During the Ordovician, Life expanded tremendously in diversification and multiplicity. There existed a wide range of reef complexes in the tropics. The early Ordovician had been considered to be quite warm, at least in the tropics.
Throughout the time of the Ordovician period, part of the Paleozoic era, a plentiful variety of marine life prospered in the vast seas and the first primitive plants started to occur on land—prior to the second-largest mass extinction of all time terminated the period.
Age of Invertebrates
The Phanerozoic is classified into 3 eras i.e.:
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the Paleozoic (550 to 250 million years ago),
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the Mesozoic (250 to 65 million years ago)
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Cenozoic (65 million years ago to the present)
The Paleozoic has been known to be the Age of Invertebrates due to the rapid development of invertebrate animals during that time.
Invertebrate
Invertebrate life became growingly complex and diverse through the Ordovician. Both calcareous and siliceous sponges are well recognized; among other types, the stromatoporoid first occurred in the Ordovician. Even the Tabulata (platform) and rugosa corals (horn corals) occurred first in the Ordovician, the solitary or horn corals being particularly distinguishing. Brachiopods (lamp shells) and Bryozoans (moss animals) have been a dominant component of many accumulations. Molluscs were also quite common and included the bivalves, chitons, cephalopods, scaphopods (tusk shells), rostroconchs (single-shelled molluscs), gastropods, and monoplacophorans (limpet-shaped, segmented molluscs).
Most of the planet’s landmasses combined to create the supercontinent of Gondwana, which included the continents of Africa, South America, Antarctica, and Australia. Gondwana floated along the south throughout the period, ultimately establishing on the South Pole. The landmass that progressed towards being North America was combined into the supercontinent of Laurentia, which was isolated from Gondwana by the narrow Iapetus Ocean.
Mass Extinction of Ordovician
Despite the extreme expansion of life during the Ordovician Period, there has been a devastating mass extinction of organisms at the end of the Ordovician. This extinction has been chronicled to be of the greatest mass extinction ever in Earth History with over 100 families going extinct.
Reason of Extinction of Ordovician
There has been majorly 2 reasons for Ordovician going extinct:-
1. One belief was that it was the breakup and movement of the massive supercontinent into many splinters. However, modern biology makes us know that this would not possibly result in extinction; instead, it would give additional niche space for groups to expand into.
2. A greater cause is that the Earth cooled, especially the oceans where the majority of the organisms lived during the Ordovician period (Remember there are still no evidence of land plants and land organisms). All the extinctions took place in the oceans.
A pronounced extinction took place in the tropical oceans. This makes sense because if the oceans cooled off due to the development of a huge ice sheet over the south polar area, the organisms adapted to warmer tropical conditions would have limited options and feasible nowhere to migrate to. There would be restricted regions warm enough to harbour all the warm-favouring organisms. This is inclined to support the idea that cooling resulted in many of the extinctions.
Silurian Period
It is the expansion of life following the mass extinction of the Ordovician. the first land plants appeared during the Silurian Period. Once again expanded the Marine organisms in diversity following the extinction of many families in the Late Ordovician.
The Silurian was possibly comparatively warm even though pCO₂ (Partial Pressure of Carbon Dioxide) may have been lower. This is believed to be since there has been no large landmass over the South Polar Region through the Silurian period.
Devonian Period
Devonian fish were a common element of the marine biological communities. Particularly important organisms during the Devonian were the jawed fish.
The first fossil evidence of terrestrial trees and insects appeared from Devonian age rocks. The Devonian is quite warm and the climate is thought to have been very dry. Evidence of this emerges from large amounts of tropical-like reefs and evaporite (salt deposits).
In current times, for example, evaporites are only limited to the mid-latitude belt where dry sinking air appearing from the Hadley cells makes these regions dry. During the Devonian period, these evaporite deposits were observed well beyond 30 degrees north and south.