Tonian — first period of the Neoproterozoic

Chronology and boundaries

The base of the Tonian is conventionally placed at 1000 million years ago, and its top at 720 million years ago, where it hands off to the Cryogenian. These numerical boundaries are used in stratigraphic charts and are constrained by radiometric dating and chemostratigraphic correlation rather than a single globally accepted fossil marker.

Tectonics and Rodinia

Tectonically the Tonian is notable for the initiation of rifting that led to the break-up of the supercontinent Rodinia. Over tens of millions of years, rift basins, volcanism and drifting continental blocks produced new oceanic gateways and reconfigured global paleogeography — processes that influenced climate, ocean circulation and sedimentation patterns.

Life and acritarch radiation

Biologically the Tonian records a conspicuous increase in diversity and abundance of organic-walled microfossils known as acritarchs. These microscopic remains, often interpreted as resting stages of eukaryotic algae or other protists, display a wider range of shapes and ornamentation during this time and are said to have radiated, providing one of the few continuous fossil records available for the interval.

Importance and distinctions

Although less famous than the glaciations of the following Cryogenian, the Tonian is important because it records the tectonic reorganization and biotic groundwork that preceded dramatic climatic and evolutionary changes. It sits after the Mesoproterozoic and before the Cryogenian, and its rock sequences preserve signals of rifting, basin development and evolving ecosystems.

Further reading and resources

• Rodinia — supercontinent overview
• Geologic period concepts
• Neoproterozoic era summary
• Geologic era definitions
• Studies of Rodinia rifting
• Acritarchs and microfossils
• Fossil radiation records