Geologic Timetable..

Precambrian Era

The Precambrian (or Pre-Cambrian) is an informal name for the eons of the geologic timescale that came before the current Phanerozoic eon. It spans from the formation of Earth around 4500 Ma (million years ago) to the evolution of abundant macroscopic hard-shelled fossils, which marked the beginning of the Cambrian, the first period of the first era of the Phanerozoic eon, some 542 Ma.

The SubDivisions of Precambrian

A diverse terminology has evolved covering the early years of the Earth's existence, but it is tending to settle out and come into greater use as radiometric dating allows plausible real dates to be assigned to specific formations and features. The terms Archean (older than about 2500 Ma), Proterozoic (2500-600 Ma), and Neoproterozoic (600-542 Ma) appear to have general currency. Some additional terms are included in the geological time line. See Timetable of the Precambrian.

• Proterozoic : Modern use is most often the period from the beginning of the lower Cambrian boundary, through 2500 Ma. The boundary has been placed at various times by various authors, but has now been settled at 542 Ma. As originally used, it was a synonym for Precambrian and hence included everything prior to the Cambrian boundary.
  
  
  
  
  
  • Neoproterozoic : the earliest subdivision of the Proterozoic roughly from the Cambrian boundary back to as far as 900 Ma, although modern use tends to represent a shorter interval : 542-600 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American geology.
    
    
    
    
    
    • Ediacaran : In March 2004, the International Union of Geological Sciences officially defined the term to describe this geologic period. The period begins at the time of deposition of a particular stratigraphic boundary, about 620 Ma. The period ends at the beginning of the Cambrian, 542 Ma. In this period the Ediacaran fauna appeared.
    
    
    
    • Cryogenian a proposed subdivision of the Neoproterozoic.
    
    
    
    • Tonian a proposed subdivision of the Neoproterozoic.
    
    
    
    
  
  
  
  • Mesoproterozoic : the middle division of the Proterozoic. Roughly from 900-1600 Ma. Corresponds to Precambrian Y rocks of older North American geology.
  
  
  
  • Paleoproterozoic : The oldest subdivision of the Proterozoic. Roughly from 1600-2500 Ma. Corresponds to Precambrian X rocks of older North American geology.
  
  
  
  



• Archaean : Roughly from 2500-3800 Ma.



• Hadean : Prior to 3800 Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although a very few old rock beds seem to be slightly older than 3800 Ma. Some zircon crystals from about 4400 Ma demonstrate the existence of crust in the Hadean Eon. Other records from Hadean time come from the moon and meteorites.

It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by GSSPs. The Precambrian could be divided into five "natural" eons, characterized as follows.

1. Accretion and differentiation: a period of planetary formation until giant Moon-forming impact event.



2. Hadean: the Late Heavy Bombardment period.



3. Archean: a period defined by the first crustal formations (the Isua greenstone belt) until the deposition of banded iron formations due to increasing atmospheric oxygen content.



4. Transition: a period of continued iron banded formation until the first continental red beds.



5. Proterozoic: a period of modern plate tectonics until the first animals.
   

Paleozoic Era

-a major division (era) of geologic time (see Geologic Timescale, table) occurring between 570 to 240 million years ago. It is subdivided into six periods, the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian (see each listed individually). During the hiatus between the late Precambrian and Paleozoic eras most of the evidence of the earth's early history was destroyed by erosion. From the beginning of the Paleozoic, shallow seas began to encroach on the continents. In North America, the era began with submerged geosynclines, or downward thrusts of the earth's crust, along the eastern, southeastern, and western sides of the continent, while the interior was dry land. As the era proceeded, the marginal seas periodically washed over the stable interior, leaving sedimentary deposits to mark their incursions. During the early part of the era, the area of exposed Precambrian, or shield, rocks in central Canada were eroding, supplying sediment to the geosynclines from the interior. Beginning in the Ordovician period, mountain building intermittently proceeded in the eastern part of the Appalachian geosyncline throughout the rest of the era, bringing in new sediments. Sediments washing from the Acadian Mts. filled the western part of the Appalachian geosyncline to form the famous coal swamps of the Carboniferous period. Uplift of the Appalachians caused the region to be never again inundated by vast marginal seas. Paleoclimatic studies and evidence of glaciers indicate that central Africa was most likely in the polar regions during the early Paleozoic. During the early Paleozoic, the huge continent Gondwanaland had either formed or was forming. By mid-Paleozoic, the collision of N America and Europe produced the Acadian-Caledonian uplifts, and a subduction plate uplifted eastern Australia. By late Paleozoic, continental collisions formed the supercontinent Pangaea and resulted in some of the great mountain chains, including the Appalachians, Urals, and Tasmans. The most noteworthy feature of Paleozoic life is the sudden appearance of nearly all of the invertebrate animal phyla in great abundance at the beginning of the Cambrian. A few primitive fishlike invertebrates, and then vertebrates, appeared in the Cambrian and Ordovician, scorpions in the Silurian period, land invertebrates and amphibians in the Devonian, land reptiles in the Carboniferous, and marine reptiles in the Permian. All reptiles increased in number and in variety by the late Permian. The plant life of the Paleozoic era reached its climax in the Carboniferous and was much contracted in the Permian.

Mesozoic Era

The Mesozoic Era is one of three geologic eras of the Phanerozoic eon. The division of time into eras dates back to Giovanni Arduino, in the 18th century, although his original name for the era now called the 'Mesozoic' was 'Secondary' (making the modern era the 'Tertiary'). Lying between the Paleozoic and the Cenozoic, Mesozoic means 'middle animals', derived from Greek prefix meso-/μεσο- for 'between' and zoon/ζωον meaning animal or 'living being'. It is often called the 'Age of Medieval Life' or the 'Age of the Dinosaurs', after the dominant fauna of the era.
The Mesozoic was a time of tectonic, climatic and evolutionary activity. The continents gradually shifted from a state of connectedness into their present configuration; the rifting provided for speciation and other important evolutionary developments. The climate was exceptionally warm throughout the period, also playing an important role in the evolution and diversification of new animal species. By the end of the era, the basis of modern life was in place.

Mesozoic Era

Geologic periods

Following the Paleozoic, the Mesozoic extended roughly 180 million years: from 251 million years ago (Mya) to when the Cenozoic era began 65 Mya. This time frame is separated into three geologic Periods. From oldest to youngest:

• Triassic (251.0 Ma to 199.6 Ma)



• Jurassic (199.6 Ma to 145.5 Ma)



• Cretaceous (145.5 Ma to 65.5 Ma)

The lower (Triassic) boundary is set by the Permian-Triassic extinction, during which approximately 90% to 96% of marine species and 70% of terrestrial vertebrates became extinct. It is also known as the "Great Dying" because it is considered the largest mass extinction in history. The upper (Cretaceous) boundary is set at the Cretaceous-Tertiary (KT) extinction, which may have been caused by the meteor that created the Chicxulub Crater on the Yucatán Peninsula. Approximately 50% of all genera became extinct, including all of the non-avian dinosaurs.

Life in the Mesozoic Period

The extinction of nearly all animal species at the end of the Permian period allowed for the radiation of many new lifeforms. In particular, the extinction of the large herbivorous and carnivorous dinocephalia left those ecological niches empty. Some were filled by the suriving cynodonts and dicynodonts, the latter of which subsequently became extinct. Animal life during the Mesozoic was dominated, however, by large archosaurian reptiles that appeared a few million years after the Permian extinction: dinosaurs, pterosaurs, and aquatic reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs.
The climatic changes of the late Jurassic and Cretaceous provided for further adaptive radiation. The Jurassic was the height of archosaur diversity, and the first birds and placental mammals also appeared. Angiosperms radiated sometime in the early Cretaceous, first in the tropics, but the even temperature gradient allowed them to spread toward the poles throughout the period. By the end of the Cretaceous, angiosperms dominated tree floras in many areas, although some evidence suggests that biomass was still dominated by cycad and ferns until after the KT extinction.
Some have argued that insects diversied with angiosperms because insect anatomy, especially the mouth parts, seems particularly well-suited for flowering plants. However, all major insect mouth parts preceded angiosperms and insect diversification actually slowed when they arrived, so their anatomy originally must have been suited for some other purpose.
As the temperatures in the seas increased, the larger animals of the early Mesozoic gradually began to disappear while smaller animals of all kinds, including lizards, snakes, and perhaps the ancestor mammals to primates, evolved. The KT extinction exacerbated this trend. The large archosaurs became extinct, while birds and mammals thrived, as they do today.

Cenozoic Era

The Cenozoic Era (IPA pronunciation: sometimes Caenozoic Era in the United Kingdom) meaning "new life" (Greek kainos = new + zoe = life) is the most recent of the three classic geological eras. It covers the 65.5 million years since the Cretaceous-Tertiary extinction event at the end of the Cretaceous that marked the demise of the last dinosaurs and the end of the Mesozoic Era. The Cenozoic era is ongoing.
The Cenozoic is divided into two periods, the Palaeogene and Neogene, and they are in turn divided into epochs. The Palaeogene consists of the Paleocene, Eocene, and Oligocene epochs, and the Neogene consists of the Miocene, Pliocene, Pleistocene, and Holocene epochs, the last of which is ongoing. Historically, the Cenozoic has been divided into periods (or sub-eras) named the Tertiary (Paleocene to Pliocene) and Quaternary (Pleistocene and Holocene), although most geologists no longer recognize them.

Cenozoic Era's Life

The Cenozoic is the age of new life. During the Cenozoic, mammals diverged from a few small, simple, generalized forms into a diverse collection of terrestrial, marine, and flying animals. The Cenozoic is just as much the age of savannas, or the age of co-dependent flowering plants and insects. Birds also evolved substantially in the Cenozoic.
Geologically, the Cenozoic is the era when continents moved into their current positions. Australia-New Guinea split from Gondwana to drift north and, eventually, abut South-east Asia; Antarctica moved into its current position over the South Pole; the Atlantic Ocean widened and, later in the era, South America became attached to North America.

Geologic Timetable