Preservation of mandibular zinc in a beetle from the Eocene Kishenehn Formation of Montana, USA

Fossil insects of the 46 million-year-old Coal Creek Member of the Kishenehn Formation exhibit exceptional preservation as evidenced by the preservation of color and the blood-derived pigment heme in a blood-engorged mosquito. In the present study, analysis of a fossil rove beetle (Coleoptera: Staphylinidae) from the Kishenehn Formation document preservation of zinc, a metal often used to harden the cutting surfaces of mandibles in extant insects, localized to the mandibles of the fossil insect. Scanning electron microscopy and energy-dispersive X-ray spectroscopy demonstrate that the carbonaceous bodies of preserved insects are physically homogeneous, composed primarily of carbon, and are distinct from the adjacent microbial mat within which the insects are thought to have been preserved. The microbial mat that covered the fossil insects is shown to consist of, in part, well-consolidated silicates. This thin layer, while completely transparent when wet, obscures the fossil when dry. The in situ preservation of components such as mandibular zinc and mosquito host blood-derived heme demonstrate that the carbonaceous bodies of Kishenehn Formation fossil insects contain some portion of their original contents. The thin layer of silicate-embedded mat may function to stabilize the fossil and its molecular components and may explain, in part, the exceptional preservation of the Kishenehn Formation fossils.