Hirnantian

System (period)	Series	Stage (age)	Lower boundary, Ma
Silurian	Llandovery	Rhuddanian	443,8±1,5
Ordovician	Upper	Hirnantian	445.2±1.4
		Katian	453.0±0.7
		Sandbian	458.4±0.9
	Middle	Darriwilian	467.3±1.1
		Dapingian	470.0±1.4
	Lower	Floian	477.7±1.4
		Tremadocian	485.4±1.9
Cambrian	Furongian	Stage 10	older
Subdivisions and "golden spikes" according to IUGS as of September 2023

The Hirnantian is the last stage of the Ordovician Era. It didn’t last long, only from about 445.2 to 443.1 million years ago. In its early stages it would have cold temperatures, severe glaciation and a high drop in sea level. In its later stages the temperatures rose, the glaciers melted and sea levels dropped.

Some scientists believe this change in the environment triggered the Ordovician Extinction Event. It represents one of the largest mass extinction events on earth; about 85% of all species died. Most of them were sea creatures, but unlike other major extinction events this one was quite small.

After the extinction event the climate changed to it’s original state and the survivors of the extinction would evolve into new creatures sort of like the ones that lived before them.

Naming and history

It was named in 1933 by B.B. Bancroft. It was named for the Cwm Hirnant south of Bala, in northern Wales. Bancroft found that there was Hirnant Limestone and other sedimentary rock formations. Said formations included things like trilobites, brachiopods and other “Shelly” animals.

It was originally broken up into two segments. However, after some time, it was found there was no point to divide them. So it was called the Hirnantian and the name was approved in the early 2000.

GGSP

There are two graptolite biozones in the Hirnantian and these are of approximately equal length. The start of it, the Wangjiawan, is marked by the appearance of the graptolite Normalograptus extraordinarius. It is also marked by the graptolite Normalograptus persulptus, which marks the end of the Ordovician period. There is also the Hirnantia Fauna 39 cm above the boundary.

Major Events

There was a large extinction event at the end of the age. Theories as to why it happened range but most scientists believe on huge climate change and sea level rise. Which could have caused all the extinctions of many of the oceans creatures. But when the climate changed and the levels changed it was a major factor on the creatures that survived the first extinction.

Dating

The Hirnantitan was about 445 to 438 million years ago.