The top of the Serravallian (the base of the Tortonian stage) is at the last common appearance of calcareous nannoplanktons Discoaster kugleri and planktonic foram Globigerinoides subquadratus.
The base of the Tortonian stage is at the last common appearance of calcareous nannoplankton Discoaster kugleri and planktonic foram Globigerinoides subquadratus.