A consortium dominated by archaeocyaths and calcified microorganisms (calcimicrobes) constructed the first metazoan reefs during an eleven-million-year interval of the Early Cambrian. However, archaeocyaths were not the first metazoan reef dwellers; the weakly calcified organism Namacalathus, and the more heavily biomineralized organism Namapoikia, occupied microbial reef environments during the late Neoproterozoic, but they were not involved in reef construction. Throughout the late nineteenth century, and during most of the twentieth century, the biological affinities of archaeocyaths were unsettled, which caused paleobiologists to avoid including them in analyses of the Cambrian fauna. However, in the late twentieth century the discovery of living, aspiculate sponges led to a consensus among archaeocyathan workers that these fossils represent an extinct class of aspiculate, calcareous sponges. The majority of archaeocyathan-calcimicrobial reefs are relatively small, lenticular mounds, typically about a meter thick and a few meters in diameter, but the archaeocyathan-calcimicrobial consortium also constructed massive, ecologically zoned, wave-resistant, framework reefs. The Great Siberian Cambrian Reef Complex is 200-300 km wide and stretches for 1500 km across northern Siberia. A consensus has not yet been found among Cambrian reef workers concerning photosymbiosis, which is such an important aspect of the ecology of modern coralgal reefs. Two extinction events hit during the Early Cambrian, the second of which is associated with a eustatic sea-level drop. The attendant marine regression eliminated reefs, and the archaeocyathan-calcimicrobial reef community disappeared. While the disappearance of reefs at this time is perfectly understandable, it is nevertheless surprising that the metazoan-calcimicrobe reef-building consortium was not able to recover within a few million years. Approximately forty million years passed æ from the end of the Early Cambrian to the beginning of the Middle Ordovician æ before metazoans finally returned to reef-building. We present seven hypotheses to explain this metazoan-reef-free window. The testing of these hypotheses will, in part, be the challenge of the next phase of research on Early Cambrian reefs.