The Devonian appearance of 1-10 meter long armored fish (placoderms) coincides with geochemical evidence recording a transition into fully oxygenated oceans.1 A comparison of extant fish shows that the large individuals are less tolerant to hypoxia than their smaller cousins. This leads us to hypothesize that Early Paleozoic O(2) saturation levels were too low to support >1 meter size marine, predatory fish. According to a simple model, both oxygen uptake and oxygen demand scale positively with size, but the demand exceeds supply for the largest fish with an active, predatory life style. Therefore, the largest individuals may lead us to a lower limit on oceanic O(2) concentrations. Our presented model suggests 2-10 meter long predators require >30-50% PAL while smaller fish would survive at <25% PAL. This is consistent with the hypothesis that low atmospheric oxygen pressure acted as an evolutionary barrier for fish to grow much above ∼1 meter before the Devonian oxygenation.