structural:reformatting
Miaolingian

The Miaolingian is the third series/epoch of the Cambrian, lasting from c. 509 Ma to c. 497 Ma, and roughly identical to the middle Cambrian in older literature. It is divided into three stages: the Wuliuan c. 509 Ma to 504.5 Ma; the Drumian c. 504.5 Ma to c. 500.5 Ma; and the Guzhangian c. 500.5 Ma to c. 497 Ma. The name replaces Cambrian Series 3 and was ratified by the IUGS in 2018. It is named after the Miaoling Mountains in southeastern Guizhou Province, South China, where the GSSP marking its base is found. This is defined by the first appearance of the oryctocephalid trilobite Oryctocephalus indicus. Secondary markers for the base of the Miaolingian include the appearance of many acritarchs forms, a global marine transgression, and the disappearance of the polymerid trilobites, Bathynotus or Ovatoryctocara. Unlike the Terreneuvian and Series 2, all the stages of the Miaolingian are defined by GSSPs.

The olenellids, eodiscids, and most redlichiids trilobites went extinct at the boundary between Series 2 and the Miaolingian. This is considered the oldest mass extinction of trilobites.