Early Ordovician

The Early Ordovician is the first epoch of the Ordovician period, also known as the Lower Ordovician series. It came after the Age 10 of the Furongian epoch of the Cambrian period. This important time in Earth's history lasted from about 486.85 to 471.3 million years ago. It ended just before the start of the Dapingian age of the Middle Ordovician period.

During the Early Ordovician, life on our planet continued to grow and change. Many new kinds of animals appeared in the oceans. Two main ages make up this epoch: the Tremadocian and the Floian. Scientists study rocks and fossils from this time to learn more about how Earth and its living things were developing long ago.

History

The International Commission on Stratigraphy started working on the boundaries of the Ordovician subdivisions in 1974. These boundaries were set using the fossils of conodonts and graptolites. By 1995, they decided to split the Ordovician into three parts: Lower, Middle, and Upper. Each part was then divided into two stages.

The Tremadocian and Floian stages are part of the Lower Ordovician. Before these international standards, different areas used their own ways to divide the Ordovician time. In 2008, new international stages replaced the old British ones, making the system more consistent around the world. In 2011, Russia also changed to match these new international standards.

Definition

The Early Ordovician is the first part of the Ordovician period, starting after the Cambrian period and lasting from about 486.85 to 471.3 million years ago. It includes two ages: the Tremadocian and the Floian.

The main reference point for marking the start of this time is found in a place called Green Point in western Newfoundland, Canada. This point is based on the first appearance of a tiny, ancient creature called Iapetognathus fluctivagus. Another reference point for the later part of the Early Ordovician is in Diabasbrottet Quarry in Sweden, marked by the first appearance of a different ancient creature called Tetragraptus approximatus.

Additional reference points were set up in Millard County, Utah, US, and in North China, but ideas about how to use these points have changed over time.

Biostratigraphy

The Early Ordovician is divided into smaller time slices based on fossils like conodonts and graptolites. In addition to global stages, some regions have their own zones. For example, the Tetragraptus approximatus Zone is used in the UK and Australia, while the Didymograptus protobifidus Zone is used in North America, Australia, and Baltoscandia.

In North America, scientists use different conodont zones to study Early Ordovician deposits. In Asia, both graptolite and conodont zones help divide the time periods. In Australia, the Early Ordovician matches several local stages including the Lancefieldian, Bendigonian, Chewtonian, and lower Castlemainian.

Paleogeography

The Iapetus Ocean was a vast ocean that lay between three large landmasses: Gondwana, Laurentia, and Baltica. At the start of the Early Ordovician, this ocean stretched over 4000 kilometers wide. To the north, the Iapetus Ocean connected with the Panthalassa Ocean, which covered about half of the Earth.

The Rheic Ocean had formed during the late Cambrian period and lay between Gondwana and a smaller landmass called Avalonia. Another land area, Cuyania, sat near the northwest of what is now Argentina. Gondwana was already a huge continent, made up of parts of today’s South America, Africa, the Arabian Peninsula, India, eastern Australia, and eastern Antarctica. Laurentia, which included regions such as Mexico, the US, Canada, Greenland, Scotland, and parts of Ireland, sat in the tropical latitudes of the southern hemisphere. By the end of the Early Ordovician, sea waters covered areas that are now central Nevada and western Utah.

Climate

Scientists have studied the climate of the Early Ordovician by looking at oxygen in ancient rocks and fossils. They found that tropical sea temperatures could have been as warm as 37 °C or even 42 °C during this time. The atmosphere had high levels of carbon dioxide, while oxygen levels changed between about 10% to 13%.

Later in the Early Ordovician, the climate began to shift gradually from very warm to cooler conditions. By the time the Middle Ordovician began, sea surface temperatures in low-latitude areas had cooled down to levels similar to today's equatorial seas. These changes happened over long periods, ranging from 10,000 to 10 million years.

CO₂ level oxygen isotopes apatites from greenhouse to icehouse Middle Ordovician

Major events

See also: Tremadocian § Major events

See also: Floian § Major events

The Great Ordovician Biodiversification Event was a time when many types of sea creatures appeared and grew more diverse. This was one of the biggest times for new life in Earth's history, along with the Cambrian explosion and changes in the early Mesozoic. During this event in the Early Ordovician, many different kinds of tiny sea plants and animals, as well as bottom-dwelling sea life, began to spread and change. Scientists have found special rock layers with fossils from this time in South Korea, showing that these communities lived in deeper parts of the ocean. These changes in sea life happened because the oceans became less poisonous and more friendly for animals to live in.

There were also signs of new life appearing in places like South China, although scientists are not sure if these changes happened all over the world. During this time, there was also a big event where lots of molten rock poured out in an area that is now part of the Iberian Peninsula, called the Ollo de Sapo magmatic event.

Paleontology

The Early Ordovician period saw many interesting ancient life forms. In places like Argentina, scientists found tiny microfossils called calcisphers or calcitarchs. These were likely simple algae and lived in shallow ocean areas near reefs. Another type of algae, Amsassia, lived along the coasts of ancient lands such as Laurentia and Cuyania.

Echinoderms, star-like animals, became more diverse during this time. New groups appeared, such as starfish-like animals, sea cucumbers, and feather stars. Fossils of these and other creatures have been found in Utah, Morocco, and France. Large arthropods like Aegirocassis, which could grow over 2 meters long, also lived during this period. Other interesting fossils include sponges, worms, and trilobites, showing the variety of life in the Early Ordovician oceans.

Mineral resources

Oil and gas exploration is happening in the Early Ordovician Tongzi and Meitan formations in Sichuan Basin and the Tarim Basin in China. Near Tazhong in the Tarim Basin, oil can be found deep underground, at depths of up to 9000 meters.