Calibrating Rates of Early Cambrian Evolution

Abstract

An explosive episode of biological diversification occurred near the beginning of the Cambrian period. Evolutionary rates in the Cambrian have been difficult to quantify accurately because of a lack of high-precision ages. Currently, uranium-lead zircon geochronology is the most powerful method for dating rocks of Cambrian age. Uranium-lead zircon data from lower Cambrian rocks located in northeast Siberia indicate that the Cambrian period began at approximately 544 million years ago and that its oldest (Manykaian) stage lasted no less than 10 million years. Other data indicate that the Tommotian and Atdabanian stages together lasted only 5 to 10 million years. The resulting compression of Early Cambrian time accentuates the rapidity of both the faunal diversification and subsequent Cambrian turnover.

Integrated chronostratigraphy of Proterozoic–Cambrian boundary beds in the western Anabar region, northern Siberia

“…The sub-Tommotian unconformity at Dvortsy is shown as a break in section that spans the interval from the negative excursion above Peak I to a point above the peak here desig-s= In our preferred correlation, the unconformity between the U st'-Yudoma and Pestrotsvet formations corresponds in time to a section of rocks minimally 48 m thick in the Kotuikan River section. Correlation of the Anabar succession with sections that contain volcanic rocks dated by U-Pb geochronometry (Bowring et al 1993), thus, permits us to estimate the duration of the sub-Tommotian hiatus in southeastern Siberia. As noted above, the beginning of Manykai deposition correlates bio-and chemostratigraphically with the lower Kessyusa Formation in the Olenek Uplift (Knoll et al 1995a); a basal Kessyusa volcanic breccia has a radiometric age of 543.9 ± 0.3 Ma (Bowring et al 1993).…”

“…1). This largely carbonate section preserves an exceptionally complete sedimentary record of terminal Proterozoic (Upper Vendian) and basal Cambrian time, contains diverse SSF assemblages, includes the stratotype of the pre-Tommotian NemakitDaldynian Stage, was never deeply buried (based on the colour of organic-walled microfossils), and can be correlated by means of bio-, chemo-and lithostratigraphy to other northern Siberian sections that contain well-dated volcanic rocks (Bowring et al 1993 ). The western Anabar section has also played an important role in the development of the concept of the Yudomian as the Siberian correlative of the Vendian System (Zhuravleva & Komar, 1962;Komar, 1966;Semikhatov, Komar & Serebryakov, 1970).…”

“…Basal Cambrian acritarch zones were developed in siliciclastic succes- Bowring et al 1993); C marks the basal Tommotian stratotype section and point.…”

Integrated Ediacaran chronostratigraphy, Wernecke Mountains, northwestern Canada

“…The age of this boundary (ca. 543 Ma) is based on U-Pb zircon dates from Namibia , Siberia (Bowring et al, 1993) and Oman (Amthor et al, 2003). Testing the timing of multiple, globally synchronous glacial events by radiometric dating has not been possible (Evans, 2000).…”

“…Lithologies of sections are greatly simplified and no thicknesses are implied. Sections: Siberia: absolute date from Bowring et al (1993), stratigraphy after Knoll et al (1995), Pelechaty et al (1996), and Pelechaty (1998); Namibia: absolute dates from Grotzinger et al (1995), stratigraphy after Saylor et al (1998) and Saylor (2003) and references therein; India: stratigraphy and preliminary isotopic data from Jiang et al (2002); Southeast USA: Nd = Noonday Dolomite, sequence stratigraphy after Fedo and Cooper (2001), stratigraphy and chemostratigraphic data after Corsetti and Hagadorn (2000) and Prave (1999); Mackenzie Mountains: Rt = Ravensthroat cap carbonate, BBR = Backbone Ranges Formation, stratigraphy and sequence boundaries after Narbonne and Aitken (1995), MacNaughton et al (2000) and Dalrymple and Narbonne (1996); isotopic trends after Narbonne et al (1994) and Kaufman et al (1997); Wernecke Mountains based on this present study, BBR-V = Backbone Ranges-Vampire Formation.…”