Although reefs often are expressed as single structures in the geological record, they are in fact composites of many superimposed communities. Therefore, an understanding of the processes of reef formation is largely dependent upon description of the life cycle of the dominant reef-building organisms. Modern reef corals commonly show synchronous gamete release (mass spawning) but, to date, such a phenomenon has not been reported from the fossil record. The first example of where the life cycle and reproductive ecology of an ancient reef-builder can be tentatively reconstructed is presented herein. Exceptionally preserved specimens of the widespread Late Paleozoic phylloid alga Eugonophyllum (Family Halimedaceae, Order Bryopsidales, Division Chlorophyta) allow the recognition of simultaneously ruptured reproductive structures. This may offer evidence for the synchronous release of gametes. In some modern bryopsidaleans, such as Halimeda and Udotea, synchronous reproduction is followed frequently by mass mortality of the community. It is speculated that phylloid algal reefs may be understood as having formed by the density-dependent recruitment of many self-seeded generations, which produced short-lived populations that reached reproductive maturity after six months to two years, but then suffered mass mortality on spawning the next generation. Those algae that formed a reef framework, as opposed to a bioclastic bed, were associated with an encrusting medium, most commonly microbialite. Recognition of such a life cycle may offer important insights into the expression of such reefs in the geological record that often are dominated by sheets of densely-packed, fragmentary material.