Type - Diplomystus Fish

Period - Eocene

37 to 53 million years ago

Location - Wyoming, North America

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The Green River Formation, located in southwestern Wyoming, is made up of ancient lake beds that once formed a lush, subtropical environment. These calm, mineral-rich waters were the perfect conditions for fossilization.

When fish died, they sank to the bottom of the lake where oxygen levels were low and sediment quickly covered their bodies, helping preserve even the most delicate features like scales, fins and bones!

Unlike many fossil sites where you’re lucky to find a single bone or partial imprint, here you regularly find complete fossils that look as if the fish died only yesterday.

Diplomystus is another well-known fossil fish from the Green River Formation, this extinct genus is closely related to modern-day herrings but can be distinguished by its upturned mouth and more robust build.

With its distinctive jaw structure and impressive size, often reaching over 20 inches in length, Diplomystus was a mid-level predator that likely fed on smaller schooling fish like Knightia, and it’s not uncommon to find fossils showing a Diplomystus in the act of swallowing its prey!

Learn more about fish fossils from the Green River Formation here - The Green River Formation, Wyoming: A Hotbed Of Fossilised Fish