Eocene era

The Eocene was an era of Earth history.

While Silurians were named after the Silurian era, the Third Doctor insisted this was a misnomer, and they were actually from the Eocene period and should be named Eocenes. (TV: The Sea Devils, PROSE: The Eight Doctors) The Eleventh Doctor listed Eocenes as one of their argued names. (TV: The Hungry Earth)

One alternate version of the Doctor was stranded in the Eocene era. (PROSE: So Vile a Sin)

Behind the scenes
While referred to as a period in TV: The Sea Devils and PROSE The Eight Doctors, the Eocene is most accurately an epoch.