Microscale analysis of the fish Knightia eocaena taphonomy: implications for preservation by bacterial mats

Abstract

Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive Spectroscopy System (EDS) examination of well-preserved Knightia eocaena from the Eocene Green River documents the presence of bacteriomorphs. Bacteriomorphs in samples from the_ Fossil Butte Member consists of fibers with two types and spherical structures, all with associated framboidal Fe-oxides. Fibers and spheres consist of external molds composed of nano-scale calcium carbonate spheres. Fish bone surfaces are corroded by framboids and euhedral rhombohedral molds. Fe-oxide framboids have an amorphous exterior and a sieve-like internal structure. The sieve-like structure consists of nannometer-scale pores with wall composed of 100 nm scale spheres. Fe-oxides are consistent with bacterial-mediated precipitation of pyrite followed by an oxidation event. The presence of the preserved bacteriomorphs is consistent with degradation of soft tissue degradation into adipocere limiting the preservation of soft tissue and leaving refracted bone.