The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species Undulograptus austrodentatus around 469.4 million years ago.
[8] The name Darriwilian is derived from Darriwil, a parish in County of Grant, Victoria (Australia).
The lower boundary of the Darriwilian is defined as the first appearance datum of the graptolite species Undulograptus austrodentatus in that section.
The base also lies in the upper part of the North American Histiodella altifrons conodont zone.
[12] The Undulograptus austrodentatus graptolite zone is known from outcrops around the world, making the base of the Darriwilian easily correlatable.