See text Diplomystus is an extinct genus of freshwater and marine clupeomorph fish distantly related to modern-day extant herrings, anchovies, and sardines.

[1] It was among the last surviving members of the formerly-diverse order Ellimmichthyiformes, with only its close relative Guiclupea living for longer.

The Green River Formation is the remnant of a large freshwater lake whose mud would eventually be transformed into soft calcite-bearing shale.

D. dentatus was a voracious predator on smaller fishes and is commonly found in close association with the extinct clupeid Knightia.

[6] The freshwater species D. shengliensis is known from the slightly younger (Middle Eocene, likely Bartonian)[4] Shahejie Formation of Shandong, China, where full specimens have been collected from boreholes.

It has been suggested that a brief exposure of Beringia during the Late Paleocene and early Eocene may have allowed for a rapid dispersal event between Asia & North America of Diplomystus and several other freshwater fish genera shared by both continents during the Paleogene.

[7] Despite their differing habitat, distribution, appearance and much earlier occurrence, morphological analyses have found them to be the closest relatives to the two Eocene species.

primotinus Uyeno, 1979) were previously placed in this genus, but morphological studies indicate that they are not true members of Diplomystus; however, they have not yet been reclassified.