Tonian

The Tonian is the first geologic period of the Neoproterozoic Era. It lasted from 1000 to 720 million years ago. Unlike other time periods, its dates are set using special science methods instead of studying rocks and layers in the ground.

Before the Tonian came the Stenian Period, part of the earlier Mesoproterozoic Era. After the Tonian, the Cryogenian Period began. The name "Tonian" comes from a Greek word meaning "stretch," showing how Earth’s history stretches far back in time.

History

The Tonian period was first described in 1990. It was set to be from 1000 to 850 million years ago. The name "Tonian" comes from an Ancient Greek word meaning "stretch." This refers to the time when a large piece of Earth’s surface broke apart.

In 2014, scientists updated the end date of the Tonian to 720 million years ago. They found that the earlier dates did not fit with new discoveries about ancient cold periods on Earth. This change is temporary until scientists choose a final marker for the start of the next period.

Geology

During the early Tonian period, from about 1 billion to at least 900 million years ago, the supercontinent Rodinia and the superocean Mirovian stayed stable. Scientists think that sedimentary rocks that appeared between 850 and 800 million years ago might mark the start of Rodinia breaking apart, but most agree that Rodinia didn’t begin to break up until around 750 million years ago, near the end of the Tonian.

In the first part of the Tonian, carbon levels changed only a little, moving from lower levels in the Mesoproterozoic to higher levels in the Tonian. In the later part of the Tonian, carbon levels were much higher, with big drops happening at least once after 740 million years ago, right before the Sturtian glaciation in the Cryogenian.

Biology

See also: Huainan biota

The Tonian period marks an exciting time for early life on Earth. Around 890 to 800 million years ago, the first possible fossils of animals appeared. One example is Otavia antiqua, which scientists think might be an early type of sponge. Even older sponge-like fossils have been found, but scientists are still discussing what they really are.

During this time, scientists also found the earliest fossils of large algae, including green algae. One special algae called Arctacellularia tetragonala even had traces of chlorophyll, the green color in plants, preserved in it. Another algae, Sinosabellidites huainanensis, was first thought to be an animal but was later identified as algae.

This period also saw the first big increase in tiny fossils called acritarchs. Many vase-shaped microfossils appeared in sediments from this time, showing the earliest forms of a group of simple organisms. During the Tonian, the number of special rock formations made by ancient bacteria called stromatolites began to drop.

Fungi also left behind visible traces in rocks from this time, showing thin thread-like structures.