The Third Interregnum was a period of history from 41,000 BCE (reference stardate -430/00) to 16,000 BCE (stardate -180/00), though these dates are not precisely known. It was the final period of the Emergent Era.
It began with the fall of the Querat and was punctuated by the Preserver Era, and finally ended with the appearance of Orions on Rigel VIII. It was characterized by a general retrogression, primitivism and a lack of interstellar travel. (FASA RPG module: The Orions: Book of Common Knowledge)