# Diplomystus

Diplomystus is an extinct genus of freshwater and marine clupeomorph fish distantly related to modern-day extant herrings, anchovies, and sardines. It is known from the United States, China, and Lebanon from the Late Cretaceous to the middle Eocene. Many other clupeomorph species from around the world were also formerly placed in the genus, due to it being a former wastebasket taxon. It was among the last surviving members of the formerly-diverse order Ellimmichthyiformes, with only its close relative Guiclupea living for longer.

## Taxonomy

The genus contains the following species:
* D. birdi Woodward, 1895 - Late Cretaceous (Cenomanian) of Lebanon (Sannine Formation)
* D. dentatus Cope, 1877 - Early Eocene of Wyoming, USA (Green River Formation)
* D. dubertreti Signeux, 1951 - Late Cretaceous (Santonian) of Lebanon (Sahel Alma)
* D. shengliensis Zhang, Zhou & Qing, 1985 - Middle Eocene (Bartonian) of China (Shahejie Formation)
The type species of the genus, and likely the most well-known ellimmichthyiform overall, is Diplomystus dentatus, due to its abundance in the famous lower Eocene lagerstätte of Fossil Butte National Monument, an exposure of the Green River Formation. They are also present but significantly less common in other members of the formation. The Green River Formation is the remnant of a large freshwater lake whose mud would eventually be transformed into soft calcite-bearing shale. Specimens of D. dentatus range from larval size to 65 cm, and reach their largest sizes in the Fossil Butte deposits. Some fossilized eggs are also known. D. dentatus was a voracious predator on smaller fishes and is commonly found in close association with the extinct clupeid Knightia. Many Diplomystus specimens are preserved with a Knightia lodged in their mouth, indicating that Diplomystus fed on the smaller clupeomorph despite it growing to nearly half the size of Diplomystus.
The freshwater species D. shengliensis is known from the slightly younger (Middle Eocene, likely Bartonian) Shahejie Formation of Shandong, China, where full specimens have been collected from boreholes. Despite occurring on the opposite side of the Pacific from the North American D. dentatus, it physically appears very similar to it. 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.
Two marine Diplomystus species, D. birdi and D. dubertreti, are known from Late Cretaceous-aged (Cenomanian and Santonian respectively) formations in Lebanon, and have much deeper bodies than the two Cenozoic freshwater species. 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.
The closest relative of Diplomystus was Guiclupea, an Oligocene genus from China and the last known ellimmichthyiform overall. Despite its late occurrence, a reconstructed phylogeny suggests it likely diverged from Diplomystus during the Early Cretaceous.

### Former species

Diplomystus was formerly used as a wastebasket taxon for many different species of fossil clupeomorphs.
A trio of Early Cretaceous (late Valanginian to early Barremian-aged) freshwater species that inhabited lakes in what is now Japan and Korea ('D.' altiformis Yabumoto, 1994, 'D.' kokuraensis Uyeno, 1979 , and 'D.' 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. These species are abundant enough to lend their names to an entire species assemblage (the "Diplomystus-Wakinoichthys Fauna"). Another tentatively assigned species, 'D.' trebecianensis Bannikov & Sorbini, 2000 from the Early Paleocene of Italy is among the last known marine ellimmichthyiforms, but likely does not belong to Diplomystus. The species 'D.' coverhamensis from the Late Cretaceous of New Zealand is considered an indeterminate clupeomorph.
The species 'D.' soligacni Gaudant & Gaudant, 1971 from the Late Cretaceous of Tunisia was previously assigned to this genus, but is now placed in Paraclupea. The former species 'D.' dartevellei Casier, 1965 from the Cenomanian of the Democratic Republic of the Congo is now placed in its own genus, Kwangoclupea. The Early Cretaceous species 'D.' longicostatus from Brazil and 'D.' goodi from Equatorial Guinea are placed in Ellimmichthys. The species 'D.' elatus from Italy is now placed in Armigatus. The species 'D.' vectensis from the Oligocene of the Isle of Wight may potentially belong to Knightia. The species "D." marmorensis from the Miocene of Turkey is now thought to belong to Clupeonella.