The Ordovician Expansion

The Ordovician covers the time period between 485.4/1.9 and 443.8/1.5 million years before now. It follows the Cambrian period and is followed by the Silurian period.

The sediments of the Ordovician contain abundant fossils, in particular: trilobites, brachiopods, cephalopods, crinoids, graptolites, corals. In some regions the Ordovician sediments contain hydrocarbon deposits. The Ordovician began with a minor extinction event about 485.4 million years ago. The end of the Ordovician, 443.8 million years ago, coincides with one of the major mass extinction events, which led to the disappearance of about 60% of marine animal genera. The air had an abundance of carbon dioxide and the sea levels were much higher than they are now.

The International Commission of Stratigraphy recognizes for the Ordovician a subdivision into three epochs and seven floors:

Upper Ordovician which includes three floors:

* Hirnantian, (445.2 ± 1.4 - 443.8 ± 1.5 Ma)
* Katiano, (453.0 ± 0.7 - 445.2 ± 1.4 Ma)
* Sandbiano, (458.4 ± 0.9 - 453.0 ± 0.7 Ma)

Middle Ordovician which includes two floors:

* Darriwilian, (467.3 ± 1.1 - 458.4 ± 0.9 Ma)
* Dapingian, (470.0 ± 1.1 - 467.3 ± 1.1 Ma)

Lower Ordovician which includes two floors:

* Floiano, (477.7 ± 1.4 - 470.0 ± 1.1 Ma)
* Tremadocian, (485.4 ± 1.9 - 477.7 ± 1.4 Ma)

## Massive Changes

The Ordovician was characterized by a very important adaptive radiation, even if less famous than the Cambrian explosion; the genera of marine fauna practically quadrupled, giving life to 12% of all known marine fauna of the Phanerozoic. The trilobites, the non-articulated brachiopods, the archaeocyathids and the eocrinoid fauna of the Cambrian were replaced by the species destined to dominate the remaining period of the Paleozoic, such as the articulated brachiopods, the cephalopods and the crinoids. Articulated brachiopods in particular replaced trilobites in continental shelf communities. Their success is an example of the great growth recorded in the Ordovician by all organisms capable of secreting a shell of a carbonate nature.

In North America and Europe the Ordovician was characterized by shallow continental seas full of life. Trilobites and Brachiopods in particular were numerous and differentiated. The first Bryozoans and the first coral reefs appeared during the Ordovician, increasing the biodiversity of the carbonate shelf communities. Mollusks, which appeared as early as the Cambrian as aermatipic (non bioconstructive) corals, became common and differentiated, especially bivalves, gastropods and cephalopod nautyloids. It was long believed that the first true vertebrate (fish - Ostracoderma) appeared in the Ordovician but recent discoveries in China have revealed that they probably originated in the early Cambrian. The early fishes were jawless and used their mouth to suck and sift through sand through the seabed as filter feeders. Now, extinct marine organisms called graptolites thrived in the oceans and a few crinoids and cystoids appeared.

One of the most essential development in the meta that came from this expansion was the proliferation of life onto the land. The fungi were the first life on land before plants. Meanwhile, arthropods began transitioning into shallow waters.

## Extinctions

The Ordovician ended with a series of extinctions which, collectively considered, make up the second of the five largest extinctions in the history of the Earth in terms of the percentage of species that have disappeared. The only major one was that which occurred in the passage between the Permian and the Triassic. It is theorised to be caused by supercontinent, Gondwana, shifting down with the continental shelves and habitats along with it before resting in the south pole. This can be paired with the rapid cooling of Earth.

The extinction occurred between 444-447 million years ago and marks the separation between the Ordovician and the following period, the Silurian. At that time many complex multicellular organisms that lived in the sea (about 49% of the fauna species) disappeared forever; brachiopods and bryozoans were decimated, along with many families of trilobites, conodonts and graptolites.

The most accepted theory is that, due to massive glaciations, the sea level dropped drastically, the chemical composition of the oceans changed and there was the extinction of many marine species including graptolites. Glacial deposits from this period have been found in the Sahara Desert.