Period· The columns of time

Stenian

1.2 Ga → 1 Ga

The Stenian, from 1.2 to 1.0 billion years ago, is the period of the narrow mountain belts that welded Rodinia together and of the oldest fossil assignable to a living group.

Period of the narrow orogenic belts that welded Rodinia together, and of the oldest fossil assignable to a group still living.

What the evidence shows

Its collisions left belts of metamorphic rock so narrow that the period is named for them, from the Greek stenos. One ran from Labrador to Alabama, and its counterparts occur on the other continents: these are the sutures of Rodinia, the supercontinent assembled between about 1.1 and 0.9 billion years, which would persist some 150 million years before being broken apart by mantle plumes. In Arctic Canada, in rocks dated to 1.05 billion years, a tuft of branching filaments named Bangiomorpha is the oldest fossil assignable to a living group, the red algae. It combines three characters that no older fossil presents together: multicellularity, differentiation of cells into more than one type, and sexual reproduction.

Bounds and conventions

The limits of the period were fixed on the chart and not read in the rocks; what is measured here belongs to the mountains, the Grenville collision being dated between 1.09 and 0.98 billion years. The configuration of Rodinia is a reconstruction and not an observation: the arrangement of its blocks is inferred from palaeomagnetism and from the correspondence of orogenic belts, and competing configurations remain in print. The elevation of those mountains is likewise debated, the most recent Grenville synthesis treating it as a broad, hot orogen rather than a high plateau. The age of Bangiomorpha, long given as 1.2 billion years, has been reduced to 1.05 billion by dating of the enclosing beds.