Early Ordovician sponge-bearing microbialites from Peninsular Malaysia: The initial rise of metazoans in reefs

Abstract Introduction Early Ordovician (Floian) lithistid sponge- Calathium -microbial reefs were discovered in the lower part of the Lower Setul Limestone in Perlis, northwestern Malaysia Peninsula. Results With a thickness of more than 30 m, massive microbial boundstone is surrounded by thick-bedded oncoid-intraclast rudstone/grainstone. The reefs were constructed of cylindrical stromatolites and maze-like thrombolites, accompanying a small proportion of lithistid sponges (anthaspidellids) and hypercalcified sponges ( Calathium ). The earliest Floian age is based on a conodont fauna including the zonal index species Serratognathus bilobatus . Conclusions The microbialites might have grown in shallow subtidal environments, indicated by the high aspect ratio of the stromatolites and the associated coarse-grained sediments. The initial growth of stromatolites was sufficient to inhibit hydrodynamic removal of sediment and to create microhabitats in which environmental conditions were favourable for later accretion of thrombolites and colonization by anthaspidellids and calathids. These sponge-bearing microbialites represent the initial rise of metazoans in reefs at the dawn of the Ordovician Radiation, providing crucial information for understanding the transition from microbial- to metazoan-dominant reefs during this unique interval.