Filling a gap in the evolution of charophytes during the Turonian to Santonian: Implications for modern physiognomy

Abstract

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.