Hirnantian

The Hirnantian is the last stage of the Ordovician Period in the Paleozoic Era. It lasted about 2.1 million years, from 445.2 to 443.1 Ma (million years ago). During this time, the Earth's climate changed a lot.

Early in the Hirnantian, the world grew very cold because of big glaciation. This made sea levels drop a lot. Later, it got warmer, the glaciers melted, and the seas returned to their old levels.

These climate changes may have caused a big extinction event. Many sea animals disappeared. Only the End-Permian mass extinction was larger. After things calmed down, the remaining animals evolved into new shapes, much like the ones that had lived before.

Naming and history

The Hirnantian was named after Cwm Hirnant south of Bala, in northern Wales. Cwm Hirnant means the "valley of the long stream" in Welsh.

The stage was introduced in 1933 by B.B. Bancroft. In 2003, the International Commission on Stratigraphy officially added the Hirnantian to its international time scale. In 2006, the ICS chose the Wangjiawan section as the official Global Boundary Stratotype Section and Point for the Hirnantian Stage.

GSSP

The reference point for the start of the Hirnantian is located at the Wangjiawan section near the village of Wangjiawan, about 42 km north of Yichang in Hubei, China. This spot marks the beginning of the Hirnantian. It is found in rock layers made of shale and chert. The exact beginning is identified by the first appearance of a specific type of graptolite called Normalograptus extraordinarius.

Major events

The Hirnantian was a time of big climate changes that led to a major mass extinction event. It began with a hot climate and high sea levels. Then the Earth cooled, glaciers grew, and sea levels fell. This drop in water level caused many species that needed shallow water to die out.

Later, the climate warmed again, glaciers melted, and sea levels rose. This change caused more extinctions among the species that had survived the first drop in sea level.

Dating

Scientists use special ways to find out how old rocks and fossils are. For the Hirnantian time, they studied rocks from the Dob's Linn area in the United Kingdom. They looked at tiny crystals called zircon in ash to learn two key dates.

The older date, from Hartfell shale rocks, shows the Hirnantian began about 445.7 million years ago. The younger date, from Birkhill shale rocks, shows it ended about 438.7 million years ago. These dates help scientists know when things happened in this time.

Subdivisions

The Hirnantian Stage has two main parts. Each part is marked by a special type of graptolite. Graptolites are tiny fossils found in rocks.

The first part starts when the graptolite Normalograptus extraordinarius first appears. The second part begins when Normalograptus persulptus appears. These markers help scientists date events from this time.

These graptolite zones help us learn more about what happened during the Hirnantian Stage and the end of the Ordovician Period.

Paleogeography

During the Hirnantian, most of Earth’s land was part of a huge supercontinent named Gondwana. This land covered the south pole and included places like South America, Africa, most of Australia, India, and Antarctica. To the north of Gondwana was a vast ocean called the Panthalassic ocean.

Other land areas included parts of what are now the southeastern United States, such as Florida, Mississippi, Alabama, and South Carolina, which were near the South pole. The rest of North America, known as Laurentia, sat to the north with warmer climates. Farther north was Baltica, made up of parts of modern Norway, Sweden, Finland, Denmark, Germany, and Russia. An island group called Avalonia stretched west and included parts of Britain, Ireland, and eastern areas of Newfoundland, Nova Scotia, New Brunswick, and New England.

Correlations to regional stages

The Hirnantian is a stage of Earth's history known around the world. It started as a regional stage in the United Kingdom. Different places split geological time in their own ways. Here are some matches between regional stages and the Hirnantian:

• Australasia – The Hirnantian matches the top 20% of the Bolindian epoch.
• Baltica – The Hirnantian equals the upper 50% of the Porkuni stage.
• China – The Hirnantian is about the upper 25% of the Wufeng stage.
• North America – The Hirnantian matches all of the Gamachian stage.
• United Kingdom – The Hirnantian is nearly the same as the regional Hirnantian stage, starting a little earlier and making up about the upper 20% of the Ashgill epoch.