The Bear Gulch Limestone in Montana is fossiliferous strata dating to the Mississippian epoch of the Carboniferous period, at some 318 million years ago. The site is sometimes considered to meet the criteria for a Lagerstatt. Sediment studies suggest that fossil formation occurred in mudflats and lagoons having fresh to brackish waters. The fossils comprise diverse fishes and less abundant invertebrates, sometimes preserved in exquisite detail consistent with inhibited decay in a silty and oxygen-starved bottom. The limestone is some 30 meters thick, and was likely deposited during periodic episodes of heavy siltation that buried deceased members of the diverse ecosystem.

Bear Gulch Fossils
Caridosuctor populosum Coelacanth Heath Shale Formation, Montana