Extinction Line: Definition & Explanation

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

The earth's geologic history is complex; however, geologists attempt to use clues to piece together what has happened. This lesson will examine the five mass extinctions and will give clues, based on geology, as to why these extinctions occurred.

Extinctions and Extinction Lines

The world looks a lot different today than it did 200 million years ago. In fact, it is estimated that 99 to 99.9% of every species that has lived on the earth is now extinct, meaning the species has no remaining members. An extinction line, or extinction boundary, shows a change in either the type or number of fossils present, suggesting an extinction took place. As you'll see, an extinction line may provide clues as to why the extinction took place. Fossils are the remains or evidence of an organism (like tracks or feces) that have been preserved in rock.

In the Badlands of Alberta there is a change in the layers of earth that shows the Cretaceous Paleocene boundary

For example, in the Badlands of Canada there is a change in the rock layers. This Cretaceous-Paleocene extinction line, which dates to about 66 million years ago, shows the boundary between two different points in the earth's history (the Cretaceous and the Paleocene). Geologists also look at how substances like rock and sediment become layered. The assumption is that the oldest layers are on the bottom and the youngest layers on top. When geologists look at rocks, they can determine which time period these rocks came from.

A few final thoughts before we delve into the five major mass extinctions. For starters, mass extinctions occur over long periods of time, sometimes millions of years. It is also important to remember that the time frames and the number of species that went extinct can vary depending on the source of your data. Keep this in mind as you note these numbers. Okay, let's start with the first mass extinction and work our way to the most recent one.

Ordovician-Silurian Boundary

The first known extinction occurred approximately 443 million years ago and had two main extinctions that were separated by hundreds of thousands of years. During the Ordovician time frame, most species lived in the ocean and about 85% of marine species died during this extinction. There are competing ideas on why all of these critters when extinct (as is true for all of the mass extinctions), but evidence suggests glaciation caused sea levels to fall, thus changing the habitat and killing off the marine species that lived there. Glaciation is marked by decreased temperature and an increase in glaciers, which cover the landscape with ice.

And how do geologists know this? In the 1970s glacial deposits were found for the Ordovician time period in the Sahara Desert, which supports the glaciation theory. So those glaciation deposits may be the extinction line for this mass extinction. Okay, onto our next mass extinction!

Geologists found evidence of glaciation in the layers associated with the Ordovician time period

Late Devonian Boundary

Nearly 3/4ths of all life on earth went extinct during the late Devonian time period. In reality, many believe that this was actually many extinctions lasting millions of years. This occurred around 359 million years ago and geologists have struggled to find a single event that caused the death of all of these species. They have found iridium in the rocks from this time and iridium is often linked to meteoroids, which would suggest an impact. However, they have also found black shale, which is associated with low oxygen levels. Some believe that the earth's temperature increased, which caused low oxygen levels in deep water, resulting in the death of many species.

Black shale is associated with low oxygen levels

Permian-Triassic Boundary

Even though the extinction of the dinosaurs gets the most attention, the extinction that occurred during the Permian-Triassic Boundary is actually the most dramatic known extinction the earth has faced. Many believe that this mass extinction was actually two extinctions separated by millions of years. This occurred about 248 million years ago and is often referred to as the 'Great Dying' because 90% of marine species and 70% of land species went extinct.

Geologists have suggested volcanoes, meteors, and changing continents as culprits. One train of thought is based on buckyballs, or a molecule made up of carbon atoms, that were found in the extinction layers of this time. Buckyballs are able to trap gases inside of them and these specific ones had abnormal amounts of helium and argon trapped in them. These elements are more common in space; so one assumption is a meteoroid hit. Some suggest this may have been the final straw for some species that were already struggling due to high volcanic activity and moving continents that were changing coastlines.

Buckyballs were found in the layers associated with this mass extinction. These buckyballs trapped gases that suggest a meteor impacted the earth

Triassic-Jurassic Boundary

Just two more mass extinctions to go! The Triassic-Jurassic extinction occurred 200 million years ago and may have actually been several extinctions (which seems to happen a lot in the world of 'mass extinction'). Many species of marine reptiles, amphibians, and mollusks died out, with approximately 50% of the earth's species kicking the bucket. Many believe that this extinction opened up the environment for dinosaurs to later flourish.

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