Tonian

Tonian is a geological period within the Neoproterozoic Era of the Proterozoic Eon. It spans roughly from about 1.0 billion years ago to about 720 million years ago, placing it as the earliest formal subdivision of the Neoproterozoic. The name Tonian comes from the Tonian stratigraphic unit and was established to organize the chronological framework of the later Precambrian. The lower and upper boundaries of the Tonian are defined in relation to the surrounding Neoproterozoic periods, with the Tonian succeeded by the Cryogenian.

During the Tonian, the global configuration of continents was affected by the fragmentation of the supercontinent Rodinia and accompanying tectonic activity. Climate and ocean chemistry varied, and the period precedes the two major Neoproterozoic climate events that follow, the Cryogenian glaciations. The Tonian is primarily known through the sedimentary record, including microfossils and chemical signatures that inform interpretations of ancient environments.

The fossil record for the Tonian includes early, simpler signs of eukaryotic life, notably microfossils such as acritarchs, and other indicators of biological diversification that set the stage for later Neoproterozoic life. While full-bodied metazoan fossils become more common in the subsequent Ediacaran, the Tonian represents a time of early diversification and increasing complexity in unicellular and small multicellular life.

Subdivisions within the Tonian are generally recognized as Early, Middle, and Late Tonian, corresponding to progressively younger intervals within the 1.0 Ga to 720 Ma span. Research into Tonian stratigraphy and biostratigraphy continues to refine its durations and the precise timing of key evolutionary and tectonic events.