The earliest evolutionary link of metazoan bioconstruction: Laminar stromatoporoid-bryozoan reefs from the Middle Ordovician of Korea

Sub-metre-scale patch reefs composed primarily of stromatoporoids and bryozoans are reported from the Duwibong Formation (upper Middle Ordovician), Taebaeksan Basin, Korea, in the eastern margin of the Sino-Korean Block. The reef framework is constructed of alternating thin laminae of the primitive labechiid stromatoporoid Cystostroma, the bryozoan Nicholsonella, subordinate Solenopora and minor siliceous sponges. Alternating laminae of stromatoporoids and bryozoans are largely responsible for the formation of a globular framework composed of columnar, branching, bulbous to irregular masses. Solenopora sporadically occurring as tiny patches or thin laminae attached to the stromatoporoid-bryozoan framework is considered to be subordinate encrusters. Siliceous sponges occur within the stromatoporoid-bryozoan framework and within the growth framework and bored cavities, interpreted as subordinate encrusters and cryptic dwellers. The compact globular framework of the Duwibong stromatoporoid-bryozoan consortium represents a new type of Ordovician skeletal bioconstruction, but with a certain structural similarity to Lower Ordovician bryozoan reefs in the South China Block. Together with coeval labechiid reefs occurring near the current study area, the Duwibong reefs suggest that incursion of the primitive stromatoporoids into the earliest bryozoan reefs resulted in the dominance of reef-building stromatoporoids in peri-Gondwana in contrast to coeval reefs in Laurentia, which commonly contain tabulate corals.