GHOST PRESERVATION: FOSSIL PRESERVATION FOSTERED BY BLEACHING

We report an unusual mode of fossil preservation that predominantly occurs in fine-grained volcanoclastic strata deposited in lacustrine to lagoonal paleoenvironments. This “ghost preservation” is rare but characterizes insects, spiders, crustaceans, and leaves in at least a dozen deposits ranging from Carboniferous to Miocene in age. Ghost preservation is unusual for two reasons: 1) because fossils are preserved both in high fidelity, such as the wing venation and antennae of lacewings; and 2) because fossils are preserved as nothing – that is, they are preserved as the absence of a stain on a bed surface.

Fossils that exhibit a transition from original cuticle to ghost preservation give a clue as to the means of fossilization. The original cuticle appears to have originally been preserved as carbonaceous material, and then later replaced by manganese oxides. The manganese oxides appear to have infilled the original cuticle and cellular structure of even the tiniest parts of insect exoskeletons, arachnid hairs, leaf veins, and the like. Manganese oxides, including birnessite, pyrolusite and related forms, are potent bleaching agents, and their presence could account for the bleached white pattern that mimics each fossil’s original morphology. In most ghosted specimens, the manganese oxides have completely dissolved away, leaving only small remnants of their presence behind.

On some bedding surfaces, there are specimens with ghost preservation that are completely surrounded by other specimens that are still preserved as cuticle or carbonaceous material. Why do some fossils exhibit this style of preservation when others don’t? Do ghosted fossils have preburial characteristics that increase their susceptibility to ghosting? If you know of specimens with this style of preservation, or have further insights, we would like to hear from you.