# How are stratigraphic limits defined before the Phanerozoic?

International Commission on Stratigraphy defines the limits of geological ages and they create this chart:

Click on image for full view.

Acording to Wikipedia:

The standard stratigraphic nomenclature is a chronostratigraphic system based on palaeontological intervals of time defined by recognised fossil assemblages.

Source: chronostratigraphy, Wikipedia.

However, until Cambrian, there are not fossils (except chemical traces of unicells).

How are the limits between units defined at Precambrian?

• I changed a bit the question. It was maybe too broad before the edit.
– user18590
Feb 21 '20 at 7:17
• looks great, thanks!
– uhoh
Feb 21 '20 at 7:44
• For Precambrian most of periods are defined using absolute dating. Periods themselves are defined more for convenience to easily refer to a relatively narrow time period. Feb 21 '20 at 9:00