Diplomystus is an extinct genus of freshwater fish distantly related to modern-day extant herrings, alewives, and sardines
There are seven species of Diplomystus: D. dentatus (Cope, 1877), D. birdii, D. dubetreiti, D. shengliensis (Chang 1983), D. kokuraensis (Uyeno 1979), D. primotinus (Uyeno 1979), and D. altiformis.
Specimens range from larval size to 65 cm and are commonly found in close association with the extinct herring Knightia. The Green River Formation is the remnant of a large lake whose mud would eventually be transformed into soft calcite-bearing shale. Diplomystus were dominant members of an Early Cretaceous lake fauna.
The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of inter-mountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The sediments are deposited in very fine layers, a dark layer during the growing season and a light-hue inorganic layer in the dry season. Each pair of layers is called a varve and represents one year. The sediments of the Green River Formation present a continuous record of six million years.