Proterozoic - Definition and Overview

 id:eon    value:rgb(1,0.7,1)  legend:Proterozoic_eon  # light purple
 id:era    value:rgb(1,1,0.7)  legend:Era   # light yellow
 id:black  value:black

 bar:eon
 bar:era

 width:100 align:center textcolor:black mark:(line,black)

 bar:eon color:eon

 from:542  till:2500 # text:Proterozoic~(542-2500)

 bar:era color:era

 from:542  till:1000  text:Neoproterozoic~(542-1000)
 from:1000  till:1600  text:Mesoproterozoic~(1000-1600)
 from:1600  till:2500  text:Paleoproterozoic~(1600-2500)

 tabs:(25-left)

 pos:(15,350)
 text:"Preceded by:"
 text:^Archaean eon
 text:^and Neoarchean era

 pos:(15,90)
 text:"Followed by:"
 text:^Phanerozoic eon
 text:^and Paleozoic era

In geology, the Proterozoic is an eon prior to the first abundant complex life on earth.

Classically, the boundary between the Proterozoic and the Paleozoic was set at the base of the Cambrian period when the first fossils of animals known as trilobites and archeocyathids appeared. In the second half of the 20th century, a number of fossil forms have been found in Precambrian rocks, but the boundary of the Proterozoic has remained fixed at the base of the Cambrian -- currently thought to be around 545 Million years before the present.

Proterozoic consists of 3 geologic periods (eras) called, from youngest to oldest:

• Neoproterozoic
• Mesoproterozoic
• Paleoproterozoic.

The Hadean is thought to occupy the time from 4500Ma to 3800Ma. Conditions in that time span are subject to considerable debate. The debate extends into the early Archaean, however it is clear that by roughly 3600Ma temperatures were stable at near modern levels, continental crusts were in place, oceans of liquid water existed, and simple lifeforms either had appeared or would appear shortly. Conditions of that sort existed throughout most of the remaining Proterozoic. The major well identified events were the transition to an Oxygenated atmosphere that probably occurred in the Mesoproterozoic and several glaciations the most severe of which took place in the late Neoproterozoic.