Diplomystus is an extinct freshwater fish related to modern herrings, alewives, and sardines. First described by paleontologist Edward Drinker Cope in 1877, the genus includes seven known species found across ancient lake deposits in North America and Asia.
The most famous species, Diplomystus dentatus, comes from Wyoming's Green River Formation. These fish ranged from tiny larvae to specimens up to 65 cm long and are often found fossilized alongside their relative, Knightia. They lived in a vast ancient lake system during the early Eocene, approximately 50 million years ago.
The Green River Formation preserves one of Earth's most complete records of ancient lake life. For six million years during the Eocene, sediments accumulated in intermountain lakes across present-day Colorado, Wyoming, and Utah.
Each year left behind two paper-thin layers of sediment—a dark layer from the growing season and a light layer from the dry season. These paired layers, called varves, average just 0.18 millimeters thick. The fine-grained limestone shale created perfect conditions for preserving fish and other organisms in exquisite detail.
The Eocene Epoch lasted from 56 to 33.9 million years ago. Its name means "dawn of the new" in Greek, referring to the emergence of many modern animal groups during this time.
Earth's climate was dramatically warmer than today, with average temperatures around 27°C (81°F). This "greenhouse world" had no polar ice caps and supported palm trees near the Arctic Circle. The Eocene ended with a major extinction event called the Grande Coupure, possibly triggered by asteroid impacts in Siberia and the Chesapeake Bay region.
This specimen was acquired in March 2024 from Stones & Bones Collection, a fossil dealer serving museums, educational institutions, and private collectors since 1989.