(Riedel and Sanfilippo, 1970, emend. 1971)
The top of the zone is defined by the morphotypic first appearance of Diartus petterssoni and is coincident with the lower limit of the Diartus petterssoni Zone.
The base of the zone is defined by the evolutionary transition from Dorcadospyris dentata to Dorcadospyris alata and is coincident with the upper limit of the Calocycletta costata Zone.
Although equivalent to the low-latitude zone of the same name, the aspect of the fauna in middle latitudes is often different and important marker species are frequently missing.Reference slides R 59.1, R 59.2 and R 59.3 from Sample 18-173-26-3, 37-43 cm.