Eocene Green River Mioplosus and Diplomystus
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Mioplosus labracoides with Diplomystus dentatus
Green River Formation
Middle Eocene
Lincoln County, Wyoming
Sold Scott Beech
This is an very interestin example of a multiple species specimen from the Fossil Butte Member of the Green River Formation. This specimen has excellent preservation and detail for this locality. Often Mioplosus specimens have significant disarticulation but this example has perfect articulation. The Mioplosus fish body is 18 cm long. The Diplomystus skeleton is 13 cm. There is also a fry of probably a diplomystus visible on the slab. There is no repair or restoration.

Mioplosus is the largest perciform fish found in the Green River Formation. The genus makes up about 2 to 4 percent of the total fish fauna. Mioplosus is easy to distinguish from the other perciform genera in the formation because of its very elongate fusiform body shape, the presence of two distinct dorsal fins, and a moderately forked tail fin. The maximum size for this species is about 20 inches but very large examples are quite rare. Mioplosus was evidently a very predaceous species as indicated by dozens of known specimens with fishes in their jaws or stomachs. Even as very your recently hatched individuals, this species was already cannibalistic, feeding on other small Mioplosus.

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.
$325