Insights into reef evolution: the first sphinctozoan‐dominated reef facies from the lower Katian (Upper Ordovician) of the Tarim Basin, China

During the transition from the Middle to Late Ordovician, the reef community shifted significantly from microbial dominated to metazoan (stromatoporoids, bryozoans and corals) and algal dominated. Sphinctozoans began contributing to reef formations in the late Katian but did not become dominant reef builders until the Permian, exhibiting an evolutionary lag relative to other metazoans. Here, we report abundant sphinctozoan fossils in reef facies limestone of the lower Katian Lianglitag Formation in the Tarim Basin, northwestern China. Due to limited perforation information, these fossils were classified as Colospongiinae gen. et sp. indet., potentially representing a transitional form between Cambrian Blastulospongia and Carboniferous Colospongia . As dominant reef builders, Colospongiinae gen. et sp. indet. co‐constructed this reef together with the binding organism Archaeolithoporella and other associated organisms. This sphinctozoan– Archaeolithoporella reef facies community thrived in a low‐energy and shallow marine environment on an open carbonate platform; a combination that provided an ideal habitat for this community's development. This sphinctozoan‐dominated reef facies exhibits substantial similarities to Permian sphinctozoan‐dominated reefs, indicating that a simple form of Permian‐like sphinctozoan reef had already emerged in the early Katian. This reef facies provides the earliest documented evidence of sphinctozoan involvement and dominance in the construction of these reef systems. Furthermore, this discovery shows that dominant reef building by sphinctozoans, stromatoporoids, bryozoans and corals emerged nearly synchronously during the Middle to Late Ordovician, thus shedding new light on the overall pattern of reef evolution.