Tonian
1000 – ~720 Ma Pha. Proterozoic Archean Had.
A map of the world as it appeared during the late Tonian, c. 750 Ma
Chronology
−1000 — – −980 — – −960 — – −940 — – −920 — – −900 — – −880 — – −860 — – −840 — – −820 — – −800 — – −780 — – −760 — – −740 — – −720 — – M ♇ N e o p r o t e r o z o i c Stenian Tonian Cryogenian       ← First sponge-like animal?[1] ← Beginning of Sturtian glaciation ← Sharp drop in Stromatolite Population[2] ← End of Boring Billion
Etymology
Name formality	Formal
Usage information
Celestial body	Earth
Regional usage	Global (ICS)
Time scale(s) used	ICS Time Scale
Definition
Chronological unit	Period
Stratigraphic unit	System
Time span formality	Formal
Lower boundary definition	Defined chronometrically
Lower GSSA ratified	1991[3]
Upper boundary definition	Redefined chronometrically with an interim calibrated age of c. 720 Ma. GSSP is in progress.[4] (Previously defined chronometrically as 850 Ma)[3]
Upper boundary definition candidates	The first appearance of widespread glaciation.[5]
Upper boundary GSSP candidate section(s)	To be determined

The Tonian (from Ancient Greek: τόνος, romanized: tónos, meaning "stretch") is the first geologic period of the Neoproterozoic Era. It lasted from 1000 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.

Biology

The first putative metazoan (animal) fossils are dated to the middle to late Tonian (c. 890-800 Mya). The fossils of Otavia antiqua, which has been described as a primitive sponge by its discoverers and numerous other scholars, date back to about 800 mya. Even earlier sponge-like fossils have been reported in reefs dating back to 890 million years before the present, but their identity is highly debated.^[6] 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.^[7]

The first large evolutionary radiation of acritarchs occurred during the Tonian.