The Bartonian is, in the International Commission on Stratigraphy's (ICS) geologic time scale, a stage or age in the middle of the Eocene Epoch or Series.
[5] The distinction between group and stage was made in the second part of the 20th century, when stratigraphers saw the need to distinguish between litho- and chronostratigraphy.
[citation needed] The base of the Bartonian is at the first appearance of the calcareous nanoplankton species Reticulofenestra reticulata.
[needs update][citation needed] The top of the Bartonian Stage (the base of the Priabonian) is at the first appearance of calcareous nanoplankton species Chiasmolithus oamaruensis (which forms the base of nanoplankton biozone NP18).
[citation needed] The Auversian regional stage of France is coeval with the Bartonian and is therefore no longer used.