Filling the middle Floian–early Darriwilian gap in the stromatoporoid record: Discovery of an early Darriwilian stromatoporoid from Qaidam, China

The early history of Palaeozoic stromatoporoids is interrupted by a pronounced middle Floian to early Darriwilian (Ordovician) hiatus of stromatoporoid records. This hiatus has posed a challenge in understanding the early dispersal pathway and the sudden late Darriwilian diversification of stromatoporoids on a global scale. In this study, we report an early Darriwilian stromatoporoid, Cystostroma sp., from the topmost Duoquanshan Formation (lower Darriwilian; Histiodella holodentata-Tangshanodus tangshanensis conodont biozone) in Qaidam, Qinghai Province, China, thereby filling a major gap in the early stromatoporoid record. Although the skeletal details of the specimen have been obscured by recrystallization and silicification, but part of the skeleton reveals a simple architecture consisting only of cyst plates with low convexity. Considering that the earliest known stromatoporoids have been confirmed from the Lower Ordovician of South China, the occurrence of this early Darriwilian stromatoporoid in Qaidam indicates a possible palaeobiogeographical connection between South China and Qaidam during the Early to Middle Ordovician. Our finding also provides evidence that the middle Floian to early Darriwilian gap in the stromatoporoid record is either an artefact resulting from inadequate sampling, particularly in less explored difficult-to-access regions such as the Qinghai–Xizang (Tibetan) Plateau in China, or a result of a global sea-level fall during the Early to Middle Ordovician, which would have led to the increasingly heterogeneous preservation of stromatoporoids in Lower to lower Middle Ordovician worldwide. During this time interval, sea-level fluctuations most likely governed the distribution of stromatoporoid habitats across the peri-Gondwanan regions, facilitating their dispersal. The present discovery, therefore, emphasizes the importance of palaeontological studies in less-explored remote areas to help fill gaps in the early evolutionary history of benthic organisms such as stromatoporoids.