Hirnantian

The Hirnantian was the last part of the Ordovician Period during the Paleozoic Era. It lasted only about 2.1 million years, from 445.2 to 443.1 Ma (million years ago). During this time, the Earth went through big changes in temperature and ice cover.

Early in the Hirnantian, the world became very cold. Huge glaciers grew, and the level of the oceans fell. Later, the climate warmed up again, the glaciers melted, and sea levels rose back to where they had been before.

Many scientists think these climate changes caused a big loss of sea animals, known as the extinction event. This loss was the second biggest in geologic history, with about 85% of sea species disappearing. Only the End-Permian mass extinction was larger. After the climate settled down, the surviving species evolved into new forms, and life in the oceans began to recover.

Naming and history

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

This time period was first described in 1933 by B.B. Bancroft, who included special rock layers called the Hirnant Limestone and similar formations. Later, in 2003, it was officially added to the international time scale by the International Commission on Stratigraphy, which helps scientists decide how to divide Earth's history into parts.

GSSP

The official start of the Hirnantian is found at the Wangjiawan section near the village Wangjiawan, north of Yichang in Hubei, China. This spot shows layers of rock called the Wufeng and Lungmachi Formation, made mostly of shale and chert. Scientists use a special type of fossil called a graptolite named Normalograptus extraordinarius to mark the beginning of this time period.

Major events

The Hirnantian was a time when the Earth's climate changed a lot. It started with very hot weather and high sea levels, but then it got very cold, and big glaciers formed. This cold weather made the sea level drop by about 80 meters around the world. Many sea creatures couldn't survive in the colder, shallower water, so a lot of them died out.

Later, the Earth got warm again, the glaciers melted, and the sea levels rose back up. This caused more changes and some more sea creatures to disappear. Through all these changes, only a few types of sea animals managed to survive and adapt to the new conditions.

Dating

Scientists have two important dates that help us understand when the Hirnantian happened. These dates come from the Dob's Linn area in the United Kingdom.

One date is from old rock layers and shows the Hirnantian started around 445.7 million years ago. The other date is from younger rock layers and shows it ended around 438.7 million years ago. Using these dates, scientists can estimate when events happened during this time.

Subdivisions

The Hirnantian had two main parts, each about the same length. The beginning of the Hirnantian is marked by the first appearance of a special fossil called Normalograptus extraordinarius. This marks the start of the first part. The second part begins when another fossil, Normalograptus persulptus, first appears and lasts until the end of the Hirnantian. These fossils help scientists figure out the exact timing of events during this time.

These two parts also show when the Ordovician Period ended.

Paleogeography

During the Hirnantian, most of Earth’s land was gathered into a huge landmass called Gondwana, which sat over the South Pole. This included South America, Africa, most of Australia, India, and Antarctica. Baltica, made up of parts of modern Norway, Sweden, Finland, Denmark, Germany, and Russia, sat to the east across a narrow sea.

To the west of Gondwana lay Laurentia, which included much of today’s North America. Farther west, an island group called Avalonia stretched out, containing parts of Britain, Ireland, and eastern areas of Newfoundland, Nova Scotia, New Brunswick, and New England.

Correlations to regional stages

The Hirnantian is a well-known time period that scientists use all around the world. Long ago, it began as a local time period name in the United Kingdom, but now it is used internationally. Different places around the world have their own ways of dividing up time, and here is how the Hirnantian matches up with some of these local time periods.

• Australasia – The Hirnantian matches the top part of the Bolindian epoch.
• Baltica – The Hirnantian matches the later half of the Porkuni stage.
• China – The Hirnantian matches the later part of the Wufeng stage.
• North America – The Hirnantian matches all of the Gamachian stage.
• United Kingdom – The Hirnantian almost matches the regional Hirnantian stage, which starts a tiny bit earlier.