Below are a couple of small plates from the Green River formation in Wyoming with the fry (juvenile) of a trout perch called Erismatopterus levatus. I bought them from a dealer at a local show this spring.
Based on the book "Paleontology of the Green River Formation with a Review of the Fish Fauna", by Lance Grande, Bulletin 63, Geological Survey of Wyoming, pg. 122-124, these fossils are found primarily in fossil lakes Gosiute and Unita. They are uncommon except for mass mortality zones possibly caused by short term lake temperature rises in the summer.
Click over to This Post I wrote two years ago about the Green River formation to see some diagrams of where the various fossil lakes that compose the formation once existed.