Coprolite-filled borings: insight into the life history of detritivorous Pennsylvanian terrestrial arthropods

Coal balls are our primary source of information about Pennsylvanian tropical peat swamps. They capture plant community composition and the complex ecological interactions between coeval terrestrial arthropods and plants. A small number of borings (or feeding tunnels) have been documented in permineralized and compression fossils from the Carboniferous Period. We report two borings filled with coprolites (fecal material) in a coal ball originating from the Mt. Rorah Coal Member (Tradewater Formation; Middle Pennsylvanian of Illinois, USA). Peels of the coal ball were prepared using the cellulose acetate peel method and digitized with a Zeiss Axio Zoom V16 microscope. The borings, measuring 3 cm and 6 cm in length, follow the transverse margin of Psaronius roots and are filled with two distinct types of uniformly shaped coprolites: type 1, ranging from 0.5–2 mm in diameter, and type 2, <60 μm in diameter. Both coprolite types consist of digested Psaronius root tissue and only occur in borings, with no signs of feeding damage observed in the surrounding plant tissues along the margins of the borings or the peel. The dimensions of the borings and morphology of type 1 coprolites suggest they were made by an early roachoid, millipede, or another terrestrial arthropod group. The distribution and content of type 2 coprolites indicate coprophagy, or secondary feeding on type 1 coprolites, likely by oribatid mites. These borings provide new insight into the life history of the organisms that created these tunnels, their role in litter decomposition, and the food webs of Pennsylvanian peat swamps.