The Dresbachian is the lower (or earlier) stage of the Late or Upper Cambrian Period in North America, equivalent to the Chinese Guzhangian which spans about 4 million years, from . The term comes from the town of Dresbach which is located in southeastern Minnesota

Minnesota

Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...

 on the Mississippi River

Mississippi River

The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...

.

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

Extinction

In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

s during the first period of the Paleozoic era; the first being the End Botomian extinction event during the Middle Cambrian. According to data on extinction intensity (see below), both extinction event

Extinction event

An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...

s slashed approximately 40 percent of marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 genera

Genera

Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...

. However, the two are poorly documented due to a paucity of fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 evidence so early in the evolution of life.

The Dresbachian stage overlies the Middle Cambrian Albertan series, and is the lowest stage of the Upper Cambrian Croixan series, followed by the Franconian series. The Dresbachian event was followed by the Cambro-Ordovician extinction event

Cambrian-Ordovician extinction events

‎The Cambrian–Ordovician extinction event occurred approximately 488 million years ago . This early Phanerozoic Eon extinction event eliminated many brachiopods and conodonts, and severely reduced the number of trilobite species....

about .
‎

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 all 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. http://stratigraphy.science.purdue.edu/resources/geowhen/