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Bear Gulch Paleoenvironment

detail of fossil SquatinactisThe Bear Gulch Limestone of central Montana has produced a large assemblage of vertebrate and invertebrate fossils since the inception of quarrying operations. The preservation of the Bear Gulch flora and fauna is remarkable in several ways. Vertebrate fossils are preserved along a spectrum that ranges from scattered scales to the extreme of venous systems, internal organs, and skin pigments. Fine traces of soft tissues are also preserved in some invertebrates.

Classical explanations for the preservation of such diverse and high quality fossil fauna invoke persistent anoxic or dysoxic bottom waters or sediments. Yet these explanations cannot be applied to the Bear Gulch, for in all lithologies and throughout the basin there is evidence of a ubiquitous bottom-living fish fauna, and including probable burrowing components. The preservation of such organisms indicates an aerobic benthic environment, albeit in very fine mud, sufficient to support vertebrate as well as invertebrate life.

The fossiliferous exposures of the Bear Gulch lens are visible in outcrop over about 70 square km and measure about 14 km east-west by 9 km north-south at it's widest, with a maximum sedimentary accumulation of about 30 m near its northeastern margin.

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