Abstract

The Cambrian diversification of animals was long thought to have begun with an explosive phase at the start of the Tommotian Age. Recent stratigraphic discoveries, however, suggest that many taxa appeared in the older Nemakit-Daldynian Age, and that the diversification was more gradual. We map lowest Cambrian (Nemakit-Daldynian through Tommotian) records of δ13CCaCO3 variability from Siberia, Mongolia, and China onto a Moroccan U/Pb–δ13CCaCO3 age model constrained by five U/Pb ages from interbedded volcanic ashes. The δ13CCaCO3 correlations ignore fossil tie points, so we assume synchroneity in δ13C trends rather than synchroneity in first appearances of animal taxa. We present new δ13Corg, 87Sr/86Sr, uranium, and vanadium data from the same carbonate samples that define the Moroccan δ13CCaCO3 curve. The result is a new absolute time line for first appearances of skeletal animals and for changes in the carbon, strontium, and redox chemistry of the ocean during the Nemakit-Daldynian and Tommotian ages at the beginning of the Cambrian. The time line suggests that the diversification of skeletal animals began early in the Nemakit-Daldynian, with much of the diversity appearing by the middle of the age. Fossil first appearances occurred in three pulses, with a small pulse in the earliest Nemakit-Daldynian (ca. 540–538 Ma), a larger pulse in the mid- to late Nemakit-Daldynian (ca. 534–530 Ma), and a moderate pulse in the Tommotian (ca. 524–522 Ma). These pulses are associated with rapid reorganizations of the carbon cycle, and are superimposed on long-term increases in sea level and the hydrothermal flux of Sr.

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