Bartonian

An unnamed fossil from Argentina proves that, by the Bartonian (Middle Eocene ), some 39–38 mya, Contra Baker et al.

The emergence of the Subantarctic lineage at the end of the Bartonian corresponds with the onset of the slow period of cooling that eventually led to the ice ages some 35 million years later.

The Ypresian stage constitutes the lower, the Priabonian stage the upper; and the Lutetian and Bartonian stages are united as the middle Eocene.