Santonian

The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 86.3 ± 0.7 mya (million years ago) and 83.6 ± 0.7 mya. The Santonian is preceded by the Coniacian and is followed by the Campanian.
Stratigraphic definition.
The Santonian stage was established by French geologist Henri Coquand in 1857. It is named after the city of Saintes in the region of Saintonge, where the original type locality is located.
The base of the Santonian stage is defined by the appearance of the inoceramid bivalve "Cladoceramus undulatoplicatus". Its top (the base of the Campanian stage) is marked by the extinction of the crinoid "Marsupites testudinarius". In 2009, a GSSP (official reference profile) for both base and top had not yet been appointed.
Subdivision.
The Santonian is sometimes subdivided into Lower, Middle and Upper substages. In the Tethys domain the Santonian is coeval with a single ammonite biozone: that of "Placenticeras polyopsis". Biostratigraphy based on inoceramids, nanoplankton or forams is more detailed.