The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper OrdovicianSandbian Stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species Undulograptus austrodentatus around 469.4 million years ago. It lasted for about 10.8 million years until the beginning of the Sandbian around 458.2 million years ago.[7] This stage of the Ordovician was marked by the beginning of the Andean-Saharan glaciation.[8]
A secondary fossil marker is the graptolite Arienigraptus zhejiangensis.[12]
Biostratigraphy
The base of the Darriwilian is also the base of the Undulograptus austrodentatus graptolite zone. This zone lies just above the North Atlantic Microzarkodina parvaconodont zone. 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.[10]
Regional stages
The Darriwilian overlaps with the upper Arenig and the Llanvirn.[13] The base of the Darriwilian can be correlated with a level in the Fennian stage of the Arenig.[12]
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