Ordovician-Silurian Extinction: Ice Age & Mass Extinction

The first major global extinction on the planet

Dinosaurs and an asteroid the greatest natural disaster in the history of planet Earth? Well, not at all. They are one of the five natural disasters that have occurred on the planet, but they are not the biggest. The biggest one was the Permian-Triassic extinction, when most of the animals on the planet died, only a handful of them survived. And its cause was volcanic activity. But at the beginning of all these mass extinctions is the Ordovician-Silurian extinction, when about 85% of all life died.

This natural disaster took place about 443 million years ago. At that time, the supercontinent Gondwana “ruled” the world. Animals inhabited mainly the sea, where there was a really diverse fauna. Trilobites, brachiopods, corals, fish and other marine predators lived here. Only fungi, lichens and algae were present on land. No animals walked here. And it was the underwater life that was to be interrupted by a huge disaster that wiped out approx. 85% of all animal species.

The end of the Ordovician, the beginning of the end of life for the first time

The mass extinction occurred gradually at the end of the Ordovician period and continued into the Silurian. First, over the course of 1 million years, there was a huge cooling of the planet, everything was glaciated, the ocean levels dropped, the shoals disappeared. The supercontinent of Gondwana was moving towards the South Pole, which meant that it was gradually cooling, the glaciers were getting bigger. Carbon dioxide decreased in the atmosphere due to the active activity of sea ranges and the weathering of rocks. The oceans cooled, the sea currents stopped moving. The acidity also changed, so animals that were dependent on a certain water temperature and depth gradually died out due to the drop in water level, change in temperature and environment. Mainly trilobites, reef-building organisms, brachiopods, bryozoans and invertebrates on the coasts died. In summary, it could be described as first phase – the onset of the ice age.

It took an incredibly long time for this time to end. However, even its end did not mean relief for the planet, or its then inhabitants. While in the first phase the extinction was caused by glacial cooling and the loss of the environment, in the second phase it was caused by anoxia and melting.

Even ice does not last forever, so after a long time the ice began to melt. And it happened the second phase. Glacial melting and gradual sinking into water caused the ocean to receive an involuntary contribution of cold fresh water. This caused the density of the sea water to be disturbed and the sea currents to stop moving. For previously living underwater animals, this meant that oxygen did not reach them. The lack of oxygen was even more fundamental to underwater life than anything else, so various life forms slowly died out. Plankton, oxygen-dependent invertebrates, select predators, and the few animals that survived the cold die-off died out. You could say that this was a time of warmth in a certain sense of the word. The period of heat, which, however, was killing with a significant contribution of anoxia.

Although the planet was experiencing climatic disasters, the animals did not go without a fight and tried to survive these changes. Many animals that were able to make a longer journey moved to an area where the water was warmer. But not everyone was able to do that.

Others tried to adapt to the current conditions, which was also not the easiest. But at least someone did it. As for the lack of oxygen, some animals were also able to adapt to it so that it did not endanger them so much and they could survive. Predators did not have this skill. Rapid reproduction of organisms was also an advantage for survival. Those with a shorter life cycle adapted more quickly.

The end of an old life, the beginning of a new life

They say that every ending is actually the beginning of something new. It was no different in this case as well. After the planet experienced a giant cooling and subsequent “oxygen-free” warming, many animal species lost their lives, but after the ecosystem recovered, new life began. Both brachiopods and echinoderms experienced a new phase of evolution.

Everything that was destroyed, including the shoals, was gradually restored in such a way that the conditions for the development of new life were created. New predators appeared and reef systems also experienced further growth.