Sibumasu as a dispersal bridge for Middle Ordovician stromatoporoids in peri-Gondwana

Reconstructing early stromatoporoid diversification across Gondwana and peri-Gondwanan terranes is hindered by incomplete and weakly constrained records — particularly those from Sibumasu Terrane despite its close palaeobiogeographical affinity with North China, where the first major radiation of labechiid stromatoporoids occurred. The timing of the first stromatoporoids in Sibumasu has remained uncertain, maintaining a longrecognised temporal and palaeogeographical gap between their earliest occurrences in South China and the subsequent diversification that took place in North China. Here, we provide the first systematic description of Middle Ordovician (Darriwilian) stromatoporoids from Thailand, documenting five genera and six species (two new and one in open nomenclature) from the unnamed Darriwilian strata in Satun (southern Thailand) and the upper Darriwilian unit of the Tha Manao Formation in Kanchanaburi (western Thailand), comprising Rosenella woyuensis Ozaki, 1938, Cystostroma satunense sp. nov., Labechia variabilis Yabe and Sugiyama, 1930a, Labechiella regularis (Yabe and Sugiyama, 1930a), Labechiella sp., and Thamnobeatricea kanchanaburiensis sp. nov. Integration of the new taxonomic data with conodont biostratigraphy indicates that, to our knowledge, stromatoporoids first appeared in Sibumasu by the early Darriwilian, represented by the Histiodella holodentata–Tangshanodus tangshanensis Conodont Biozone, earlier than their first occurrence in North China. This occurrence indicates that stromatoporoids had already expanded across peri-Gondwanan terranes by the early Middle Ordovician and partially narrows the long-recognised temporal gap between the earliest Early Ordovician records and their later Middle Ordovician diversification. Relatively higher affinities between Sibumasu and North China compared with other continents and terranes support a shallow marine connection across peri-Gondwana during the Middle Ordovician. These results help fill a long-standing regional gap in stromatoporoid research and provide a clearer temporal framework for evaluating early palaeobiogeographical relationships among peri-Gondwanan terranes