The Ingersoll shale, an estuarine clay lens within the Upper Cretaceous Eutaw Formation of eastern Alabama that hosts a well-preserved terrestrial biota (plants, feathers, and amber), contains abundant narrow, discontinuous, straight to meandering, rarely branched, virtually flat and mostly horizontal ribbons of essentially pure, mainly framboidal pyrite. These structures likely originated as mucus films secreted by epibenthic or shallow endobenthic organisms, perhaps grazing gastropods and errant worms. Whatever their progenitors, mucus films served as labile hosts for sulfate-reducing bacteria and, consequently, were primary sites for very early iron-sulfide mineralization in Ingersoll substrates. Without mucus secretion and subsequent pyritization, evidence for relatively pervasive biogenic activity and environmental conditions would not have been preserved. By contrast, pyritization of body fossils in the Ingersoll shale terrestrial biota was limited, reflecting inhibited degradation of constituent refractory organic compounds in rapidly deposited, oxygen-deficient sediments.