Bear Gulch: An Exceptional
Upper Carboniferous Plattenkalk
James W. Hagadorn
THE UPPER MISSISSIPPIAN BEAR GULCH BEDS OF CENTRAL Montana constitute one of the lesser-known conservation Lagerstätten, but are of importance because they contain one of the most diverse fossil fish assemblages in the world, as well as a range of exquisitely preserved soft-bodied and skeletonized nektonic and benthic organisms. Soft tissues, phosphatic fossils, cartilaginous fossils, and molds of carbonate skeletal elements are preserved in this deposit. The Bear Gulch Beds are a lensoidal unit of fine-grained limestone, or Plattenkalk, very similar in nature to the famous Solnhofen Limestone of the Bavarian Jurassic (Chapter 18), but differing somewhat in depositional style. Sediments in this unit grade laterally and downward into black shales and were likely deposited in a quiescent, oxygenated shallow basin or estuary with poorly oxygenated fine-grained sediments (Williams 1983; Factor and Feldmann 1985). The Bear Gulch basin was periodically subjected to microturbidite sedimentation and, perhaps, rapid shifts in salinity—factors that may have killed resident and vagrant Bear Gulch faunas. For example, the best-preserved vertebrate fossils occur in the inferred depocenter of the basin, where rapid burial by fine-grained carbonate, restriction of bioturbation, and minimal transport are thought to have mitigated sealing of soft tissues in alternating thin and thick flat tabular limestones. The preservation of veins, skin pigments, organ pigments, gut contents, sexual organs, and other soft tissues has allowed reconstruction of the life habits, feeding strategies, sexual dimorphism, trophic structure, and evolutionary history of numerous taxa from this deposit.