End–Ordovician extinction event facts for kids

The End-Ordovician extinction event was a huge event in Earth's history. It was the third-largest extinction event of the Phanerozoic eon, which is a very long time period. This event happened at the end of the Ordovician period. Before the Ordovician was the Cambrian period, and after it came the Silurian period. During the Ordovician, there were no living things on land except for tiny bacteria and maybe some simple algae. Almost all life was found in the oceans.

What Happened During the Extinction?

The End-Ordovician extinction happened in two main steps. Both steps occurred during the Hirnantian stage. This was the very last part of the Ordovician period.

Step 1: The Big Freeze

The first step started when the Earth's climate changed a lot. It went from being very warm to very cold. Huge sheets of ice, called glaciers, began to cover large parts of the planet.

• Ocean Changes: The cold weather caused the ocean water to mix differently. Water that had very little or no oxygen (called anoxic water) rose from the deep parts of the ocean.
• Loss of Life: This anoxic water killed many tiny ocean creatures called plankton. Also, as more water turned into ice, sea levels dropped. This made the oceans shrink, destroying the habitats (homes) of many sea animals.

Step 2: The Big Melt

The second step happened when the climate warmed up again. The glaciers began to melt, and sea levels rose once more.

• More Oxygen Problems: As the oceans warmed, the anoxic conditions spread. This time, the oxygen-poor water reached the shallower areas where many animals lived.
• Second Wave of Extinction: This second change in ocean conditions caused another wave of deaths among sea creatures. They simply could not survive the new environment.

Why Did This Happen?

The main reason for the End-Ordovician extinction was a huge change in Earth's climate. It went from being very warm, then very cold, and then back to very warm. These climate changes caused big shifts in how ocean water moved around. Animals living on the ocean bottom (called benthic animals) and those swimming in the open water (called pelagic animals) faced conditions they could not handle.

More than 100 groups of animals without backbones (called invertebrate families) died out. Almost half of all the different types of animals (genera) disappeared. Animals like brachiopods (shellfish), bryozoans (tiny colonial animals), trilobites (ancient arthropods), conodonts (eel-like creatures), and graptolites (tiny colonial animals) were greatly reduced in number.

Scientists think the cause was probably linked to the rise and wearing away (erosion) of the Appalachian Mountains. When these mountains first formed, they might have released a lot of carbon dioxide (CO₂) gas into the atmosphere. Later, as the mountains eroded, they might have taken a lot of CO₂ out of the atmosphere. These changes in CO₂ levels could have led to the big climate shifts.

Related pages

• List of extinction events

Images for kids

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  An illustration showing Cameroceras shells sticking out of the mud as sea levels dropped during the Ordovician-Silurian Extinction event.

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  Cyanobacteria blooms likely happened after the cold period. These blooms probably caused the widespread lack of oxygen in the oceans, which was a big reason for the second extinction wave.

See also

In Spanish: Extinciones masivas del Ordovícico-Silúrico para niños