Dresbachian

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 501 to 497 million years ago. The term comes from the town of Dresbach which is located in southeastern Minnesota on the Mississippi River.

The Dresbachian extinction event during the Late Cambrian was the second of two severe extinctions 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 events slashed approximately 40 percent of marine genera. However, the two are poorly documented due to a paucity of fossil 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 about 488 million years ago.

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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. [1]