The oldest sponge reef is a small Early Cambrian bioherm at the base of the Tommotian Stage (∼535–540 Ma) in southeast Siberia. The mainly archaeocyath construction may be a response to turbid conditions. Cambrocyathellus bowls fused to create a rigid cavernous frame colonized by cryptic Archaeolynthus and calcified microbes (Renalcis). In addition to these constructors and binders, other reef guilds are present: bafflers (other archaeocyaths, spiculate sponges, and hyoliths) and dwellers (hyoliths, mollusks, and many others). This is the oldest known reef possessing an open skeletal frame structure built by animals and a mixed animal-autotroph composition. It provides a blueprint for younger Phanerozoic reefs.