Stromatoporoids have played a role in reef construction globally since the late Middle Ordovician. The diachronous and patchy distribution of stromatoporoids has limited our understanding of their diversification and palaeoecological involvement in early reef ecosystems during the Ordovician. This study re-examines calcimicrobe-stromatoporoid bioherms from the upper Darriwilian Moyero Formation, exposed along the Moyero River in Siberia. The bioherm is dominated by calcimicrobes and stromatolitic fabric that intermittently alternate with stromatoporoid sponges. Unlike other contemporaneous reef structures from peri-Gondwana and Laurentia, the stromatoporoids in the Moyero River locality are accessory components with a subordinate role in reef construction, represented by a single stromatoporoid species, Cystostroma insuetum. This difference of stromatoporoid occurrence from other Middle Ordovician examples is interpreted to reflect the progressive absorption of stromatoporoids into reef communities, contrasting with their more abrupt appearances in contemporary reefs in other regions of peri-Gondwana and Laurentia. This study highlights the importance of recognizing regional variability in reef development and emphasizes the role of stromatoporoids in the transition from microbial-to metazoan-dominated reef ecosystems. The re-investigation provides new insights of palaeogeographic variations and the environmental conditions that favoured the proliferation of stromatoporoids during the Middle Ordovician. (c) 2024 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights are reserved, including those for text and data mining, AI training, and similar technologies.