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Filling a gap in the evolution of charophytes during the Turonian to Santonian


Lacustrine systems began to expand worldwide during the Late Jurassic and Early Cretaceous on many continents due to the subsidence of intraplate sedimentary basins in the context of a low relative sea level (Ziegler et al., 2001). This scenario increased the radiation of charophytes during this period, and two charophyte families, the Characeae and the Clavatoraceae, expanded, with the second family being dominant on the Central Tethyan Archipelago (Martín-Closas and Serra-Kiel 1991, Martín-Closas 2003). After a significant gap in the charophyte fossil record from the Cenomanian to the Santonian, a renewal of the charophyte assemblages occurred in the latest Cretaceous (Grambast 1974, Martín-Closas and Soulié-Märsche 2016). As a result, in the Campanian to Maastrichtian, the charophyte floras were dominated by the Characeae. What happened in the beginning of the Late Cretaceous that provoked this change in floras? The “SK-1” scientific drilling project in Songliao Basin is the first continental Cretaceous scientific drilling under the framework of the International Continental Scientific Drilling Program, which provides a rich source of fossils for this research. Recent investigation conducted by the assistant researcher Sha Li, Professor Qifei Wang, Professor Haichun Zhang from “Modern terrestrial ecosystems origin and early evolution research team” at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, cooperated with Professor Xiaoqiao Wan and Professor Yiyi Zhang from the China University of Geosciences and Professor Carles Martín-Closas from the University of Barcelona, studied the Turonian to Santonian charophyte floras in the Songliao Basin.

The fossil record of Turonian to Santonian charophytes is very poor worldwide, probably due to the limited non-marine records from this age, when there was a global high stand in the relative sea level and, thus, a reduction in non-marine habitats. After this gap, the charophyte floras changed to their modern physiognomy, with dominance of the Characeae. Charophytes from the Turonian to Santonian in the Songliao Basin help to fill this significant gap in the evolution of charophytes. The recorded flora is formed by one member of the Clavatoraceae, Atopochara trivolvis var. restricta, and a greater diversity of Characeae, including Nodosochara (Turbochara) heilongjiangensis comb. nov., Lamprothamnium ellipticum, Lychnothamnus barbosai and Lychnothamnus quantouensis comb. nov. These findings showed that the Characeae began to dominate charophyte floras during the global sea level high stand, up to 8 Ma earlier than previously reported. The occurrence of the genus Lychnothamnus in the Turonian in the Songliao Basin represents the oldest record in Eurasia of this still-living genus. The occurrence of A. trivolvis var. restricta in the Turonian to Santonian in China in comparison with its previous records in the Albian?–Cenomanian in Europe indicates that this variety survived as a relict in North China, while the more-derived variety Atopochara trivolvis var. multivolvis had evolved elsewhere. The charophyte flora thrived in large, shallow, permanent freshwater lakes and sometimes in palustrine settings of the Quantou, Qingshankou and Yaojia formations, which explains its higher biodiversity in contrast with the other records that belong mainly to brackish settings.

This study is a contribution to projects Grant nos. 41972018, 41602003, 41688103, funded by the National Natural Science Foundation of China, Strategic Priority Pre-Research Program (B) of the Chinese Academy of Sciences (XDB26000000), and project CGL2015-69805-P of the Spanish Ministry of Economy and Competitiveness (MINECO) and the European Regional Development Fund (ERDF-EU).

Reference: Li, S.*, Wang, Q.F., Zhang, H.C., Zhang, Y.Y., Wan, X.Q., Martín-Closas, C.*, 2020. Filling a gap in the evolution of charophytes during the Turonian to Santonian: Implications for modern physiognomy. Review of Palaeobotany and Palynology, https://doi.org/10.1016/j.revpalbo.2019.104154

Charophytes from the Songliao Basin

Charophytes from the Songliao Basin

Stratigraphic log of the SK-1 (South) borehole showing position of samples and of charophytes, calibrated to the GPTS (Deng et al. 2013)

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