WOOD DECOMPOSITION IN PALEOZOIC AND MODERN MIres

Wood is an ideal medium to explore decomposition processes in ancient and modern mires. The anatomical structure of wood, particularly of conifer and cordaitean wood, has not changed significantly since the Late Paleozoic. Fungal attack of modern wood produces taphonomic patterns that can be identified to broad groups of fungi, i.e. brown rot fungi - mostly basidiomycetes, and white rot fungi - ascomycetes and basidiomycetes. Organisms that consume wood or tunnel in wood for shelter frequently leave recognizable borings that may reveal the size of the boring organism, and reflect the purpose of the tunnel (i.e. shelter only, or feeding and shelter). Finally, wood occurs in permineralized peat, lignite, and coal; and it may be possible to trace patterns of wood decomposition from peat to coal. Wood has an extremely high C/N ratio (commonly 500 or more), making it a poor nutrient source for detritivores. Because fungal attack lowers the C/N ratio of wood, making it a better resource, many wood detritivores target fungally infested wood. Wood in Pennsylvanian permineralized peat (coal balls) shows widespread evidence of brown rot fungi, in the form of tracheids with collapsed cell walls, and of selective decomposition by white-rot fungi, in the form of ‘pulled apart’ tracheids in which the compound middle lamella has been destroyed. Evidence of simultaneous white rot (areas of wood with broken cell walls surrounded by undamaged wood) is comparatively rare in Pennsylvanian wood. In Pennsylvanian wood, most tunnels occur on the edges of wood pieces, and appear to be feeding and shelter traces. The most likely tunneling organisms are oribatid mites. Few tunnels in Pennsylvanian wood penetrate well-preserved wood. The common occurrence of tunnels on the fungally attacked edges of wood pieces in Pennsylvanian mires suggests that the tunneling organisms were soil-dwelling detritivores. In contrast, modern terrestrial arthropods (mostly insects) tunnel into wood both for shelter (carpenter ants) and for food and shelter (termites). Compared to Pennsylvanian wood-tunneling organisms, modern wood-tunneling organisms construct larger tunnels (up to 1.5 cm diameter), tunnel deeper into wood, and may tunnel into living wood, suggesting that rates of wood decomposition in mires may have increased through geologic time.