The stratigraphic record of shallow-water shoreline paleoenvironments is characterized by significant facies heterogeneity and laterally discontinuous stratal geometries. In contrast, we investigate a uniquely extensive and microstratigraphically spatially uniform interval of upper Famennian (Upper Devonian) marginal marine strata in the Rocky Mountains. This transgressive deposit (< 5 m thick) rests on a depositional sequence boundary, and is composed of a thin, discontinuous basal transgressive sandstone bed, two thin fossiliferous shale beds, and several meters of oncolite-bearing carbonate wackestone. The lower shale is a consistently thin (~ 0.1 m) bed with a fossil fauna of spinicaudatans (clam shrimp or conchostracans), which are extant, bivalved, chitinous, benthic crustaceans that live in fresh to brackish water, in ephemeral ponds, estuaries, and other shoreline settings. Isotopic data for the oncolitic unit record deposition in an epicontinental seaway with restricted circulation and locally brackish conditions. Sulfur isotope data may also reflect short-term changes in redox conditions, consistent with our interpretation of temporary hypoxia during deposition of the spinicaudatan bed, based on its faunal assemblages. The remarkably large areal extent (~ 1600 km north to south, and ~ 1000 km east to west) of this uniformly thin interval with marginal marine fauna is one of the most unusual paleoecological events of the latest Devonian in Laurentia. It is considered an artifact of exceptional depositional processes including passive transgression (i.e., little or no wave or tidal ravinement) along with rapid opportunistic takeover of habitats during transgression. The radiation, possibly associated with temporary hypoxia, was aided by the spinicaudatan reproductive strategy of numerous offspring, high growth rate, and rapid reproduction.