Eocene–Oligocene extinction event

The Eocene–Oligocene extinction event, also called the Eocene-Oligocene transition (EOT), happened between 33.9 and 33.4 million years ago. It marks the time when the Eocene ended and the Oligocene began. During this time, many plants and animals disappeared, and new ones appeared.

This event caused a big change in the kinds of animals and plants living on Earth, but it was not as severe as some of the largest extinctions in history. In Europe, this period is known as the Grande Coupure, which means "great cut" in French. Many animal groups that lived in Europe during the Eocene went extinct, and new groups came from Asia to live there.

Causes

Glaciation

The change from the Eocene to the Oligocene happened when Antarctica became icy, starting the Late Cenozoic Ice Age. This big change in climate is thought to be the main reason for many animals and plants disappearing at that time. Before this, Antarctica might have had some ice, but now it stayed icy all the time, ending the warm weather of the early Palaeogene. In North America, temperatures dropped a lot over many years. Near Tibet, it even got close to freezing.

One idea is that there was less carbon dioxide in the air, which made the Earth cooler. This cooling happened slowly and then got worse about 34 million years ago, leading to big ice sheets forming in East Antarctica. The drop in carbon dioxide may have been caused by the Indian Subcontinent moving into warmer areas, which changed the weather. Another reason could be the opening of the Drake Passage, creating a current that kept Antarctica colder.

Extraterrestrial impact

Some scientists think that large meteorite impacts, like the ones that created the Chesapeake Bay crater and the Popigai impact structure in Siberia, might have played a role. But not all studies agree that these impacts were connected to the extinctions.

Solar activity

Studies of sunspots from the Bohai Bay Basin do not show any big changes in the Sun’s activity during this time.

Extinction patterns

In central North America, many reptiles, amphibians, and snails changed a lot because the temperature dropped quickly. Some small mammal groups, like multituberculates, went completely extinct. Lemurs also lost many members during this time. Australia saw changes in its animal life, especially in the east.

South American mammals were less affected. Their changes happened more slowly and were linked to things like cooling temperatures, the rising of the Andes mountains, and the spread of grasslands. In East Asia, animal changes happened but not all at once and not only because of global climate shifts.

The Grande Coupure, meaning "great break" in French, happened about 33.5 million years ago in Europe. It marked a big change in mammal life, with many local species disappearing and new ones arriving from Asia. Before this break, Europe had many special mammals, but after, it had more types that came from Asia. This change was likely caused by climate shifts linked to ice forming at the poles and sea levels dropping, or perhaps competition with new species moving in from Asia. Another possible reason could be impacts from space rocks in North America.

Later, around 31 million years ago, another group of Asian animals spread into Europe through a different route. In the Caribbean, a big change happened in the plants that grow in salty water, shifting from one main type to another that is still common today.

In the oceans, some sea creatures changed a lot. Some types of small sea animals that drill into others became more common. Some big single-celled creatures called orthophragminid foraminifera went extinct, and other sea animals like bryozoans became more widespread. Diatoms, a type of tiny ocean plant, may have become more important, although this idea is debated. Some fish groups also grew quickly in variety during this time.

Some places show that the changes happened over millions of years rather than all at once. For example, near Eugene, Oregon, plants began to disappear 33.4 million years ago, and sea creatures changed about 33.2 million years ago, both later than the main extinction event.