Cambrian–Ordovician extinction event

The Cambrian–Ordovician extinction event happened about 485 million years ago during the Paleozoic era in the early Phanerozoic eon. It marked the end of the Cambrian period and the start of the Ordovician period.

Many kinds of small sea animals did not survive this event. It greatly reduced the numbers of brachiopods, conodonts, and trilobite species. These changes helped shape the world’s oceans in the time that followed.

Causes

Scientists think that big volcanic eruptions might have changed the environment and caused this extinction event. These eruptions happened around the same time, but we do not know for sure if they caused the changes in plant and animal life.

Other ideas include changes in the amount of oxygen in the water and shifts in sea level. During this time, many plants and animals were disappearing, possibly because the water did not have enough oxygen. Later, conditions changed again, which helped new kinds of life to grow and spread.

Effects on life

Trilobites were strongly affected by this event. Those in shallow water had a harder time surviving than those in deeper water. Many trilobites had special ways to live in water with low oxygen, working with tiny organisms that could use sulfur.

Controversy

Scientists found fossils in Morocco that look like animals from the Cambrian period. These fossils are 20 million years newer than when many animals disappeared. In 2010, some researchers thought these findings showed that Cambrian animals might have lived longer than we believed. They think the idea of many animals disappearing from the Cambrian to the Ordovician might be because we haven’t found the right rocks to look in yet. These special fossils were only kept in places where conditions were just right for preserving soft bodies.