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 and joined an Earth Reptile lab. (PROSE: So Vile a Sin)
Behind the scenes
While referred to as a period in The Sea Devils and The Eight Doctors, the Eocene is most accurately an epoch.
Doctor Who and the Silurians made the error of suggesting that the reptilian Silurians were from the Silurian era. The earliest reptiles evolved in the late Carboniferous, over 100 million years after the Silurian. The correction in The Sea Devils was insufficient, as the Eocene is millions of years before the earliest ape-like humans. The dating issue of the Silurians has persisted throughout Doctor Who media.
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