The Ellimmichthyiformes, also known as double-armored herrings, are an extinct order of ray-finned fish known from the Early Cretaceous to the Oligocene.

[1] A highly successful group throughout the Cretaceous, they were found worldwide and are known to have inhabited both marine and freshwater habitats.

Following this, they saw an explosion in diversity during the Cenomanian, possibly due to the high sea levels of the Tethys at the time and the resulting impact on geography and food distribution, with the group evolving numerous different body plans.

Only a few freshwater species in the genera Diplomystus and Guiclupea survived primarily in North America and China (although the unusual Gasteroclupea of South America appears to have briefly survived into the Paleocene), including Diplomystus dentatus of the famous Green River Formation.

The last surviving ellimmichthyiform, Guiclupea, was a freshwater species that occurred in southern China during the Oligocene, representing the youngest member of the group.