It has been divided into three eras: Unknown The Precambrian Era comprises all of geologic time prior to million years ago.
What little is known has been discovered largely in the past four or five decades. The Precambrian fossil record is poor, and what fossils are present such as those of stromatolites—formed by microbial biofilms whose calcarious slime mixed with sedimentary grains hardens and over time accumulates in layers forming a kind of nodule are of limited use for biostratigraphic work Monroe and Wicander Many Precambrian rocks are heavily metamorphosedobscuring their origins, while others have either been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata Monroe and WicanderGore It is thought that the Earth itself coalesced from material in orbit around the sun roughly mya and may have been struck by a Mars-sized planetesimal shortly after it formed, splitting off material that came together to form the Moon.
A stable crust was apparently in place by mya, since zircon crystals from Western Australia have been dated at mya. The term "Precambrian" is somewhat dated, but is still in common use among geologists and paleontologists. It was briefly also called the Cryptozoic eon.
It seems likely that "Precambrian" will eventually be replaced by the preferred terms ProterozoicArchaeanand Hadean, and become a deprecated term. Life before the Cambrian It is not known when life originated, but carbon in rocks dated at mya from islands off western Greenland may be of organic origin.
Well-preserved bacteria older than million years have been found in Western Australia. Probable fossils million years older have been found in the same area.
There is a fairly solid record of bacterial life throughout the remainder of the Precambrian. Excepting a few contested reports of much older forms from Texas and India, the first complex multicellular life forms seem to have appeared roughly mya. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between mya and mya the beginning of the Cambrian period.
These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that time span.
A very diverse collection of forms appeared around mya, starting in the late Precambrian with a poorly understood "small shelly fauna" and ending in the very early Cambrian with a very diverse, and quite modern "Burgess shale fauna," giving evidence of a rapid radiation of forms called the Cambrian explosion of life.
Mayr concludes that the apparent explosion of new phyla in the early Cambrian was possibly due to the skeletonization of a variety of soft-bodied organisms that already existed in the Precambrian. Planetary environment and the oxygen catastrophe A proposed reconstruction of the supercontinent Rodinia million years ago, with 1.
Geologic evidence suggests that Rodinia formed about mya during the Mesoproterozoic era and broke apart during the Neoproterozoic era about mya. Continental land masses protruding above the surface of the waters covering Earth have assembled, broken apart, and re-assembled several times during the eons of geologic time, but details of plate motions in the Precambrian are only hazily known.
It is generally believed that most of Earth's landmasses collected into a single supercontinent, Rodinia, around mya, and then broke apart around mya. A number of glacial periods have been identified going as far back as the Huronian epoch, roughly mya. The best studied of the most ancient glaciations is the Sturtian-Varangian glaciation, around mya, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth.
The young planet likely had a reddish tint, and its seas were thought to be olive green. Many materials with insoluble oxides appear to have been present in the oceans for hundreds of millions of years after the Earth's formation.
When bacteria developed the biochemical machinery for performing photosynthesisthey began to produce oxygen in large quantities, causing an ecological crisis sometimes called the Oxygen Catastrophe. In an early phase of the high volume production of oxygen, the oxygen was soon tied up in chemical reactions, primarily with ironuntil the supply of oxidizable surfaces ran out.
|Links The Precambrian is sometimes referred to as an "eon. It is simply Precambrian time.|
After that, the modern high-oxygen atmosphere developed. Older rocks contain massive banded iron formations that were apparently laid down as iron and oxygen first combined.
Subdivisions of the Precambrian Diverse terminologies and frameworks for characterizing the early years of the Earth's existence have been developed in the past century. With improvements in and greater use of radiometric dating methods, however, scientists are assigning plausible real dates to specific formations and features, and the field is becoming more settled.
In modern use, the Proterozoic eon is most often the period extending back from the beginning of the earliest Cambrian boundary to mya. The earliest Cambrian boundary has been placed at various times by various authors, but that boundary has now become settled at mya.
As originally used, the term Proterozoic was a synonym for Precambrian, and hence included everything prior to the Cambrian boundary.
The Neoproterozoic era is considered to be the earliest subdivision of the Proterozoic, extending back from the earliest Cambrian boundary to as far as mya, although modern use tends to represent a shorter interval from mya.
Ediacaran part of the Neoproterozoic era of the Proterozoic eon.In Cambrian Period: Fossil record of the Precambrian-Cambrian transition. The preservation of the record of the Precambrian-Cambrian transition was significantly affected by global changes in sea level.
The Precambrian is the earliest of the geologic ages, which are marked by different layers of sedimentary rock. Laid down over millions of years, these rock layers contain a permanent record of. Precambrian definition, noting or pertaining to the earliest era of earth history, ending million years ago, during which the earth's crust formed and life first appeared in the seas. See more. The Cambrian period, part of the Paleozoic era, produced the most intense burst of evolution ever known. The Cambrian Explosion saw an incredible diversity of life emerge, including many major.
During latest Precambrian time, the sea level was relatively low, resulting in spatially restricted oceans and expanded continents. Stratigraphic scale of the ICS subdivisions and Precambrian/Cambrian boundary.
The Cambrian Period (/ ˈ k æ m b r i ə n / or / ˈ k eɪ m b r i ə n /) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon.
. The Precambrian is the earliest of the geologic ages, which are marked by different layers of sedimentary rock. Laid down over millions of years, these rock layers contain a permanent record of. The Precambrian (or Pre-Cambrian, sometimes abbreviated pЄ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon.
The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, which is named after Cambria, the Latinised name for Wales, where rocks from this.
The Cambrian Period is the first geological time period of the Paleozoic Era (the “time of ancient life”). This period lasted about 53 million years and marked a . The Cambrian period, part of the Paleozoic era, produced the most intense burst of evolution ever known.
The Cambrian Explosion saw an incredible diversity of life emerge, including many major.