Megamastax (meaning "big mouth") is a genus of lobe-finned fish which lived during the late Silurian period, about 423 million years ago, in China.

Before the discovery of Megamastax, it was thought that jawed vertebrates (gnathostomes) were limited in size and variation before the Devonian period.

In addition, Megamastax uniquely has a row of large, blunt teeth fused to four coronoid bones on the inside edge of each mandible.

[1] Various hard-shelled animals were present in the late Silurian of China, including brachipods, molluscs, trilobites, and even armored fish like placoderms and galeaspids.

Due to its large size and predatory lifestyle, Megamastax could be considered the first vertebrate apex predator.