Cambrian Series 3 is the still unnamed 3rd Series of the Cambrian. It lasted from about ~509 to ~497 million years ago and is divided into 3 stages: the unnamed Stage 5, the Drumian, and the Guzhangian. Cambrian Series 3 is preceded by also unnamed Cambrian Series 2 and succeeded by the Furongian series.
The International Commission on Stratigraphy still has to decide on the official name of the 2nd series of the Cambrian. The new name will also replace the older term “Middle Cambrian”.
The lower boundary of Series 3 has the same definition as Cambrian Stage 5. This boundary has not been formally defined yet by the ICS but a number of proposals for fossils and type sections have been made. The most promising fossil markers are the respective first appearances of either trilobite species Ovatoryctocara granulata or Oryctocephalus indicus. Either species should have an age close to ~509. The Series 3-Furongian boundary has the same definition as the Paibian stage. It is defined as the first appearance of Glyptagnostus reticulatus around ~497 million years ago.
Cambrian Series 3 is subdivided into the following stages:
Note : The above story is based on materials provided by Wikipedia