Meishan GSSP, Changxing, Zhejiang

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A quarried limestone section in Changxing County, Zhejiang Province, that holds the golden spike defining the base of the Triassic and therefore the top of the Permian — the boundary at the largest extinction in the fossil record. Meishan is the reference section for the most severe biotic crisis known, and it is also where the crisis was converted from a stratigraphic interval into a duration. The section contains several volcanic ash beds interbedded with the fossiliferous limestone within a few metres of the boundary, and those ash beds contain zircons. High-precision uranium–lead dating of them by Burgess, Bowring and Shen in 2014 placed the boundary at 251.902 ± 0.024 million years and constrained the main extinction pulse to roughly sixty thousand years, with an uncertainty of about the same size. That result is what makes the end-Permian arguable as a rate problem rather than a list of casualties: a loss of that magnitude spread over ten million years and one compressed into sixty thousand demand entirely different mechanisms.