Lower Ordovician

The Early Ordovician is the first epoch of the Ordovician period, corresponding to the Lower Ordovician series of the Ordovician system. It began after the Age 10 of the Furongian epoch of the Cambrian and lasted from to million years ago, until the Dapingian age of the Middle Ordovician. It includes Tremadocian and Floian ages.

History

International Commission on Stratigraphy (ICS) appointed working groups on the boundaries of the Ordovician subdivisions in 1974. The boundaries were established by the fauna of conodonts and/or graptolites. In 1995, the Subcommission on Ordovician Stratigraphy, with the support of 90% majority of voting members, adopted the division of the Ordovician system into three series: Lower, Middle and Upper. In the same year, it was decided to divide each of the three series into two global stages. ''Tetragraptus approximatus'' zone was chosen as the base of the upper stage of the Lower Ordovician by 95% of the votes. Before the adoption of the international standard, the series from the chart of Britain, the type locality of the Ordovician system, were accepted by default as Ordovician subdivisions.
In accordance with this scale, the Ordovician was divided into six series, of which the lower one, the Tremadocian, passed into the International Stratigraphic Chart (ISC) as the stage of the same name. The upper Arenig series corresponds to the upper part of the Lower and lower part of the Middle Ordovician global series. On the British chart, the boundaries of Ordovician subdivisions are determined by local stratigraphic and paleontological features that are poorly defined in the rest of the world. As a result, local series and stages, very different from the British standard, were identified on different paleoplates or modern continents. In 2008, the traditional British Ordovician subdivisions were replaced by seven new stages with defined GSSPs. Tremadocian and Floian stages were included in the Lower Ordovician series. In 2011, the Russian regional stratigraphic chart was changed from the British to the international standard division of the Ordovician system.

Definition

The Global Boundary Stratotype Section and Point (GSSP) of the Lower Ordovician, which is also a GSSP of the Tremadocian stage and the whole Ordovician system, is established in the Green Point section in the west of the Newfoundland, Canada () and corresponds to the first appearance datum (FAD) of the conodont ''Iapetognathus fluctivagus'' at the 101.8 m above the base of the outcrop. Doubts have been expressed about the identification of the index taxon, but it occurs below the '' Rhabdinopora'' and related graptolites. The GSSP of the Floian, the second and last stage of the Lower Ordovician, is established in the Diabasbrottet Quarry, Sweden, () and defined by FAD of the graptolite '' Tetragraptus approximatus''.

Two Auxiliary boundary Stratotype Sections and Points (ASSPs) were also established for the Lower Ordovician/Tremadocian: the first one in the Lawson Cove section in Millard County, Utah, USA, and the second one in the Xiaoyangqiao section, North China. Both ASSPs were approved by the Subcommission on Ordovician Stratigraphy in 2016 and 2019, respectively, but, in 2021, the International Union of Geological Sciences (IUGS) proposed to deny the use of specific points and replace them by Standard Auxiliary Boundary Stratotypes (SABS) for more "flexible" correlations with GSSPs.

Biostratigraphy

The global stages of the Lower Ordovician, as well as of other series, are also subdivided into stage slices (time slices), which are corresponding to the base of conodont or graptolite zones. In addition to the global stages, the ''Tetragraptus approximatus'' Zone is also defined in the regional subdivisions of the UK and Australia, and the ''Didymograptus protobifidus'' Zone in those of North America, Australia and Baltoscandia.

North America

The following conodont zones are distinguished in Early Ordovician deposits of Boothia Peninsula, Canada (from upper to lower):
* ''Oepikodus communis'' and ''Reutterodus andinus'' ( Ship Point Formation, Floian);
* ''Rossodus manitouensis'' and ''Acodus deltatus/Oneotodus costatus'' (upper part of Turner Cliffs Formation, Tremadocian);
* ''Cordylodus angulatus'' (lower part of Turner Cliffs, Tremadocian).

Asia

Graptolite (g) and conodont (c) zonal subdivisions of southern Siberia (from upper to lower):
* ''Ps. angustifolius elongatus/E. broggeri'' (g), ''D. protobifidus'' (g), ''Ph. densus'' (g), upper part of ''Oepikodus evae'' (c), plus chitinozoan ''Conochitina raymondi'' Zone (Floian);
* Lower part of ''Ph. densus'' (g), ''Ac. balticus'' (g), lower part of ''Oepikodus evae'' (c) (Floian);
* ''T. approximatus'' (g), upper part of ''Paroistodus proteus'' (c) (Floian);
* Lower part of ''Paroistodus proteus'' (c) (Tremadocian);
* ''K. kiaeri/Ad. tenellus'' (g), ''B. ramosus/Tr. osloensis/Al. hyperboreus'' (g) (Tremadocian);
* ''Iapetognathus fluctivagus'' (c) (Tremadocian).

Australia

On the Australian scale, the global Lower Ordovician roughly corresponds to the Lancefieldian, Bendigonian, Chewtonian and lower Castlemainian stages.

Paleogeography

The Iapetus Ocean, located between Gondwana, Laurentia and Baltica, reached over 4000 km in width at the beginning of the Ordovician. In the north, between eastern Laurentia and Siberia, it connected with Panthalassa, which covered about half the planet. Formed in late Cambrian, Rheic Ocean stretched between Gondwana and the microcontinent of Avalonia. Cuyania (northwest of today's Argentina) was located at low latitudes. Gondwana, already a large continent, consisted of today's South America, Africa, the Arabian Peninsula, India, eastern Australia and eastern Antarctica. Laurentia included the territories of Mexico, the USA, Canada, Greenland, Scotland and partly Ireland. Laurentia was located in the tropical latitudes of the southern hemisphere; today's central Nevada and western Utah were covered by sea waters at the end of the Early Ordovician.

Climate

In 2007, Bassett et al. analyzed the oxygen isotope values of Early Ordovician strata of the Lange Ranch section in central Texas and concluded that tropical sea temperatures at that time could have reached 37°C or 42°C. Similar results were recovered by Trotter et al. in 2008, after oxygen isotope analysis of conodonts from the four paleoplates located in the Early Ordovician at low latitudes. Authors of the 2021 article give values in accordance with the study by Song et al. (2019): from 43.9°C 485 million years ago to 37.1°C 470 million years ago. High temperatures that persisted throughout the Early Ordovician affected the biodiversification later, in the Middle Ordovician. CO₂ level in Early Ordovician atmosphere was high while oxygen levels varied from approximately 10% to 13%.

Studies of oxygen isotopes from apatites (particularly conodont apatites from Nevada and Utah) show that at the end of the Early Ordovician the climate began to change from greenhouse to icehouse. These changes occurred gradually and were accompanied by fluctuations in sea surface temperature with an approximate duration of 10⁴ to 10⁷ years. By the end of the Middle Ordovician, the surface of the low-latitude seas had cooled to temperatures comparable to today's equatorial ones.

Major events

The Great Ordovician Biodiversification Event (GOBE), manifested in an increase in the diversity of marine life, occurred in Early Ordovician. Along with the Cambrian explosion and radiation in the early Mesozoic, it is the most significant biodiversification in the Phanerozoic eon. This event occurred at different intervals depending on the groups of organisms and the geographical region, and, in fact, the GOBE refers to a whole complex of sequential and interconnected processes of evolution and migration of organisms. The first phase, associated with planktonic communities, probably began in the late Cambrian and ended in the late Early Ordovician. The Early Ordovician marks the beginning of the second phase, which affected the inhabitants of benthos

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