Abstract: The Silurian Xiaoxiang Fauna from Qujing, Yunnan is unique worldwide for its exquisitely-preserved jawed vertebrates either in diversity or in completeness. In June 2014, a new Silurian fish, Megamastax amblyodus, which is the largest Silurian vertebrate discovered to date with an estimated length up to 1.2 m, was reported from Xiaoxiang Fauna. The jaws of the new fish feature minute conical dentition on the occlusal margins and large blunt teeth that are fused to the coronoids, highlighting durophagous feeding. A global increase in oxygen levels may have facilitated the emergence and diversification of large jawed vertebrates in the mid-Paleozoic. Competing paleoatmospheric models predict the onset of near-modern oxygen levels during either the late Silurian (~420 Ma) or the Emsian (Early Devonian, ~400 Ma). The absence of pre-Emsian fishes more than half-a-metre in length was used as support for the latter model, with presumed low pre-Devonian oxygen levels imposing evolutionary constraints on maximum vertebrate body size. The new finding is consistent with previous studies documenting an increase in gnathostome diversity and functional disparity well before the advent of the Devonian along with predictions of a late Silurian oxygen rise.