# Phanerozoic/Ordovician period

Meeshaan Shah

## Ordovician Period - 490 to 443 million years ago

The Ordovician period began 490 million years ago at the end of the Cambrian, through around 443 million years ago with the beginning of the Silurian. During this time most of the world was covered in ocean, and most of land was comprised of the super-continent Gondwana.

## Tectonics

During the Ordovician the land was together forming the super-continent Gondwana. Gondwana consisted of Southern Europe, Africa, South America, Antarctica, and Australia. Throughout the Ordovician, Gondwana moved towards the South Pole where it finally stayed towards the end of the period. In the Early Ordovician, North America was approximately by the equator and submerged under water. However, North America developed into a land with exposed carbonate rocks. Western and Central Europe became separated and where located somewhere near the southern tropics. Europe shifted towards North America from higher to lower latitudes.
During the Middle Ordovician, areas that where shallow seas began to uplift, which became the precursor to glaciation. In addition, plate motions appears to have taken place which included the northward drift of the Baltoscandian Plate (northern Europe). Increased sea floor spreading and ridge activity and volcanic activity occurred during this time. Ocean currents changed as a result of lateral continental plate motions that caused the opening of the Atlantic ocean. Sea levels underwent regression and transgression globally. Because of sea level transgression, flooding of the Gondwana craton occurred as well as regional drowning.
During the Late Ordovician, a major glaciation in Africa occurred due to global disturbance, oxygenation, ocean erosion, and a severe drop in sea level resulted. This glaciation caused a dramatic regression of ocean waters which drained nearly all cratons. More specifically, this glaciation contributed to the mass extinctions. .

## Climate and Atmosphere

The Early Ordovician climate was thought to be quite warm in the tropical regions. In North America and Europe, Gondwana was largely covered with shallow seas. Shallow clear waters over brought about the growth of many organism.
A major transgression in the Middle Ordovician created widespread shallow, warm epicontinental seas. Which showed most of the Ordovician was favorable for marine life, particularly around the European and North American cratons. However, the Ordovician ended in a brief severe ice age. Gondwana, particularly Africa, moved towards the South Pole and became glaciated. There were even glaciers in what is now the Sahara. About 60% of animal genera became extinct, making this the second or third most deadly mass extinction of the Phanerozoic.
A lot of attention has been applied toward the cause of the Ordovician Ice Age. Not much evidence exists to show this ice age even occurred. Atmospheric carbon dioxide levels are believed to have been 8 to 20 times their current values. Sea levels were high through most of the Ordovician. Sea level then dropped, in connection with the ice age, but it is hard to tell if this was the cause. One factor which would affect both increase in carbon dioxide and sea level is the rate of sea floor spreading along mid-ocean ridges.

## Life

Ordovician life are characterized by numerous and diverse trilobites and conodonts (phosphatic fossils with a tooth-like appearance) found in sequences of shale, limestone, dolostone, and sandstone. In addition, blastoids, bryozoans, corals, crinoids, as well as many kinds of brachiopods, snails, clams, and cephalopods appeared for the first time in the geologic record in tropical Ordovician environments. Despite the appearance of coral fossils during this time, reef ecosystems continued to be dominated by algae and sponges, and in some cases by bryozoans.
However, there apparently were also periods of complete global reef collapse due to global disturbances.

### Fauna

The trilobite, inarticulate brachiopod, archaeocyathid, and eocrinoid faunas of the Cambrian were succeeded by those which would dominate for the rest of the Paleozoic, such as articulate brachiopods, cephalopods, and crinoids; articulate brachiopods, in particular, largely replaced trilobites in shelf communities. In North America and Europe, the Ordovician was a time of shallow continental seas rich in life. Trilobites and brachiopods in particular were rich and diverse. With that the first coral appeared in the Ordovician.
Molluscs, which had appeared during the Cambrian, became common, especially bivalves, gastropods, and nautiloid cephalopods. Now-extinct marine animals called graptolites thrived in the oceans. Some new cystoids and crinoids appeared. It was long thought that the first true vertebrates appeared in the Ordovician, but recent discoveries in China reveal that they originated in the Early Cambrian. The very first gnathostome (jawed fish) appeared in the Late Ordovician.

### Flora

Green algae were common in the Late Cambrian and in the Ordovician. Terrestrial plants probably evolved from green algae, first appearing in the form of tiny non-vascular mosses. Fossil spores from land plants have been identified in uppermost Ordovician sediments layers. Among the first land fungi may have been arbuscular mycorrhiza fungi (Glomerales), which played a crucial role in front running the colonization of land by plants.

## Extinction

For most of the Late Ordovician, life continued to flourish, but at and near the end of the period there were mass-extinction events that seriously affected planktonic forms like conodonts, graptolites, and some groups of trilobites which completely died out. Brachiopods, bryozoans and echinoderms were also heavily affected, and the endocerid cephalopods died out completely, except for possible rare Silurian forms. The Ordovician-Silurian Extinction Events may have been caused by an ice age that occurred at the end of the Ordovician period as the end of the Late Ordovician was one of the coldest times in the last 600 million years of earth history.
The Ordovician–Silurian extinction event or quite commonly the Ordovician extinction, was the third-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life. Between about 450 Ma to 440 Ma two extinctions occurred. This was the second biggest extinction of marine life, ranking only below the Permian extinction. At the time, all known life was confined to the seas and oceans. More than 60% died, particularly affected were brachiopods, bivalves, echinoderms, bryozoans, and corals. The immediate cause of extinction appears to have been the continental drift of a significant landmass into the south pole, causing a global temperature drop, glaciation, and consequent lowering of the sea level, which destroyed species' habitats around the continental shelves. Evidence for this was found through deposits in the Sahara Desert. When Gondwana passed over the south pole in the Ordovician, global climatic cooled to such a degree that there was widespread continental glaciation. This glaciation event also caused a lowering of sea level worldwide.