Eocene Green River Diplomystus
Diplomystus dentatus
Green River Formation
Middle Eocene
Lincoln County, Wyoming
This interesting specimen contains baby and juvenile examples of Diplomystus. This slab probably represents an example from a shallow, warm-water facies of the Eocene lake that had good conditions for hatchlings. There are 5 Diplomystus fry skeletons and one fingerling skeleton. The slab is 27 cm by 17 cm.

Diplomystus is an extinct genus of freshwater clupeomorph fish distantly related to modern-day extant herrings, alewives, and sardines. The genus was first named and described by Edward Drinker Cope in 1877. D. dentatus (Cope, 1877) is well known from lower Eocene deposits from the Green River Formation in Wyoming. Specimens range from larval size to 65 cm and are commonly found in close association with the extinct herring Knightia sp. The Green River Formation is the remnant of a large lake whose mud would eventually be transformed into soft calcite-bearing shale.