Period· The columns of time

Tonian

1 Ga → 720 Ma

The Tonian, from 1 billion to 720 million years ago, saw Rodinia break up, oxygen rise a second time and cells prey on cells for the first time on record.

Opening period of the Neoproterozoic, marked by the break-up of Rodinia, by the second rise of oxygen and by the first direct evidence of predation between cells.

What the evidence shows

Rifts tear the supercontinent apart, and the period is named for that stretching, from the Greek tonos. The break-up has chemical consequences: fresh rock exposed to rainfall consumes carbon dioxide as it weathers, and the shallow seas of the new margins bury carbon in their sediments. Oxygen begins a second rise, a billion and a half years after the first, and it is slow, the deep ocean remaining anoxic and iron-rich until late in the Neoproterozoic. In the shales the Grand Canyon now cuts, microfossils 780 to 740 million years old carry round perforations through the wall, 0.1 to 3.4 microns across and always one per cell: the oldest direct evidence of predation on eukaryotes. The period also carries a contested claim on the origin of animals, structures observed in Canadian reefs 890 million years old and interpreted as the tissue of horny sponges.

Bounds and conventions

The base is a conventional round number; the top, by contrast, is tied to the onset of the Sturtian glaciation and moves as that dating is refined. The claim of sponges at 890 million years remains contested and commands no consensus; molecular clocks place the first animals between 890 and 635 million years, across most of this period and the next.