10.1 4.2.2 geological timescale

Geological timescale

Note that the time scale above shows the Carboniferous period as represented by the Pennsylvanian and Mississippian separately. This is not always done.

Geological time: interactive site: http://learn.genetics.utah.edu/content/variation/time/

Geological time: http://www.pbs.org/wgbh/evolution/change/deeptime/index.html

The three eras of the geological time scale

The earth’s history can be traced over MANY millions of years, so scientists have developed a geological time scale to help visualize theseperiods. This vast amount of time is divided into eons, eras and periods for easier reference. You MUST know the names of the three ERAS:

So, for example, the most recent eon is divided into 3 eras, called the

Paleozoic (meaning ancient life)

Mesozoic (meaning middle life)

Caenozoic (meaning recent life)

You don’t have to memorize the periods, only the eras . It is important that you become familiar with these names, so that you can use such information in a test or exam. The end of each era is marked by a seriesof catastrophic extinctions, which wiped out many of the previously successful species. Examine the diagram below, showing eras&periods.

(Ma = million years)

Examine the pie chart and the table, which has the main events of each period

The following are the major events in each era of the time scale:

• During the Pre-Cambrian time, life in general consisted of bacteria, simple algae and simple unicellular organisms. The best examples of such early life are the stromatolites, large mounds of cyanobacteria in sediment along the continental shelf that oxygenated the early atmosphere and allowed other aerobic life forms to exist.

• Paleozoic : Started with an “explosion” of multicellular life, called the Cambrian Explosion. Marine trilobites were common. The first invertebrates and fish are found, later the first amphibians and insects. Diverse land plants develop and coal swamps form incertain areas. The first reptiles form. This era ended with the massive Permian Extinction, wiping out many successful species

  

• Mesozoic : The Mesozoic era starts with the Triassic period, which saw the rise of the dinosaurs as the world’s dominant organisms. The dominant plants were Gymnosperms (cone bearers, like cycads and pine trees). Later, the first mammals develop and flowering plants or Angiosperms are formed. Birds develop, as well as the firstplacental mammals. This era ends with the Cretaceous Extinction (the so-called KT-boundary), that wiped out the dinosaurs 65 million years ago.