Labechiid stromatoporoids from the Middle Ordovician Machiakou Formation of North China and their implications for the early development of stromatoporoids

Abstract

The Darriwilian stromatoporoid assemblage of North China represents a critical stage in early stromatoporoid development across the peri-Gondwanan regions, reaching the highest generic diversity than elsewhere as part of the Great Ordovician Biodiversification Event. We describe four labechiid stromatoporoid species, Labechia variabilis Yabe & Sugiyama, 1930, Labechiella mingshankouensis (Ozaki, 1938), Thamnobeatricea suxianensis (Dong, 1982), and Sinodictyon columnare Yabe & Sugiyama, 1930 from the Middle Ordovician Machiakou Formation in the Xiaonanhai section near Anyang City in northern Henan Province of China. The conodont Plectodina onychodonta occurs in association with these labechiids, implying a middle to late Darriwilian age. This assemblage extends the stratigraphic range of early stromatoporoids in North China to as early as the middle Darriwilian, thus slightly older than the previously known late Darriwilian earliest records. Mutual encrustations of Labechia variabilis and Labechiella mingshankouensis in the present assemblage constructed a microbioherm, together with unidentified tubular organisms, Ortonella-like calcimicrobes, a tubeworm-like organism, and borings. The assemblages of the Darriwilian labechiid stromatoporoids vary from place to place palaeogeographically in the shallow epicontinental marine environment of North China, i.e., in a patchy manner. The significant diversification of Darriwilian labechiid stromatoporoids in North China was likely promoted by a combination of environmental and ecological factors, which include: (1) the occurrence of a very shallow epicontinental marine environment, (2) a possible increase in the carbonate saturation state in seawater, (3) favourable seawater temperature for stromatoporoids during the Middle Ordovician and (4) the scarcity of bryozoans and corals as ecological competitors in North China.