The base is defined as the first appearance of the trilobite Glyptagnostus reticulatus around  million years ago. The top, or the base of the Jiangshanian is defined as the first appearance of the trilobite Agnostotes orientalis around  million years ago.
The base and top of the trilobite species Glyptagnostus reticulatus and Agnostotes orientalis are defined as their first appearances in geological time, with a specific timeframe.