THE INVERTEBRATE PALEOECOLOGY, SEDIMENTOLOGY AND TAPHONOMY OF THE MISSISSIPPIAN BEAR GULCH LIMESTONE: A MARINE KONSERVAT LAGERSTäTTEN

The Late Mississippian Bear Gulch Limestone (BGL) is well exposed in central Montana and stands as one of the outstanding marine konservat lagerstätten in the United States. The BGL is best known for soft-tissue preservation of fish, but there are abundant and diverse invertebrates remaining to be studied. The BGL is a plattenkalk, a finely laminated micritic limestone. The processes controlling the alternating resistant flinz and weathered fäule laminasets are not known but may record tidal, seasonal, climatic or differential diagenesis. Discovery of an intraformational conglomerate at the top of the BGL in 2013 suggests subaerial exposure, which may explain some of the unique lithographic and taphonomic conditions. For example, mollusk shells are often completely dissolved. However, internal structures such as cephalopod mandibles and molds/casts of bivalves and gastropods are preserved. Preservation of these identifiable fossil signatures suggests that paleoecological trends can still be elucidated. Detailed paleoecological study of invertebrate benthic macrofauna may reveal if flinz and fäule laminasets represent environmental change or differential diagenesis.

Current stratigraphic models place the BGL within or very close to the Serpukhovian stage, globally a time of great biotic and climatic fluctuation. The BGL likely preserves a unique snapshot into this critical time in Earth’s history, which may further our understating of processes underway during the Serpukhovian Biodiversity Crisis (SBC).