Very Rare Eohiodon (Mooneye) with Diplomystus dentatus | Green River Formation | Wyoming

Eohiodon approx. size: 4"

Diplomystus approx. size: 5.5"

Matrix approx. size: 8" x 6.5"

Order Hiodontiformes, Family Hiodontidae

Mooneyes are considerably rare in the Green River Formation.  They fed mostly on insects, but larger individuals also fed on crayfishes, shrimps, smaller fish, and frogs. Eohiodon falcatus closely resembles the osteoglossid Phareodus, but is distinguished from it by the lack of an enlarged pectoral fin, smaller scales, the presence of smaller dorsal and anal fins, smaller teeth, and a smaller head.

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.

Ray-finned Fish - Diplomystus dentatus

Order Ellimmichthyiformes, Family Paraclupeidae

The last known species of the Ellimmichthyiformes order went extinct sometime in the middle Eocene. D. dentatus is a primitive relative of the modern day herring. The genus Diplomystus is also known from fossil deposits in China.

D. dentatus is the 2nd most common fossil fish found from Fossil Lake. Specimens ranging from embryonic size (about 0.7 inches) to full-grown adults (about 26 inches) are common. Smaller-sized specimens are more commonly found in mid-lake than near-shore deposits. This suggests that D. dentatus spawned in open water.

This species' upturned mouth indicates it fed at the surface of Fossil Lake. D. dentatus specimens are commonly found with other fish stuck in their mouths, including other D. dentatus specimens.