Stromatoporoids from the Upper Ordovician (Katian) Koumenzi Formation of North Qilian Mountains, China

Abstract

Stromatoporoid faunas in the peri-Gondwanan regions during the Katian of the Late Ordovician exhibit a greater diversity of taxa compared with those from other contemporaneous continents and terranes. Geographical separation due to the numerous microplates comprising peri-Gondwana was a potential driver of this high diversity, but stromatoporoid faunas in many of these microplates have not been studied in detail because of their remote present-day locations. In this study, we report 12 genera and 16 species of stromatoporoids from the Upper Ordovician (Katian) Koumenzi Formation, exposed in the upper reaches of the Tianbao River, Qilian County, Qinghai Province, Northwest China, representing a unique occurrence in the North Qilian Mountains. This fauna includes Cystostroma inconstans Jeon n. sp., Pseudostylodictyon poshanense Ozaki, 1938, Labechia conglomerata Dong and Wang, 1984, Labechia shanhsiensis Yabe and Sugiyama, 1930 (Yabe and Sugiyama, 1930a), Labechia sp., Labechiella gondwanense Jeon in Jeon et al., 2022a, Radiostroma astroqilianus Jeon and Zhan n. sp., Aulacera arbuscula Jeon n. sp., Sinabeatricea crassicentra Jeon n. sp., Lophiostroma leptolamellatum Jeon n. sp., Clathrodictyon idense Webby and Banks, 1976, Clathrodictyon sp. cf. Cl. mammillatum (Schmidt, 1858), Ecclimadictyon tiewadianensis (Jiang et al., 2011), Ecclimadictyon nestori Webby, 1969, Plexodictyon qilianense Jeon n. sp., and Simplexodictyon uniplexum Jeon n. sp. In comparison with other peri-Gondwana regions and other adjacent areas, this fauna consists of a relatively high proportion of endemic taxa and some pandemic taxa, with species-level affinities most closely linking the North Qilian Mountains to North China, and to a lesser extent, South China. The occurrence of genera such as Radiostroma, Plexodictyon, and Simplexodictyon, previously restricted to Baltica and North China extends their known palaeogeographical ranges. The stromatoporoid assemblage of the North Qilian Mountains represents a transitional fauna between the Ordovician and Silurian, characterised by persistent labechiid dominance alongside the appearance of clathrodictyids and stromatoporellids, and the presence of both long-ranging Ordovician taxa and morphologically advanced genera, reflecting diversification trends also seen in the contemporaneous North China assemblage. Relatively high endemism of this stromatoporoid fauna is attributed to the island arc volcanic setting during the Late Ordovician, emphasizing the role of island arc setting in stromatoporoid diversification during the Ordovician.