Diplomystus: A Fossil Fish of the Green River Formation

Content written by Mike Bloodsworth

Diplomystus is one of the most iconic fossil fishes from the Eocene Epoch, preserved in the world-famous Green River Formation of Wyoming, Utah, and Colorado. Its striking appearance, frequent association with other fossil fishes, and abundance in museum collections and private hands make it a centerpiece in paleontology and fossil collecting.

Geological Context

Diplomystus thrived during the Eocene Epoch (approximately 56–34 million years ago), a time when the Earth was much warmer and vast freshwater lakes filled parts of what is now the western United States. Fine-grained lake sediments in the Green River Formation preserved entire ecosystems in remarkable detail, capturing fish, plants, insects, and even crocodiles.

Physical Characteristics

Family: Clupeomorpha (the group that includes modern herrings, sardines, and anchovies).

Body Form: Deep-bodied with a distinctive upturned mouth, giving it a profile different from its close relatives.

Size Range: Most fossils measure between 6 and 12 inches, though rare specimens exceed 20 inches in length. Our museum has several specimens in the 20 inch range.

Preservation: Fossils often display scales, fins, and skeletal details with astonishing clarity, due to the oxygen-poor lake bottoms where they were buried.

Feeding Habits and Ecology

Diplomystus’s upturned mouth suggests that it fed on prey near the surface of the lakes. It likely consumed smaller fish such as Knightia, another common Green River genus. Some fossils dramatically capture Diplomystus with a Knightia lodged halfway in its throat, preserving a snapshot of ancient predation.

While Mioplosus, a fellow predator of the Green River ecosystem, was a solitary ambush hunter, Diplomystus may have formed schools, resembling the feeding behaviors of modern herring-like fish.

Fossil Abundance and Significance

Diplomystus fossils are widespread in the Green River quarries, making them among the most accessible vertebrate fossils in the world. Their abundance, coupled with spectacular preservation, makes them invaluable for scientific research and popular with collectors. In absolute numbers, they are the second most common fossil fish in the Green River Formation.

Be sure to ask to see our collection Diplomystus and other fishes on your next visit to The Museum Of Natural History of Wyoming located on the campus of Sheridan College.