INSECT TAPHONOMY IN THE GREEN RIVER FORMATION OF COLORADO: PRESERVATION AS CARBONACEOUS COMPRESSIONS AND IRON OXIDES

The original organic composition, sedimentary context, and surrounding microbial community are important in determining the preservation quality and mineralogy of non-biomineralized fossils in marine settings. At first glance, it seems that the mineralogies and mechanisms preserving non-biomineralizing fossils in marine and lacustrine settings can often be similar, but it is not known if this actually reflects comparable taphonomic processes. To determine if the details of non-biomineralized fossil preservation in lacustrine environments are similar to those in marine environments, we examined the preservation of insects from three localities in the Green River Formation of Colorado by studying the relative abundance of insect taxa and by using scanning electron microscopy and energy dispersive x-ray spectroscopy (EDS) to assess fossil insect composition and quality.

Results thus far indicate that insect fossils in the Green River Formation are preserved similarly to their marine counterparts, with some interesting caveats. In two of the localities, insect fossils are preserved as carbonaceous compressions with a bias toward the preservation of recalcitrant insect cuticles, as is seen in arthropods in marine rocks. At a micrometer scale, the carbonaceous compressions are organized into a series of fine layers, although the ultrastructure of the chitinous precursor of the carbonaceous compressions is not preserved. Insects from the Paleoburn locality of the Green River Formation, however, are preserved as iron oxides. The taphonomic bias toward more robust insects is stronger here, with insects like beetles being preserved most frequently. These iron oxide insects are preserved similarly to pyritized arthropods in the marine realm. The lack of pyritization in the Green River Formation may be attributable to a lack of sulfate in the lake system. What is unknown at this point is whether preservation in iron oxides was through direct permineralization, or microbially mediated via iron-reduction bacteria. Future studies will concentrate on characterizing the microbial component of fossilization in the Green River Formation, as well as whether the sedimentological conditions which promote non-biomineralized fossilization in marine circumstances operate in the Green River Formation as well.