Ordovician in the Paleozoic

The four eras of geological time

There are 4.55 billion years, the dust of ancient giant stars are proto solar nebula rotated about what will become of our Sun.
The Earth is not constituted.
The disk surrounding the proto-sun heats up at birth, size reached 200 AU (astronomical unit). It begins to solidify and the atoms stick to each other to form grains of material. Thus sets up the accretion will lead to the formation of planets. Of dust grains condense, gravity increases attracting more and more objects, large and small. Over time a large ball covered with lava rock, is formed.

At this point resembles the young Earth, a huge ball molten rock.
Since the first microorganisms to man through the first oxygenated atmosphere, the first aerobic organisms, the first animals to external skeleton in the seas, the first fish, the first marine and terrestrial reptiles, the first plants, first insects, dinosaurs, mammals and primates, the Earth has left traces that have allowed us to define the characteristics eras in its history.
The Earth is only half of his life and the geological eras to come, remain very long.

Geological time span of today, there are 4.55 billion years, time of the birth of the Earth.
This story begins with the first geological eon of Earth, the Hadean through the Archean, Proterozoic, and the ends of the Phanerozoic. The Hadean is 700 million years, the Achaean my 1300, the Proterozoic and Phanerozoic 2000 my 540 my. Eras are best defined naturally on the last million years.

Image: The geological eons of Earth, the Hadean to now, the Phanerozoic.

Ordovician

The Ordovician is the second of six geological systems constituting the Paleozoic located himself in the last geological eon, the Phanerozoic. It takes 45 million years and extends from -488.3 ± 1.7 to -443.7 ± 1.5 million years.
The term Ordovician, from the name of the Celtic tribe of Ordovices, was created in the 19th century by Charles Lapworth to designate a stratigraphic sequence from north Wales.
During the Ordovician sea level reached a maximum, however, there is a supercontinent, Gondwana from the fracture of an even older supercontinent, Rodinia.
It is in the Ordovician that the diversity of marine life explodes with the so-called "events of the Great Ordovician Biodiversification" (Great Ordovician Biodiversification Event: GOBE).

Life in the Ordovician

The first explosion of life date from the Cambrian (-542 to -488 million years) with the emergence of the main current animal living in the seabed. But it was in the Ordovician that the oceans are full of life with a truly exceptional increase in the number of marine organisms.
The study of reasons for this "Great Ordovician Biodiversification" was the subject of an international program, the International Geoscience Program (IGCP) Project No. 503, Ordovician Palaeogeography and Palaeoclimate, in which several French teams participated.
The synthesis of these results, published in the journal GSA Today in 2010, explains that the key factor in the expansion of marine life in the Ordovician, was the increase in areas where the continental shelf seas are shallow and rich in life.

The Ordovician period is known as one of particular interest to life.
After the breakup of Rodinia, Precambrian supercontinent on the separation of continents has reached its maximum, before coming together again to form Pangaea, Permian, in the late Paleozoic.
The Ordovician period is the Paleozoic that had the largest number of continents.
Source: CNRS / INSU

Image: Limestone in trilobites, brachiopods, corals, gastropods (Lower Silurian), Anticosti Island, Quebec, Canada.
© Axel Munnecke, Universität Erlangen-Nürnberg

Mass extinction of the Ordovician

The Ordovician was a time of giving shallow inland sea rich in life, animal life existed only in the sea but there are about 440 to 445 million years, at the boundary between the Ordovician and Silurian two mass extinctions occur.
Although the geological eras are all marked by more or less massive extinctions, the five major extinctions in the Phanerozoic are known, including that of the transition between the Ordovician and Silurian.
During the Ordovician, nearly all the submerged lands were located south of the equator.
A period of glaciation would have resulted in climatic and ecological disturbances, it seems to be the cause of this extinction.
The diversity of marine life would have declined because of lower sea level restricting ecosystems.
Older brands streaks left by glaciers, indicating that a large Gondwana ice cap at the end of the Ordovician.
This ice age would have made it difficult for the adaptation of species and ecosystems with a decline of the sea hundreds of kilometers.
Like all great chapters in the history of the Earth, beginning with the Ordovician species that had survived several extinctions.
The mass extinction that would end the Ordovician, touched mainly trilobites, which became the most important arthropod of the time. Insects, scorpions, spiders and crustaceans belong to the phylum arthropods.
Extinction dates are only approximate because in ancient times, they are set by the fossil record, with dates in broad ranges, and refined over time by other scientific discoveries.

Image: Mass extinctions according Raupp, D.M. 1993.
L'extinctions des espèces. Gallimard.

Image: Biodiversity evolution in the Phanerozoic.
Mass extinctions have always been followed by explosions radiative (rapid evolution of species). Species disappearing free ecological niches for other species will benefit.