stenian
The Stenian Period ( STEE-nee-ən, from Ancient Greek: στενός, romanized: stenós, meaning "narrow") is the final geologic period in the Mesoproterozoic Era and lasted from 1200 Mya to 1000 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically. The name derives from narrow polymetamorphic belts formed over this period.
Preceded by the Ectasian Period and followed by the Neoproterozoic Era.
The supercontinent Rodinia assembled during the Stenian. It would last into the Tonian Period.
This period includes the formation of the Keweenawan Rift at about 1100 Mya.Fossils of the oldest known sexually reproducing organism, Bangiomorpha pubescens, first appeared in the Stenian.

See also
Boring Billion – Earth history, 1.8 to 0.8 billion years ago
Riphean (stage) – stage in the geological timescale named after the UralsPages displaying wikidata descriptions as a fallback

Notes
References
"Stenian Period". GeoWhen Database. Archived from the original on May 12, 2006. Retrieved January 5, 2006.
James G. Ogg (2004). "Status on Divisions of the International Geologic Time Scale". Lethaia. 37 (2): 183–199. doi:10.1080/00241160410006492.