Bear Gulch Beds

The Bear Gulch Limestone in Montana is a fossiliferous lagerstätte, a limestone layer laid down in the Mississippian epoch of the Carboniferous period, about 318 mya. This lens of limestone was laid down in a surrounding matrix that indicates a landscape of mudflats and braided channels (linear sandstone seams) in fresh and brackish water, in an arid climate (gypsum formation). Magnetic orientation of particles relative to the bedding planes of the strata suggest a latitude of about 12°N, a tropical siting.

In the Bear Gulch Limestone, many genera of fossil fishes and some invertebrates were occasionally preserved in detail when they died and sank to the bottom of a shallow marine or estuarine lagoon. In such cases, conditions on the silty bottom preventing the usual bacterial decomposition are usually invoked, characterized as close to anoxic, thus suppressing ordinary bacterial decomposition; however, fossils of fishes of distinctly bottom-dwelling type are present, and traces of worm burrows or similar bottom disturbance show that sufficient oxygen was available in the upper levels of bottom silt. Pulses of hypoxic or anoxic conditions and perhaps periodic episodes of heavy siltation—the lagoon laid down up to 90 ft/30 m of limestone during its existence— may have been involved.

Fossils
The numerous genera and many body types represented by fossils are witnesses to a highly diversified ecology with many available niches to be filled by specialists.

This deposit has yielded one of the most diverse and well preserved fossil fish assemblages in the world. We have excavated approximately 130 species of fish from this deposit over the last 35 years. The site also contains well preserved arthropods, sponges, starfish, conulariids, worms, and other soft-bodied organisms, as well as brachiopods, bryozoans, and molluscs. The Bear Gulch fossils are so well preserved that they provide a window into the life of the Mississippian that has never been available before. This site is dedicated to bringing you and the fishes of the Mississippian together.

The fish of the Bear Gulch Limestone include a lamprey, an acanthodian, over 65 species of "sharks", six coelacanths, a rhizodont, and a host of ray-finned bony fish. Most of these species had never been seen before by science and none had ever been seen intact before. Fossils are preserved along a spectrum that ranges from scattered scales to the extreme of beautiful sharks with their venous systems and skin pigments intact.

The Bear Gulch limestone is up to 90 feet thick and approximately 8 miles in east-west extent, and was deposited in a shallow, muddy, tropical marine bay 318 million years ago. The latitude of the bay was approximately 12° from the equator at the time of deposition. The climate of the surrounding area must have been seasonally quite arid, as there are gypsum beds in adjacent parts of the Heath Formation. Fish faunal diversity is high, while invertebrate diversity is only moderate. The high diversity of fish and the wide range of body forms is evidence of a complex ecosystem most similar to modern bay or estuarine communities.