The Toarcian is, in the

ICS ICS may refer to: Computing * Image Cytometry Standard, a digital multidimensional image file format used in life sciences microscopy * Industrial control system, computer systems and networks used to control industrial plants and infrastructu ...

geologic timescale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronol ...

, an

age Age or AGE may refer to: Time and its effects * Age, the amount of time someone or something has been alive or has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...

and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toarcian Age began with the

Toarcian turnover The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

, the

extinction event An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. I ...

that sets its fossil faunas apart from the previous Pliensbachian age. It is believed to have ended with a global cooling event known as the Comptum Cooling Event, although whether it represented a worldwide event is controversial.

Stratigraphic definitions

The Toarcian takes its name from the city of Thouars, just south of

Saumur Saumur () is a commune in the Maine-et-Loire department in western France. The town is located between the Loire and Thouet rivers, and is surrounded by the vineyards of Saumur itself, Chinon, Bourgueil, Coteaux du Layon, etc.. Saumur statio ...

in the Loire Valley of

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

. The stage was introduced by French palaeontologist Alcide d'Orbigny in 1842, after examining

rock strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

of this age in a quarry near Thouars. In Europe this period is represented by the upper part of the

Lias Lias may refer to: Geology * Lias Formation, a geologic formation in France *Lias Group, a lithostratigraphic unit in western Europe * Early Jurassic, an epoch People * Godfrey Lias, British author * Mohd Shamsudin Lias (born 1953), Malaysian ...

. The base of the Toarcian is defined as the place in the stratigraphic record where the

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

'' Eodactylites'' first appears. A global reference profile (a GSSP) for the base is

located In geography, location or place are used to denote a region (point, line, or area) on Earth's surface or elsewhere. The term ''location'' generally implies a higher degree of certainty than ''place'', the latter often indicating an entity with an ...

at Peniche, Portugal. The top of the stage is at the first appearance of ammonite genus '' Leioceras''. In the Tethys domain, the Toarcian contains the following ammonite biozones: *zone of '' Pleydellia aalensis'' *zone of '' Dumortieria pseudoradiosa'' *zone of '' Phlyseogrammoceras dispansum'' *zone of '' Grammoceras thouarcense'' *zone of '' Haugia variabilis'' *zone of ''

Hildoceras bifrons ''Hildoceras bifrons'' is an extinct species of ammonite in the family Hildoceratidae. It dates from about 175 million years ago in the Early Jurassic when it was both widespread and common. Fossils have been found in North Africa and Europe, in ...

'' *zone of '' Harpoceras serpentinum'' *zone of '' Dactylioceras tenuicostatum''

References

Sources

*; 2004: ''A Geologic Time Scale 2004'',

Cambridge University Press Cambridge University Press is the university press of the University of Cambridge. Granted letters patent by Henry VIII of England, King Henry VIII in 1534, it is the oldest university press A university press is an academic publishing hou ...

. *; 1842: ''Paléontologie française. 1. Terrains oolitiques ou jurassiques'', Bertrand, Paris. *Elmi, S., Rulleau, L., Gabilly, J. & Mouterde, R. 1997: Toarcien. ''In'': Cariou, E. & Hantzpergue, P. (eds): Biostratigraphie du Jurassique ouest-européen et méditerranéen. ''Bulletin du Centre des Recherches, Elf Explor. Prod. Mém.'', 17.

External links

GeoWhen Database - ToarcianLower Jurassic timescale
at the website of the subcommission for stratigraphic information of the ICS
Stratigraphic chart of the Lower Jurassic
at the website of Norges Network of offshore records of geology and stratigraphy {{Geological history, p, m *04 Geological ages