LAUSR

Abstract As an obconical macrofossil with porous double-wall, Calathium was commonly present in reefs of Early to early Middle Ordovician age. The Calathium -bearing reefs thrived globally during the Early Ordovician, but this ecosystem collapsed in Middle Ordovician. A rare case of Calathium -microbial reefs was found from the middle part of the Yijianfang Formation (Darriwilian, late Middle Ordovician) of the Bachu area, located in the northwestern margin of the Tarim Basin, northwestern China. Surrounded by bioclastic grainstones, patch reefs here are well-developed and vary in size, with 1–4 m in thickness and 2–18 m in diameter. Three facies types are distinguished within the reef limestones: (1) Calathium -microbial framestone, (2) echinoderm- Calathium bafflestone, and (3) bryozoan-microbial bindstone. As a dominant type, the Calathium -microbial framestone shows a three-dimensional skeletal framework that is mainly constructed by Calathium and stabilized by microbialites. Although most specimens are toppled, Calathium displays well-developed lateral outgrowths, which connected individuals of the same species. Morphological characters of Calathium in thin sections show that calathids are hypercalcified sponges rather than receptaculitid algae. Unlike the Early Ordovician lithistid sponge- Calathium reefs, the Tarim reefs studied herein contain very few lithistid sponges. Instead, bryozoans are fairly common and act as the most important non-microbial encrusters, attaching to the walls of Calathium . Noticeably, the co-occurrence of pelmatozoans and Calathium is similar to those reported from Late Ordovician calathids-echinoderm reef communities from Tennessee. The presence of Calathium -microbial reefs in Tarim indicates that locally calathids were major reef builders, at least before the latest Middle Ordovician.