The Dresbachian is a

Maentwrogian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...

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 zone Trilobites are used as index fossils to subdivide the Cambrian period. Assemblages of trilobites define trilobite zones. The ''Olenellus''-zone has traditionally marked the top of the Lower Cambrian, and is followed by the '' Eokochaspis'' zone. ...

s. It overlaps with the ICS-stages Guzhangian,

Paibian The Paibian is the lowest stage of Furongian series of the Cambrian. It follows the Guzhangian ( 3rd series of the Cambrian) and is succeeded by the Jiangshanian Stage. The base is defined as the first appearance of the trilobite '' Glyptagnostus ...

and the lowest

Jiangshanian The Jiangshanian is the middle stage of the Furongian series. It follows the Paibian Stage and is succeeded by the still unnamed Stage 10 of the Cambrian. The base is defined as the first appearance of the trilobite '' Agnostotes orientalis'' wh ...

. The Dresbachian overlies the Middle Cambrian Albertan series, and is the lowest stage of the Upper Cambrian

Croixian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...

series, followed by the Franconian stage. The Dresbachian extinction event, about 502 million years ago, was followed by the

Cambrian–Ordovician extinction event The Cambrian–Ordovician extinction event occurred approximately 488 million years ago (Myr, mya). This early Phanerozoic Eon (geology), Eon extinction event eliminated many brachiopods and conodonts, and severely reduced the number of trilobit ...

about million years ago.

Naming

The term is derived from the town of Dresbach which is located in southeastern

Minnesota Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over t ...

on the

Mississippi River The Mississippi River is the List of longest rivers of the United States (by main stem), second-longest river and chief river of the second-largest Drainage system (geomorphology), drainage system in North America, second only to the Hudson B ...

Definition

The Dresbachian is defined by four trilobite zones: ''

Cedaria ''Cedaria'' is a small, rather flat trilobite with an oval outline, a headshield and tailshield of approximately the same size, 7 articulating segments in the middle part of the body and spines at the back edges of the headshield that reach halfl ...

''-, ''

Crepicephalus ''Crepicephalus'' is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived from 501 to 490 million years ago during the Dresbachian faunal stage of the late Cambrian Period The Cambrian Period ( ; som ...

''-, '' Aphelaspis''- and '' Dunderbergia''

Events

The Dresbachian extinction event during the Late Cambrian was the second of two severe

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the Endling, last individual of the species, although the Functional ext ...

s during the first period of the

Paleozoic era The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ''z ...

; 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 ( ...

. 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 change in the diversity and abundance of multicellular organisms. I ...

s slashed approximately 40 percent of

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

. However, the two 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 preserved ...

evidence so early in the

evolution of life Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variati ...

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