The Tonian Period is the earliest geological period of the last billion years, lasting from 1,000 million to 850 million years ago. It was generally a cold period with no complex life due to low oxygen levels. Fossils of small metazoans and multicellular algae have been found, and the most common fossils are acritarchs.
The Tonian Period (Greek: “stretching”) is the earliest geological period of the last billion years, extending from 1,000 million to 850 million years ago. The Tonian is the first period of the longer Neoproterozoic era, which also includes the Cryogenian and the Ediacaran. Unlike many subsequent geological periods, the Tonian is not defined on the basis of rock strata (stratigraphy), but on radiometric dates.
The tonic period, like the rest of the Neoproterozoic, was generally a cold period. The average temperature may have been 10 °C (41 °F) cooler than today, mostly in the continental interior. Possible trace fossils of small metazoans, such as 1 mm nematode-like worms, have been found dating back to the Tonian and even 200 million years earlier, in the Stenian period. While no animal fossil bodies date from the Tonian period, this may be because the organisms were very small, soft, and poorly fossilized. Fossils of multicellular algae from the Tonic period have been found.
Apart from unicellular fungi, bacteria, archaeans and some multicellular algae and possible primitive metazoans, there was no life during the tonic period. These primitive organisms probably inhabited the land immediately around water sources. In general, oxygen levels were lower in the Tonian than in the later Cryogenian and Ediacaran, making it difficult for complex life to evolve. The microbes have gathered in large, thick colonies called microbial mats. These microbial mats have no modern analogue, as any mat exposed today would quickly be devoured by animals.
The most common fossils from the Tonic Period are acritarchs, mostly the remains of planktonic algae and other unknown but related organisms. Acritarchs appear in the fossil record more than 1400 million years ago, flourishing in diversity over the next 600 million years. About 800 million years ago, shortly after the Tonian, the Sturtian-Varanger glacial event took place, which covered the planet with a layer of ice and drastically reduced the diversity of acritarch. As the planet began to warm 630 million years ago, the first complex multicellular organisms began to emerge.
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