Tonian

The Tonian (from Greek τόνος (tónos), meaning 'stretch') is the first geologic period of the Neoproterozoic Era. It lasted from 1000 Mya to 720 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined by the ICS based on radiometric chronometry. The Tonian is preceded by the Stenian Period of the Mesoproterozoic era and followed by the Cryogenian. Rifting leading to the breakup of supercontinent Rodinia, which had formed in the mid-Stenian, occurred during this period, starting from 900 to 850 Mya. The first large evolutionary radiation of acritarchs occurred during the Tonian. The first putative metazoans (animal) fossils dated to the late Tonian (c. 800 Mya). A notable example of this is the Otavia antiqua, which has been described as a sponge by its discoverers and numerous other scholars. This dating is consistent with molecular data recovered through genetic studies on modern metazoan species; more recent studies have concluded that the base of the animal phylogenetic tree is in the Tonian.