Stromatoporoids of the Katian (Upper Ordovician) Beiguoshan Formation, North China

North China was a key centre of Middle Ordovician stromatoporoid diversification. However, detailed studies of the subsequent Late Ordovician stromatoporoid development in this terrane are lacking; thus, the spatiotemporal development of North Chinese stromatoporoids is poorly understood. The Beiguoshan Formation (middle Katian age, Late Ordovician) contains the youngest Ordovician stromatoporoids in North China, comprising a unique stromatoporoid fauna, consisting of 12 species across 10 genera and three orders of traditional taxonomic groupings: labechiids Rosenella woyuense Ozaki, 1938, Pseudostylodictyon poshanense Ozaki, 1938, Pseudostylodictyon chunhuaensis (Jiang et al., 2011), Labechia sp., Labechiella gondwanense Jeon in Jeon, Liang, Kershaw, Park, and Zhang, 2022, Labechiella regularis (Yabe & Sugiyama, 1930a); clathrodictyids Clathrodictyon sp. cf. Cl. mammillatum (Schmidt, 1858), Camptodictyon amzassensis (Khalfina, 1960), Ecclimadictyon tiewadianensis (Jiang et al., 2011), Plexodictyon xibeiense Jeon sp. nov., Petriterastroma exililamellatum Jeon and Kershaw gen. et sp. nov.; and the stromatoporellid Simplexodictyon conspicus Jeon and Kershaw sp. nov. This assemblage is distinguished by: (1) succeeding Darriwilian species of North China, (2) few peri-Gondwanan species, and (3) stromatoporoid genera common in Siluro–Devonian assemblages. Plexodictyon and Simplexodictyon species (commonly found in middle Palaeozoic rocks) are here recorded in the Ordovician for the first time. Their stratigraphical occurrence in the middle Katian reveals an interval of absences before their faunal development in the middle Silurian, a stratigraphical gap caused by the Huaiyuan Epeirogeny, which largely shaped and controlled the regional stratigraphy of North China. This unique assemblage of Katian stromatoporoids is not known from other contemporary terranes, supporting the interpretation that North China was separated from the north-eastern peri-Gondwanan regions, thus constituting an independent palaeobiogeographical unit during the Late Ordovician. Early occurrence of these common Siluro–Devonian-type stromatoporoids indicates a prologue of the Ordovician–Silurian stromatoporoid faunal transition in North China, characterized by a decrease in labechiids and rapid diversification of clathrodictyids and related stromatoporoids of typical Silurian types.