The Silurian-Devonian boundary in East Yunnan (South China) and the minimum constraint for the lungfish-tetrapod split

The Silurian-Devonian interval is an essential period in Earth history for witnessing the rise of sarcopterygian fishes and terrestrial vascular plants. In addition to its implication in global stratigraphic correlation, the precise location of the Silurian-Devonian boundary (SDB) in East Yunnan closely relates to the minimal and maximal estimated dates for the lungfish-tetrapod split. Several geochemical indicators including the values and curves of δ13Corg, δ13Ccarb and TOC are obtained from the continuous SDB sequence in Dahe, Yiliang County, East Yunnan. The results reveal the significant positive δ13Corg shifts in the upper part of the Yulungssu Formation and the lower part of the Xishancun Formation, and the peak value (−20.0‰) in the sample YD-25 from the lowermost of the Xishancun Formation, replicating the δ13Corg variation trend from the uppermost Silurian to the lowermost Devonian worldwide. The δ13Corg variation across the SDB at the Dahe Section resembles the SDB curve from the borehole Klonk-1 drilled at the top of the Klonk GSSP in the Prague Basin, Czech Republic. As such, we place the SDB in the Dahe Area between the samples of YD-17 and YD-18 from the lowermost part of the Xishancun Formation. This SDB assignment is corroborated by new findings of Early Devonian thelodont Parathelodus from the lower part of the Xishancun Formation in Qujing Area. The resolution of the SDB in Dahe, coupled with available paleontological data and the biostratigraphic zonation in East Yunnan, has provided vital data for the geological ages of the fish-bearing strata in East Yunnan. The earliest rhipidistian Youngolepis from the Xishancun Formation (Lochkovian, Devonian) and earliest stem-sarcopterygian Psarolepis from the Kuanti Formation (Ludfordian, Silurian) in East Yunnan indicate that the split between lungfish and tetrapods occurred between 426.5 and 416.0 Ma.