Fishes of the modern type, with bony skeletons, date from about 415 million years ago. These Eocene (approx. 50 m.y.) specimens are from the Green River Formation. (more below)
These fossils of the fish species Knightia
are common items at any rock show or mineral shop. Fish like these, and other species like insects and plant leaves, are preserved by the millions in the creamy shale of the Green River Formation in Wyoming, Utah and Colorado. This rock unit consists of deposits that once lay at the bottom of three large, warm lakes during the Eocene Epoch (56 to 34 million years ago). Most of the northernmost lake beds, from the former Fossil Lake, are preserved in Fossil Butte National Monument
, but private quarries exist where you can dig your own.
Localities like the Green River Formation, where fossils are preserved in extraordinary numbers and detail, are known as lagerstätten. The study of how organic remains become fossils is known as taphonomy.