorite collision with the Earth – geological evidence – crater off the coast of Mexico – leading to a "long, dark period of time"

VIEW VIDEO "Mass Extinction of the Dinosaurs"

It is also during this era that PANGAEA broke apart

Cenozoic Era: Dec. 26 – Dec. 31(Age of Mammals)

this is the era in which we now live

mammals began to flourish

primates appeared 30 million years ago

modern man appeared perhaps as recently as 200,000 years ago (or around the late evening on Dec. 31)

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 extinction’s 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 radiation’s.

E-mail Mr. Galajda at:  david.galajda@sac.on.ca