Hirnantian

The Hirnantium is in Earth's history the top chronostratigraphic stage of Oberordovizium series and thus the Ordovician at all. It corresponds geochronological in about a period of about 445.2 to about 443.4 million years. The Hirnantium replaces the Katium and is in turn killed by the Rhuddanium, the lowest level of the Silurian.

Naming and history

The Hirnantium is named after the village of Cwm Hirnant near Bala in North Wales. The name was suggested by John Bevis Beeston Bancroft in 1933.

Definition and GSSP

The lower limit is defined by the first appearance of the graptolite - type Normalograptus extraordinarius. It also coincides with the base of a larger positive carbon -13 isotope and the beginning of a distinct deviation sea lowering, which is triggered by the beginning of a larger icing. The upper limit (and thus the border to Silurian ) is defined by the first appearance of the graptolite - type Akidograptus ascensus; little later the graptolites Type Parakidograptus acuminatus appears for the first time. As a Global Stratotype Section and Point ( GSSP ), the northern Wangjiawan profile in Wangjiawan, 42 km north of the city of Yichang (Hubei Province, China) is determined.