Changes in the carbon isotope composition of carbonate rocks at time-boundary zones show similar patterns in all three major boundaries: Precambrian/Cambrian, Permian/Triassic, and Cretaceous/Tertiary. The δ13C values drop from a high level, reach a minimum after the time boundary, and then increase to a new level. These changes reflect variations in the exogenic carbon cycle that may correlate with variations in the total biomass. The pattern of changes may reflect a sequence of reduction in the biomass of the old fauna, a period when the old and new faunal populations reach similar size, and a proliferation episode that follows. The similarity of the isotopic patterns at each of the boundaries suggests that there occurred at the Precambrian/Cambrian not only a radiation event, during which the shelly fauna first appeared, but also an extinction of older fauna for which the record was lost.