国際標準模式層断面及び地点
The International Commission on Stratigraphy (ICS) has assigned the First Appearance Datum of the "Psiloceras spela"-group as the defining biological marker for the start of the Hettangian, 201.3 ± 0.2 million years ago, the earliest stage of the Jurassic, with the Global Boundary Stratotype Section and Point (GSSP) located at Kuhjoch pass, Karwendel Mountains, Northern Calcareous Alps, Austria, which was ratified in 2010. The base of the Jurassic in Britain was historically defined by the first appearance of the ammonite "Psiloceras planorbis."