Green River Formation, Colorado/Wyoming
| Location | Colorado, Wyoming, and Utah, USA |
| Age | Eocene (~53–48 Ma) |
| Preservation | Lacustrine shale |
| Key Finds | Diverse Eocene insect fauna; post-Cretaceous bridge to modern forms |
| Note | Post-Cretaceous, included for context |
The Green River Formation is technically beyond the main scope of this site, which focuses on Mesozoic and Paleozoic insects. However, it is included here because it provides an important bridge between the Cretaceous insect world and the modern one. This Eocene deposit, dating to roughly 53-48 million years ago, preserves insect communities from the period shortly after the extinction of the dinosaurs, showing how insect faunas recovered and diversified in the early Cenozoic.
Geological Setting
The Green River Formation was deposited in a system of large, shallow lakes that covered parts of present-day Colorado, Wyoming, and Utah during the Eocene. The lakes were surrounded by subtropical forests, and the fine-grained lake sediments (oil shales) preserved an extraordinary range of organisms, including fish, plants, birds, mammals, and insects.
Insect Fossils
Green River insects include beetles, flies, wasps, ants, butterflies, moths, crickets, dragonflies, and many other orders. Many are identifiable to modern families, and some even to modern genera, showing that by the Eocene, the insect fauna was recognizably "modern" in composition. The site has produced some of the earliest well-preserved butterfly fossils and excellent specimens of various beetle families.
Significance
The Green River Formation shows what insect communities looked like in the aftermath of the K-Pg extinction. By the Eocene, insect diversity had recovered and in many ways surpassed pre-extinction levels, driven by the continued expansion of flowering plants. See From Ancient to Modern for context.