Hirnantian

The Hirnantian was the last part of the Ordovician Period during 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 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.

Many scientists think these climate changes caused a big loss of sea animals, known as the extinction event. This was one of the biggest losses in geologic history. 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. He talked about special rock layers called the Hirnant Limestone and similar formations. In 2003, it was added to the international time scale by the International Commission on Stratigraphy. This group helps scientists decide how to divide Earth's history into parts.

GSSP

The Hirnantian began at a spot near the village Wangjiawan, north of Yichang in Hubei, China. This place has layers of rock called the Wufeng and Lungmachi Formation. These rocks are mostly shale and chert. Scientists look at a special fossil, a graptolite named Normalograptus extraordinarius, to know when this time period started.

Major events

The Hirnantian was a time when the Earth's climate changed a lot. It began with very hot weather and high sea levels. Then it became very cold, and big glaciers formed. This cold weather made the sea level drop by about 80 meters around the world. Many sea creatures could not 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 disappeared. Only a few types of sea animals survived and adapted to the new conditions.

Dating

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

One date is from older rock layers and shows the Hirnantian began around 445.7 million years ago. The other date is from younger rock layers and shows it ended around 438.7 million years ago. With these dates, scientists can guess when events occurred during this time.

Subdivisions

The Hirnantian had two main parts. The first part starts when a special fossil, Normalograptus extraordinarius, first appears. The second part starts when another fossil, Normalograptus persulptus, first appears. These fossils help scientists understand when events happened during this time.

These two parts also show when the Ordovician Period ended.

Paleogeography

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

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

Correlations to regional stages

The Hirnantian is a time period that scientists use around the world. It started as a name used in the United Kingdom, but now it is used everywhere. Different places divide up time in their own ways. Here is how the Hirnantian matches 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.