The Dresbachian is a
Maentwrogian regional stage of North America, lasting from 501 to 497 million years ago.
It is part of the
Upper Cambrian and is defined by four
trilobite zones. It overlaps with the
ICS-stages
Guzhangian,
Paibian and the lowest
Jiangshanian.
The Dresbachian overlies the Middle Cambrian
Albertan series, and is the lowest stage of the Upper Cambrian
Croixian series, followed by the
Franconian stage. The Dresbachian extinction event, about 502 million years ago, was followed by the
Cambrian–Ordovician extinction event about million years ago.
Naming
The term is derived from the town of
Dresbach which is located in southeastern
Minnesota
Minnesota ( ) is a U.S. state, state in the Upper Midwestern region of the United States. It is bordered by the Canadian provinces of Manitoba and Ontario to the north and east and by the U.S. states of Wisconsin to the east, Iowa to the so ...
on the
Mississippi River
The Mississippi River is the main stem, primary river of the largest drainage basin in the United States. It is the second-longest river in the United States, behind only the Missouri River, Missouri. From its traditional source of Lake Ita ...
.
Definition
The Dresbachian is defined by four trilobite zones: ''
Cedaria''-, ''
Crepicephalus''-, ''
Aphelaspis''- and ''
Dunderbergia''
trilobite zones.
Events
The Dresbachian extinction event during the
Late Cambrian
The Furongian is the fourth and final Epoch (geology), epoch and Series (stratigraphy), series of the Cambrian. It lasted from to million years ago. It succeeds the Miaolingian series of the Cambrian and precedes the Lower Ordovician Tremadocia ...
was the second of two severe
extinction
Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
s during the first period of the
Paleozoic era; the first being the
End-Botomian extinction event during the
Middle Cambrian
Middle or The Middle may refer to:
* Centre (geometry), the point equally distant from the outer limits.
Places
* Middle (sheading), a subdivision of the Isle of Man
* Middle Bay (disambiguation)
* Middle Brook (disambiguation)
* Middle Creek (di ...
. According to data on extinction intensity (see below), both
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 fall in the diversity and abundance of multicellular organisms. It occ ...
s slashed approximately 40 percent of
marine genera
Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...
. They are poorly documented due to a paucity of
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
evidence so early in the
evolution of life.
References
* Chen Jun-yuan & Teichert C, 1983; "Cambrian Cephalopods", ''Geology'' Vol 11, pp647–650, Nov 1983
* Flower R.H.1964, ''The Nautiloid Order Ellesmerocerida (Cephalopoda) Menoir 12'', New Mexico Bureau of Mines and Mineral Resources, Socorro, NM
*Harland, W. B. et al. 1990. ''A Geologic Time Scale'' 1989. Cambridge University Press, Cambridge . Ref in Paleobiology Database on line.
* Moore, Lalicker, and Fischer 1952; ''Invertebrate Fossils''; McGraw-Hill; fig 1-17.
* Geowhen database
{{Geological history, p, p
Cambrian geochronology
Cambrian United States
Cambrian North America
Extinction events
Cambrian Minnesota
Guzhangian
Paibian
Jiangshanian