WHY DID METAZOAN REEFS DISAPPEAR FOR FORTY MILLION YEARS IN THE EARLY PALEOZOIC?: BIOLOGICAL VS. PHYSICOCHEMICAL MECHANISMS

Metazoans, especially archaeocyaths, were actively involved in reef building during the Early Cambrian. The Early Cambrian metazoan-calcimicrobe reef ecosystems collapsed at the end of the Early Cambrian and were replaced by microbialite reefs (stromatolites, thrombolites, and dendrolites). With very few exceptions, metazoans did not once again evolve into reef-building niches until the Middle Ordovician, about forty million years later. This is the longest time interval in the Phanerozoic during which metazoans were not conspicuously involved in building reefs. The previously proposed explanations for this long post-Early Cambrian disappearance of metazoan reefs are biological in nature: no metazoan groups were available to evolve into reef-building niches, or an absence of grazers allowed calcimicrobes to flourish, or critical photosymbiotic relationships took a long time to re-evolve. We find these biological mechanisms inadequate to account for the extraordinarily long post-Early Cambrian hiatus in metazoan reefs. Instead, we suggest that the explanation probably lies in a nexus of physicochemical phenomena which created global conditions that inhibited metazoan reefs and promoted microbialite reefs. Three phenomena that likely played important roles are: (1) very high levels of atmospheric carbon dioxide, (2) very warm sea surface temperatures, and (3) a precipitous drop in the Mg/Ca ratio of the world ocean. The hiatus in metazoan reefs coincides with the highest inferred level of atmospheric carbon dioxide in the Phanerozoic, and presumably attendant super-greenhouse conditions. High levels of atmospheric carbon dioxide and rising water temperatures have been documented to inhibit modern coralgal reef growth. We suggest that metazoan reefs suffered a similar fate in the Cambrian. Archaeocyaths, in particular, faced an additional problem of rapidly changing ocean chemistry. Having evolved under aragonite-facilitating conditions, they disappeared during a rapid swing to aragonite-inhibiting conditions.