Proterozoic
The Proterozoic (composed of the Greek words πρότερος, Proteros = previous, early and ζῶον, zoon = living things ), formerly called Algonkian or Eozoikum, is a eon of geological history in the Precambrian. It ranges from the late Archean lived about 2,500 million years ago until about 541 million years.
Atmosphere
Features Proterozoic is the presence of an oxygen- containing atmosphere. Already towards the end of the Archean, mya before 2500, the oxygen content in the atmosphere began to rise. Although the production of oxygen by photosynthetic creatures had started already about one billion years ago, oxygen was released into the atmosphere until that date. Previously, this process by the oxygen uptake during the oxidation of the dissolved inorganic substances in sea water, in particular iron ions and ions of other heavy metals can be prevented. Those present in the reduced state the metal ion was oxidized to the metal oxides in the course of time, and only then was free oxygen into the atmosphere. With increasing oxygen concentration in the atmosphere also was able to form ozone, which was capable of containing harmful for living beings UV light filtering. Only the oxygen made possible the development of animal life.
The climate was at times significantly different than it is today, there is evidence of global glaciations ( Snowball Earth, Snowball Earth ).
Evolution of life
The name Proterozoic indicates that it is the time of the early animal life. Fossil remains of living things, but these are difficult to assign the currently known animal phyla were found. Since these first animal forms hardly had skeletons or hard shells, many morphological structures have not been preserved, their shapes are difficult to reconstruct. In addition, the fossil-bearing strata were exposed to geological changes over time. Peculiar situations, as they are preserved in the fossil record of the Ediacaran fauna known, formed in shallow, warm seas near the coast out. With the completion of the Proterozoic to the turn of the Cambrian most strains of the animal kingdom seem to have already been available as an output for the following Cambrian explosion.
Subdivision of the Proterozoic
It is divided into the following Äratheme:
- Eon: Phanerozoic ( 541-0 mya )
- Aeon: Proterozoic ( 2500-541 mya ) Era: Neoproterozoic ( 1000-541 mya )
- Era: Mesoproterozoikum ( 1600-1000 mya )
- Era: Paleoproterozoic ( 2500-1600 mya )
An alternative structure is as follows:
- Sinium ( 800-545 mya )
- Riphean ( 1650-800 mya )
- Animikium ( 2200-1650 mya )
- Huronium ( 2450-2200 mya )