The “ptychopariid” Elrathia kingii is the most familiar and abundant trilobite in North America, but it occurs at only a few localities in the Middle Cambrian Wheeler Formation of Utah. E. kingii's unusual abundance and typically monospecific community association resulted from a novel, opportunistic ecological strategy. We infer that the trilobite occupied the exaerobic zone, at the boundary of anoxic and dysoxic bottom waters. E. kingii consistently occur in settings below the oxygen levels required by other contemporaneous epifaunal and infaunal benthic biota and may have derived energy from a food web that existed independently of phototrophic primary productivity. Although other fossil organisms are known to have preferred such environments, E. kingii is the earliest-known inhabitant of them, extending the documented range of the exaerobic ecological strategy into the Cambrian Period. We consider it likely that some other monospecific trilobite assemblages consisting of abundant articulated individuals may also be related to extreme, low-oxygen conditions.