The Ordovician Mass Extinction

A trilobite fossil from the Paleozoic Era. Getty/Jose A. Bernat Bacete

 The Earth is estimated to have formed about 4.6 billion years ago. When it was first formed, there was much turmoil and chaos. There were frequent meteor strikes, a hostile atmosphere, and large amounts of volcanic activity. Needless to say, conditions were not very friendly for life to form.

In fact, it took about a billion years for the first individual prokaryotic unicellular organisms to appear on Earth.

While it is not known exactly how life first began on Earth, it did evolve and start to create larger and more complex life forms. However, there were several periods throughout the history of Earth when mass extinctions occurred. There are five recognized major mass extinction events that wiped out large percentages of all species that could be found on the Earth during that time period.

The first of these major mass extinction events happened at the end of the Ordovician Period of the Paleozoic Era. This first major mass extinction happened about 440 million years ago and ended up eliminating about 85% of all of the species living on Earth at that time. It is thought that no major groups of life were completely lost, but individual species of those groups were completely wiped out.

Trilobites and Graptolites, which are now used as index fossils for relative dating, took an especially hard hit.

It is estimated about 90% of the individuals of these species died off. The largest of the predators in the oceans also took a big hit. The nautiloids, which are related to the present day squid, were mostly wiped out during this mass extinction.

So what caused all this death and destruction of life on Earth during the Ordovician Mass Extinction event?

This particular mass extinction event was caused, ultimately, by continental drift. The supercontinent Gondwanaland had drifted over the South Pole, plunging the Earth into a quick and intense ice age. This would be particularly devastating for all of the marine species. Not only did the ocean temperatures drop drastically, but large ice sheets also formed which made sea levels go down by large amounts.

Any sort of species that called the shallow waters their homes had a particularly tough time. Since the ocean levels fell by up to 100 meters, they were left uncovered as the water line did not come up high enough for them to survive. Corals were especially hard hit because of the declining water levels in the oceans.

The drastic temperature drop was also devastating to many species. Many types of life had adapted to warmer waters and would migrate when temperatures got cooler in the winter months. However, since this was an ice age, there was no warmer climates for the individuals to migrate to. These species either had to adapt or die. Unfortunately, the vast majority of them were unable to adapt quickly enough and instead went extinct.

Even if a species managed to survive this ice age, they were not in the clear.

Scientists believe the Ordovician Mass Extinction event actually happened in two waves. The first was the ice age, but the second wave of devastation happened when that ice age ended. After about a million years of the cold, the end of the ice age happened suddenly. While this made the sea levels rise once again and shallow waters returned, the ocean water was very low in oxygen concentration. Even deep waters became stagnant and life that had survived the ice age was now in jeopardy. By the end of the Ordovician Period, about 85% of all species had gone extinct.

Without as much biomass in the oceans due to the mass extinction, plants were able to create enough oxygen for life to go through somewhat of a recovery during the early Devonian Period. Many new types of life evolved and began to thrive.