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.
The Age of Weird Sharks
The sharks of Bear Gulch have a wide variety of body forms. The most famous resident is Stethacanthus, a shark up to 3 meters long with a structure on its back like a flat-topped anvil. This "spine-brush complex" was present only in mature males and was covered in enlarged, tooth-like scales called denticles. A corresponding patch of denticles on the shark's head faced backward. Scientists theorize this entire apparatus was used in mating displays or to intimidate rivals.
Another remarkable find is Falcatus falcatus, a small shark around 30 cm long. Males had a single, forward-curving spine projecting from their heads. Its function would be a mystery, except for one incredible fossil that captures a female biting a male's head-spine, preserving a 325-million-year-old courtship behavior. Other oddities include the serpentine Thrinacodus and Iniopterygians, or "flying sharks," which had huge, wing-like pectoral fins mounted high on their bodies, likely used to "fly" through the water. This diversity shows a time when sharks and their kin were the dominant and most experimental fishes in the sea.