Early Ordovician

The Early Ordovician is the first epoch of the Ordovician period, also called the Lower Ordovician series. It came after the Age 10 of the Furongian epoch of the Cambrian period. This 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 Earth kept growing and changing. 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 changing long ago.

History

The International Commission on Stratigraphy began working on the parts of the Ordovician in 1974. They used fossils of conodonts and graptolites to set the boundaries. By 1995, they split the Ordovician into three parts: Lower, Middle, and Upper. Each part was divided into two stages.

The Tremadocian and Floian stages are in the Lower Ordovician. Before these rules, different places used their own ways to split the Ordovician time. In 2008, new international stages replaced the old British ones. This made the system the same all over the world. In 2011, Russia also changed to match these new standards.

Definition

The Early Ordovician is the first part of the Ordovician period. It came after the Cambrian period and lasted from about 486.85 to 471.3 million years ago. It has two ages: the Tremadocian and the Floian.

We know when this time began from a place called Green Point in western Newfoundland, Canada. This point marks when a tiny, ancient creature called Iapetognathus fluctivagus first appeared. Another point for later in the Early Ordovician is in Diabasbrottet Quarry in Sweden. This marks when a different ancient creature called Tetragraptus approximatus first appeared.

More points were set up in Millard County, Utah, US, and in North China, but ideas about using these points have changed over time.

Biostratigraphy

The Early Ordovician is divided into smaller time slices using fossils like conodonts and graptolites. Some areas have their own special 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 conodont zones to study Early Ordovician rocks. In Asia, graptolite and conodont zones help divide the time. In Australia, the Early Ordovician matches local stages such as the Lancefieldian, Bendigonian, Chewtonian, and lower Castlemainian.

Paleogeography

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

The Rheic Ocean formed during the late Cambrian period. It lay between Gondwana and a smaller landmass called Avalonia. Another land area, Cuyania, was near the northwest of what is now Argentina. Gondwana was a huge continent made up of parts of today’s South America, Africa, the Arabian Peninsula, India, eastern Australia, and eastern Antarctica. Laurentia included places such as Mexico, the US, Canada, Greenland, Scotland, and parts of Ireland. It 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 studied the climate of the Early Ordovician by looking at oxygen in old rocks and fossils. They found that tropical sea temperatures could have been as warm as 37 °C or even 42 °C. The air had lots of carbon dioxide, and oxygen levels changed between about 10% to 13%.

Later, the climate slowly changed from very warm to cooler. By the Middle Ordovician, sea temperatures in some areas cooled to levels like today’s equatorial seas. These changes happened over very long times, 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 became more varied. 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 sea life that lived on the ocean floor, 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 had 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 is being found in the Early Ordovician Tongzi and Meitan formations in the Sichuan Basin and the Tarim Basin in China. Near Tazhong in the Tarim Basin, oil is located deep underground, at depths of up to 9000 meters.