A Carboniferous Pompeii
About 324 million years ago, what is now central Montana was a shallow, tropical bay teeming with life. This ancient body of water, located about 12 degrees north of the equator, experienced a monsoonal climate. During rainy seasons, storms washed huge amounts of fine-grained sediment into the bay, rapidly burying organisms and creating low-oxygen conditions on the bottom. This process led to the Bear Gulch Limestone, a geological formation known as a Lagerstätte—a site of exceptional fossil preservation. The rapid burial and lack of oxygen preserved skeletons and impressions of soft tissues like muscles, internal organs, and even blood vessels.
The result is a record of a complete marine ecosystem from the Mississippian subperiod of the Carboniferous. Over 130 species of fish have been identified here, with sharks and their relatives (chondrichthyans) making up nearly 60% of the fish species diversity. This differs from modern oceans, where cartilaginous fish comprise only about 3% of fish species. The fossils also include shrimp, worms, cephalopods, sponges, and one of the earliest known lampreys, Hardistiella montanensis. The entire 30-40 meter thick deposit may have formed in as little as 1,000 years.