Herbivorous and Detritivorous Arthropod Trace Fossils Associated with Subhumid Vegetation in the Middle Pennsylvanian of Southern Britain

Abstract

We describe plant arthropod associations from the Middle Pennsylvanian (late Bolsovian early Asturian) Pennant Sandstone Formation of southern Britain. Our material comprises calcified cordaitaleans and tree-fern axes, preserved in braided channel deposits, and interpreted as remains of subhumid riparian vegetation distinct from that of coeval coal swamps. The first plant arthropod association, attributed to herbivorous insects, comprises cambial damage to cordaitalean leafy branches, resulting in traumatic wound response. The second and most widespread association, attributable to detritivorous oribatid mites, includes tunnels and galleries containing widely scattered, clustered, or densely packed microcoprolites within the inner root mantle of marattialean tree ferns and cordaitalean trunks and branches. Diameter data for tunnels and microcoprolites are multimodal, recording four or five instars of oribatid mites that parallel instar-based fecal pellet and body lengths in modern taxa. The third association attributed, possibly, to an arthropleurid, comprises a single, very large (19 × 14 mm) coprolite. Included plant fragments support a previous conjecture that arborescent lycopsids formed part of this iconic arthropod's diet. Mucus-lined burrows within the macrocoprolite imply that fecal material was processed by annelids. The high diversity and frequency of plant arthropod associations are unusual for Mid-Pennsylvanian time, and may reflect previously undetected interactions in those ecosystems that lay outside coal forest swamps.