image: In the Dahe Composite Section, the significant positive δ13Corg shifts occur in the upper part of the Yulungssu Formation and the lower part of the Xishancun Formation, and the δ13Corg value reach the peak (-20.0‰) in the sample YD-25 from the lowermost of the Xishancun Formation, which replicate the δ13Corg variation trend from the uppermost Silurian to the lowermost Devonian worldwide. The SDB in the Dahe Area is better to be placed between the samples of YD-17 and YD-18 from the lowermost part of the Xishancun Formation, based on the correlation of δ13Corg variations between the Dahe Composite Section and the Klonk-1 Section close to GSSP in the Prague Basin, Czech Republic together with four sections from Turkey, Morocco, and China. view more
Credit: ©Science China Press
This study is conducted by Drs. Wenjin Zhao and Min Zhu (Institute of Vertebrate Paleontology and Paleoanthropology, the Chinese Academy of Sciences), Drs. Xiaolin Zhang and Yan’an Shen (University of Science and Technology of China), and Dr. Guodong Jia (Tongji University). The continuous late Silurian to Early Devonian strata are well exposed in East Yunnan, South China, and provide a complete record of an upward transition from open sea to near-shore environments, accompanied by the diversification and faunal shift of early vertebrates. Due to the lack of index fossils for the Silurian-Devonian interval, the ages of the fish-bearing Xishancun and Xitun formations and the exact level of the Silurian-Devonian Boundary (SDB) in East Yunnan remain contentious, resulting in the uncertainty of the minimal calibration age for the lungfish-tetrapod split. The detailed chemostratigraphic studies provide new convincing data for a better definition of the SDB in East Yunnan, resolve long-standing debates over the SDB in South China and help to calibrate the minimum and soft maximum ages for the coelacanth-rhipidistian and lungfish-tetrapod splits.
The researchers obtained several geochemical indicators including the values and curves of δ13Corg, δ13Ccarb and TOC from the continuous SDB sequence in Dahe, Yiliang County, East Yunnan. The results revealed 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 resembled the SDB curve from the borehole Klonk-1 drilled at the top of the Klonk GSSP in the Prague Basin, Czech Republic. As such, they placed 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, and the detailed chemostratigraphic studies can provide a feasible method to define the exact level of the SDB in East Yunnan.” Zhao says.
The team also dated the ages of the fish-bearing strata in East Yunnan based on the precise location of the SDB in Dahe Area, coupled with available paleontological data and the biostratigraphic zonation across the SDB sequence. The Kuanti Formation can be dated to the Ludlow (Silurian), the Miaokao and Yulungssu formations to the Pridoli (Silurian), and the Xishancun and Xitun formations to the Lochkovian of Early Devonian, which help to deepen our understanding on the early evolutionary history of sarcopterygians. The dipnomorph Youngolepis from the Xishancun and Xitun formations (Lochkovian, approximately from 419.2±3.2 Ma to 410.8±2.8 Ma) is universally regarded as the oldest representative of crown Rhipidistia, the clade uniting lungfishes and tetrapods to the exclusion of coelacanths, as well as the earliest representative of crown Sarcopterygii. The oldest-known sister taxa of crown Sarcopterygii (e.g., Psarolepis, Guiyu) have been reported from the Ludfordian of Ludlow (425.6±0.9 Ma). Thus, both the coelacanth-rhipidistian and lungfish-tetrapod splits should have occurred sometime during the interval from Ludlow to Lochkovian, which means that the minimal estimated time for two splits must be at least 416.0 (419.2-3.2) Ma, and the maximal estimated time for two splits is about 426.5 (425.6+0.9) Ma. “In the history of animal life, the crown Rhipidistia node that corresponds to the divergence between Dipnoi and Tetrapoda, is one of the most important evolutionary nodes for basal animal clades, and holds a crucial position in the evolutionary history from fishes to human beings. The definite SDB position and the abundant sarcopterygian fossils in East Yunnan help to calibrate the minimum and soft maximum ages for the coelacanth-rhipidistian and lungfish-tetrapod splits.” Zhu says.
The SDB is the first system boundary to be subject to intensive scrutiny using modern stratigraphic methods and procedures. The determination of the exact level of the SDB in East Yunnan not only has the key significance of global stratigraphic correlation, but also directly affects the minimum and soft maximum calibration ages for both the coelacanth-rhipidistian and lungfish-tetrapod splits. The research results of the team will greatly advance the study on the origin and early evolution of sarcopterygians including ourselves.
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See the article:
Zhao W, Zhang X, Jia G, Shen Y, Zhu M. 2021. The Silurian-Devonian boundary in East Yunnan (South China) and the minimum constraint for the lungfish-tetrapod split. SCIENCE CHINA Earth Sciences, 64 (10): 1784-1797, https://doi.org/10.1007/s11430-020-9794-8
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Science China Earth Sciences