Carnian


Carnian

The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic series (or earliest age of the Late Triassic epoch). Its boundaries are not characterized by major extinctions or biotic turnovers, but a climatic event (known as the Carnian Pluvial Event) occurred during the Carnian and seems to be associated with important extinctions or radiations.

Naming

The Carnian was named in 1869 by Mojsisovics. It is unclear if it was named after the Carnic Alps or after the Austrian region of Carinthia ("Kärnten" in German). The name, however, was first used referring to a part of the Hallstatt Limestone cropping out in Austria.

Dating and subdivisions

There is not an established, standard usage for the Carnian subdivisions, thus, while in some regional stratigraphies a two-substage subdivision is common:
* Julian
* Tuvalianothers prefer a three-substage organization of the stage as follows:
* Cordevolian
* Julian
* Tuvalian

The Carnian spans from 228.0 ± 2.0 to 216.5 ± 2.0 Ma in the proposed geologic time scale by Gradstein et al. (2004). These dates are interpolated, because direct radiometric dates for this stage were missing when that time scale was compiled. Recently, Upper Carnian beds in southern Italy yielded an age of 230.91 ± 0.33 Ma. The age and duration of the Carnian need thus to be reconsidered.

Definition

The base of the Carnian stage is not yet formally defined. The leading GSSP candidate is at Prati di Stuores/Stuores Wiesen, in Badia, Bolzano-Bozen, Italy. The proposed base of the Carnian is at the First Occurrence of the ammonoid "Daxatina canadensis". Other possible candidate sections are in Spiti, Indian Himalayas, and at New Pass in Nevada.

Paleogeography and climate

The paleogeography of the Carnian was basically the same as for the rest of the Triassic. Most continents were merged into the supercontinent Pangaea, and there was a single global ocean, Panthalassa. The global ocean had a western branch at tropical latitudes called Paleo-Tethys. The sediments of Paleo-Tethys now crop out in southeastern Europe, in the Middle East, in the Himalayas, and up to the island of Timor.

The extreme land-sea distribution led to "mega-monsoons", i.e., an atmospheric monsoon regime more intense than the present one.

As for most of the Mesozoic, there were no ice caps. Climate was mostly arid in the tropics, but an episode of wet tropical climate is documented at least in the Paleo-Tethys. This putative climatic event is called the “Carnian Pluvial Event”, its age being between latest early Carnian (Julian) and the beginning of late Carnian (Tuvalian). The nature of this event is still discussed; some scientists believe it is only an artifact, due to the migration of continents of the Tethyan area across the equatorial climatic belt. Following this idea, the apparent shift from arid to humid, and then back to arid climate simply testifies the continents going from southern tropical, to equatorial, and then to northern tropical latitudes.

Flora

In the marine realm, the Carnian saw the first abundant occurrences of calcareous nannoplankton, a morphological group including the Coccolithophores.

Invertebrate Fauna

There are a few invertebrates which are typical and characteristic of the Carnian. Among molluscs, the ammonoid genus "Trachyceras" is exclusive of the lower Carnian (i.e., Julian of the two-substages subdivision, see above). The family tropitidae and the genus Tropites appear at the base of the upper Carnian (Tuvalian). The bivalve genus "Halobia", a bottom-dweller of deep sea environments, differentiated from "Daonella" at the beginning of this age. Scleractinian coral reefs, i.e., reefs with corals of the modern type, became relatively common for the first time in the Carnian.

Vertebrate Fauna

The earliest dinosaur Eoraptor originated slightly before the Carnian stage began around 230 Ma, nevertheless, the oldest well documented dinosaurian assemblage, in the Ischigualasto Formation of Argentina, is most probably late Carnian in age.

Conodonts are present in Triassic marine sediments. "Paragondolella polygnathiformis" appears at the base of the Carnian stage, and is perhaps the most characteristic species.A partial list of Carnian vertebrates is given below.

Many Carnian vertebrates are found in Santa Maria Formation rocks of the Paleorrota geopark.

Temnospondyls

Crurotarsans (non-crocodylomorph)

Dinosaurs

Classic localities and lagerstätten

The lower Carnian fauna of the San Cassiano Formation (Dolomites, northern Italy) is studied since the 19th century. Fossiliferous localities are many, and are distributed mostly in the surroundings of Cortina d'Ampezzo and in the high Badia Valley, near the village of San Cassiano, after which the formation was named. This fauna is extremely various, including ammonoids, gastropods, bivalves, echinoderms, calcareous sponge, corals, brachiopods, and a variety of less common fossils. A collection of this fauna is exposed in the “Museo delle Regole”, a museum in Cortina d'Ampezzo.

The Ischigualasto Formation of northwestern Argentina yielded a very important vertebrate association, including the oldest dinosaurian assemblage.

ee also

*Paleorrota

References

*Gradstein F.M., Ogg J.G. and Smith A.G., 2004, A Geologic Time Scale 2004, Cambridge University Press.
*Broglio Loriga, C., and others, 1999, The Prati di Stuores/Stuores Wiesen section (Dolomites, Italy): a candidate Global Stratotype Section and Point for the Base of the Carnian stage. Rivista Italiana di Paleontologia e Stratigrafia, v. 105, p. 37-78.
*Furin, S., and others, 2006, High-precision U-Pb zircon age from the Triassic of Italy: Implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs. Geology, v. 34, p. 1009-1012.

External links

* [http://www.palaeos.com/Mesozoic/Triassic/Carnian.htm Palaeos Mesozoic: Carnian Age]
* [http://www.dinodata.org/index.php?option=com_content&task=view&id=885&Itemid=96 Carnian dinosaurs] at DinoData
* [http://www.stratigraphy.org/geowhen/stages/Carnian.html GeoWhen database: Carnian]


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