Wikipedia
Darriwilian
The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper Ordovician Sandbian stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species Undulograptus austrodentatus around million years ago. It lasted for about 8.9 million years until the beginning of the Sandbian around million years ago.