A U-Pb zircon age of 530.7 \textpm0.9 Ma for a volcanic ash in the lower Placentian Series of the Avalonian terrane in southern New Brunswick provides a precise point for calibration of Early Cambrian biologic evolution. The ash is from the middle Rusophycus avalonensis Zone in Saint John, New Brunswick. This date is on subtrilobitic, Lower Cambrian strata that may be approximately correlative with the sub-Tommotian Manykaian Stage of Siberia and the lowest Meishucunian Stage of the South China platform. This age, when combined with published geochronologic data from the Cambrian, suggests that the Manykaian Stage may have lasted approximately 13 m.y. and that the Tommotian may have been considerably shorter.